Shared library support for GNU ¶

1.1 Motivation for writing libtool ¶

Since early 1995, several different GNU developers have recognized the importance of having shared library support for their packages. The primary motivation for such a change is to encourage modularity and reuse of code (both conceptually and physically) in GNU programs.

Such a demand means that the way libraries are built in GNU packages needs to be general, to allow for any library type the package installer might want. The problem is compounded by the absence of a standard procedure for creating shared libraries on different platforms.

The following sections outline the major issues facing shared library support in GNU, and how shared library support could be standardized with libtool.

The following specifications were used in developing and evaluating this system:

1. The system must be as elegant as possible.
2. The system must be fully integrated with the GNU Autoconf and Automake utilities, so that it will be easy for GNU maintainers to use. However, the system must not require these tools, so that it can be used by non-GNU packages.
3. Portability to other (non-GNU) architectures and tools is desirable.

1.2 Implementation issues ¶

The following issues need to be addressed in any reusable shared library system, specifically libtool:

1. The package installer should be able to control what sort of libraries are built.
2. It can be tricky to run dynamically linked programs whose libraries have not yet been installed. LD_LIBRARY_PATH must be set properly (if it is supported), or programs fail to run.
3. The system must operate consistently even on hosts that don’t support shared libraries.
4. The commands required to build shared libraries may differ wildly from host to host. These need to be determined at configure time in a consistent way.
5. It is not always obvious with what prefix or suffix a shared library should be installed. This makes it difficult for Makefile rules, since they generally assume that file names are the same from host to host.
6. The system needs a simple library version number abstraction, so that shared libraries can be upgraded in place. The programmer should be informed how to design the interfaces to the library to maximize binary compatibility.
7. The install Makefile target should warn the package installer to set the proper environment variables (LD_LIBRARY_PATH or equivalent), or run ldconfig.

1.3 Other implementations ¶

Even before libtool was developed, many free software packages built and installed their own shared libraries. At first, these packages were examined to avoid reinventing existing features.

Now it is clear that none of these packages have documented the details of shared library systems that libtool requires. So, other packages have been more or less abandoned as influences.

1.4 A postmortem analysis of other implementations ¶

In all fairness, each of the implementations that were examined do the job that they were intended to do, for a number of different host systems. However, none of these solutions seem to function well as a generalized, reusable component.

Most were too complex to use (much less modify) without understanding exactly what the implementation does, and they were generally not documented.

The main difficulty is that different vendors have different views of what libraries are, and none of the packages that were examined seemed to be confident enough to settle on a single paradigm that just works.

Ideally, libtool would be a standard that would be implemented as series of extensions and modifications to existing library systems to make them work consistently. However, it is not an easy task to convince operating system developers to mend their evil ways, and people want to build shared libraries right now, even on buggy, broken, confused operating systems.

For this reason, libtool was designed as an independent shell script. It isolates the problems and inconsistencies in library building that plague Makefile writers by wrapping the compiler suite on different platforms with a consistent, powerful interface.

With luck, libtool will be useful to and used by the GNU community, and that the lessons that were learned in writing it will be taken up by designers of future library systems.

3.1 Creating object files ¶

To create an object file from a source file, the compiler is invoked with the -c flag (and any other desired flags):

burger$ gcc -I. -g -O -c main.c
burger$

The above compiler command produces an object file, usually named main.o, from the source file main.c.

For most library systems, creating object files that become part of a static library is as simple as creating object files that are linked to form an executable:

burger$ gcc -I. -g -O -c foo.c
burger$ gcc -I. -g -O -c hello.c
burger$

Shared libraries, however, may only be built from position-independent code (PIC). So, special flags must be passed to the compiler to tell it to generate PIC rather than the standard position-dependent code.

Since this is a library implementation detail, libtool hides the complexity of PIC compiler flags and uses separate library object files (the PIC one lives in the .libs subdirectory and the static one lives in the current directory). On systems without shared libraries, the PIC library object files are not created, whereas on systems where all code is PIC, such as AIX, the static ones are not created.

To create library object files for foo.c and hello.c, simply invoke libtool with the standard compilation command as arguments (see Compile mode):

a23$ libtool --mode=compile gcc -I. -g -O -c foo.c
gcc -I. -g -O -c foo.c -o foo.o
a23$ libtool --mode=compile gcc -I. -g -O -c hello.c
gcc -I. -g -O -c hello.c -o hello.o
a23$

Note that libtool silently creates an additional control file on each ‘compile’ invocation. The .lo file is the libtool object, which Libtool uses to determine what object file may be built into a shared library. On ‘a23’, only static libraries are supported so the library objects look like this:

# foo.lo - a libtool object file
# Generated by ltmain.sh (GNU libtool) 2.5.4
#
# Please DO NOT delete this file!
# It is necessary for linking the library.

# Name of the PIC object.
pic_object=none

# Name of the non-PIC object.
non_pic_object='foo.o'

On shared library systems, libtool automatically generates an additional PIC object by inserting the appropriate PIC generation flags into the compilation command:

burger$ libtool --mode=compile gcc -I. -g -O -c foo.c
mkdir .libs
gcc -I. -g -O -c foo.c  -fPIC -DPIC -o .libs/foo.o
gcc -I. -g -O -c foo.c -o foo.o >/dev/null 2>&1
burger$

Note that Libtool automatically created the .libs directory upon its first execution, where PIC library object files will be stored.

Since ‘burger’ supports shared libraries, and requires PIC objects to build them, Libtool has compiled a PIC object and made a note of it in the libtool object:

# foo.lo - a libtool object file
# Generated by ltmain.sh (GNU libtool) 2.5.4
#
# Please DO NOT delete this file!
# It is necessary for linking the library.

# Name of the PIC object.
pic_object='.libs/foo.o'

# Name of the non-PIC object.
non_pic_object='foo.o'

Notice the second run of GCC has its output discarded. This is done so the compiler warnings aren’t annoyingly duplicated. If you need to see both sets of warnings (you might have conditional code inside ‘#ifdef PIC’ for example), you can turn off suppression by passing the -no-suppress option to libtool’s compile mode:

burger$ libtool --mode=compile gcc -no-suppress -I. -g -O -c hello.c
gcc -I. -g -O -c hello.c  -fPIC -DPIC -o .libs/hello.o
gcc -I. -g -O -c hello.c -o hello.o
burger$

3.2 Linking libraries ¶

Without libtool, the programmer would invoke the ar command to create a static library:

burger$ ar cr libhello.a hello.o foo.o
burger$

But of course, that would be too simple, so many systems require that you run the ranlib command on the resulting library in order to generate a symbol table:

burger$ ranlib libhello.a
burger$

It seems more natural to use the C compiler for this task, given libtool’s “libraries are programs” approach. So, on platforms without shared libraries, libtool simply acts as a wrapper for the system ar (and possibly ranlib) commands.

Again, the libtool control file name (.la suffix) differs from the standard library name (.a suffix). The arguments to libtool are the same ones you would use to produce an executable named libhello.la with your compiler (see Link mode):

a23$ libtool --mode=link gcc -g -O -o libhello.la foo.o hello.o
*** Warning: Linking the shared library libhello.la against the
*** non-libtool objects foo.o hello.o is not portable!
ar cr .libs/libhello.a
ranlib .libs/libhello.a
creating libhello.la
(cd .libs && rm -f libhello.la && ln -s ../libhello.la libhello.la)
a23$

Aha! Libtool caught a common error… trying to build a library from standard objects instead of special .lo object files. This doesn’t matter so much for static libraries, but on shared library systems, it is of great importance. (Note that you may replace libhello.la with libhello.a in which case libtool won’t issue the warning any more. But although this method works, this is not intended to be used because it makes you lose the benefits of using Libtool.)

So, let’s try again, this time with the library object files. Remember also that we need to add -lm to the link command line because foo.c uses the cos math library function (see Using libtool).

Another complication in building shared libraries is that we need to specify the path to the directory where they will (eventually) be installed (in this case, /usr/local/lib)¹:

a23$ libtool --mode=link gcc -g -O -o libhello.la foo.lo hello.lo \
                -rpath /usr/local/lib -lm
ar cr .libs/libhello.a foo.o hello.o
ranlib .libs/libhello.a
creating libhello.la
(cd .libs && rm -f libhello.la && ln -s ../libhello.la libhello.la)
a23$

Now, let’s try the same trick on the shared library platform:

burger$ libtool --mode=link gcc -g -O -o libhello.la foo.lo hello.lo \
                -rpath /usr/local/lib -lm
rm -fr  .libs/libhello.a .libs/libhello.la
ld -Bshareable -o .libs/libhello.so.0.0 .libs/foo.o .libs/hello.o -lm
ar cr .libs/libhello.a foo.o hello.o
ranlib .libs/libhello.a
creating libhello.la
(cd .libs && rm -f libhello.la && ln -s ../libhello.la libhello.la)
burger$

Now that’s significantly cooler… Libtool just ran an obscure ld command to create a shared library, as well as the static library.

Note how libtool creates extra files in the .libs subdirectory, rather than the current directory. This feature makes it easier to clean up the build directory, and helps ensure other programs fail horribly if you accidentally forget to use libtool when you should.

Again, you should look at the .la file to see what Libtool stores in it. You will see Libtool uses this file to remember the destination directory for the library (the argument to -rpath) as well as the dependency on the math library (‘-lm’).

3.3 Linking executables ¶

If you choose at this point to install the library (put it in a permanent location) before linking executables against it, then you don’t need to use libtool to do the linking. Simply use the appropriate -L and -l flags to specify the library’s location.

Some system linkers insist on encoding the full directory name of each shared library in the resulting executable. Libtool has to work around this misfeature by special magic to ensure that only permanent directory names are put into installed executables.

The importance of this bug must not be overlooked: it won’t cause programs to crash in obvious ways. It creates a security hole, and possibly even worse, if you are modifying the library source code after you have installed the package, you will change the behaviour of the installed programs!

So, if you want to link programs against the library before you install it, you must use libtool to do the linking.

Here’s the old way of linking against an uninstalled library:

burger$ gcc -g -O -o hell.old main.o libhello.a -lm
burger$

Libtool’s way is almost the same² (see Link mode):

a23$ libtool --mode=link gcc -g -O -o hell main.o libhello.la
gcc -g -O -o hell main.o  ./.libs/libhello.a -lm
a23$

That looks too simple to be true. All libtool did was transform libhello.la to ./.libs/libhello.a, but remember that ‘a23’ has no shared libraries. Notice Libtool also remembered libhello.la depends on -lm, so even though we didn’t specify -lm on the libtool command line³ Libtool has added it to the gcc link line for us.

On ‘burger’ Libtool links against the uninstalled shared library:

burger$ libtool --mode=link gcc -g -O -o hell main.o libhello.la
gcc -g -O -o .libs/hell main.o -L./.libs -R/usr/local/lib -lhello -lm
creating hell
burger$

Now assume libhello.la had already been installed, and you want to link a new program with it. You could figure out where it lives by yourself, then run:

burger$ gcc -g -O -o test test.o -L/usr/local/lib -lhello -lm

However, unless /usr/local/lib is in the standard library search path, you won’t be able to run test. If instead you use libtool to link the already-installed libtool library, it will do The Right Thing (TM) for you:

burger$ libtool --mode=link gcc -g -O -o test test.o \
                /usr/local/lib/libhello.la
gcc -g -O -o .libs/test test.o -Wl,--rpath \
        -Wl,/usr/local/lib /usr/local/lib/libhello.a -lm
creating test
burger$

Note that libtool added the necessary run-time path flag, as well as -lm, the library libhello.la depended upon. Nice, huh?

Notice the executable, hell, was actually created in the .libs subdirectory. Then, a wrapper script (or, on certain platforms, a wrapper executable see Wrapper executables for uninstalled programs) was created in the current directory.

Since libtool created a wrapper script, you should use libtool to install it and debug it too. However, since the program does not depend on any uninstalled libtool library, it is probably usable even without the wrapper script.

On NetBSD 1.2, libtool encodes the installation directory of libhello, by using the ‘-R/usr/local/lib’ compiler flag. Then, the wrapper script guarantees the executable finds the correct shared library (the one in ./.libs) so it can be properly installed.

Let’s compare the two different programs:

burger$ time ./hell.old
Welcome to GNU Hell!
** This is not GNU Hello.  There is no built-in mail reader. **
        0.21 real         0.02 user         0.08 sys
burger$ time ./hell
Welcome to GNU Hell!
** This is not GNU Hello.  There is no built-in mail reader. **
        0.63 real         0.09 user         0.59 sys
burger$

The wrapper script takes significantly longer to execute, but at least the results are correct, even though the shared library hasn’t been installed yet.

So, what about all the space savings that shared libraries are supposed to yield?

burger$ ls -l hell.old libhello.a
-rwxr-xr-x  1 gord  gord  15481 Nov 14 12:11 hell.old
-rw-r--r--  1 gord  gord   4274 Nov 13 18:02 libhello.a
burger$ ls -l .libs/hell .libs/libhello.*
-rwxr-xr-x  1 gord  gord  11647 Nov 14 12:10 .libs/hell
-rw-r--r--  1 gord  gord   4274 Nov 13 18:44 .libs/libhello.a
-rwxr-xr-x  1 gord  gord  12205 Nov 13 18:44 .libs/libhello.so.0.0
burger$

Well, that sucks. Maybe I should just scrap this project and take up basket weaving.

Actually, it just proves an important point: shared libraries incur overhead because of their (relative) complexity. In this situation, the price of being dynamic is eight kilobytes, and the payoff is about four kilobytes. So, having a shared libhello won’t be an advantage until we link it against at least a few more programs.

• Wrapper executables for uninstalled programs

3.4 Debugging executables ¶

If hell was a complicated program, you would certainly want to test and debug it before installing it on your system. In the above section, you saw how the libtool wrapper script makes it possible to run the program directly, but unfortunately, this mechanism interferes with the debugger:

burger$ gdb hell
GDB is free software and you are welcome to distribute copies of it
 under certain conditions; type "show copying" to see the conditions.
There is no warranty for GDB; type "show warranty" for details.
GDB 4.16 (i386-unknown-netbsd), (C) 1996 Free Software Foundation, Inc.

"hell": not in executable format: File format not recognized

(gdb) quit
burger$

Sad. It doesn’t work because GDB doesn’t know where the executable lives. So, let’s try again, by invoking GDB directly on the executable:

burger$ gdb .libs/hell
GNU gdb 5.3 (i386-unknown-netbsd)
Copyright 2002 Free Software Foundation, Inc.
GDB is free software, covered by the GNU General Public License,
and you are welcome to change it and/or distribute copies of it
under certain conditions.  Type "show copying" to see the conditions.
There is no warranty for GDB.  Type "show warranty" for details.
(gdb) break main
Breakpoint 1 at 0x8048547: file main.c, line 29.
(gdb) run
Starting program: /home/src/libtool/demo/.libs/hell
/home/src/libtool/demo/.libs/hell: can't load library 'libhello.so.0'

Program exited with code 020.
(gdb) quit
burger$

Argh. Now GDB complains because it cannot find the shared library that hell is linked against. So, we must use libtool to properly set the library path and run the debugger. Fortunately, we can forget all about the .libs directory, and just run it on the executable wrapper (see Execute mode):

burger$ libtool --mode=execute gdb hell
GNU gdb 5.3 (i386-unknown-netbsd)
Copyright 2002 Free Software Foundation, Inc.
GDB is free software, covered by the GNU General Public License,
and you are welcome to change it and/or distribute copies of it
under certain conditions.  Type "show copying" to see the conditions.
There is no warranty for GDB.  Type "show warranty" for details.
(gdb) break main
Breakpoint 1 at 0x8048547: file main.c, line 29.
(gdb) run
Starting program: /home/src/libtool/demo/.libs/hell

Breakpoint 1, main (argc=1, argv=0xbffffc40) at main.c:29
29        printf ("Welcome to GNU Hell!\n");
(gdb) quit
The program is running.  Quit anyway (and kill it)? (y or n) y
burger$

3.5 Installing libraries ¶

Installing libraries on a non-libtool system is quite straightforward… just copy them into place:⁴

burger$ su
Password: ********
burger# cp libhello.a /usr/local/lib/libhello.a
burger#

Oops, don’t forget the ranlib command:

burger# ranlib /usr/local/lib/libhello.a
burger#

Libtool installation is quite simple, as well. Just use the install or cp command that you normally would (see Install mode):

a23# libtool --mode=install cp libhello.la /usr/local/lib/libhello.la
cp libhello.la /usr/local/lib/libhello.la
cp .libs/libhello.a /usr/local/lib/libhello.a
ranlib /usr/local/lib/libhello.a
a23#

Note that the libtool library libhello.la is also installed, to help libtool with uninstallation (see Uninstall mode) and linking (see Linking executables) and to help programs with dlopening (see Dlopened modules).

Here is the shared library example:

burger# libtool --mode=install install -c libhello.la \
                /usr/local/lib/libhello.la
install -c .libs/libhello.so.0.0 /usr/local/lib/libhello.so.0.0
install -c libhello.la /usr/local/lib/libhello.la
install -c .libs/libhello.a /usr/local/lib/libhello.a
ranlib /usr/local/lib/libhello.a
burger#

It is safe to specify the -s (strip symbols) flag if you use a BSD-compatible install program when installing libraries. Libtool will either ignore the -s flag, or will run a program that will strip only debugging and compiler symbols from the library.

Once the libraries have been put in place, there may be some additional configuration that you need to do before using them. First, you must make sure that where the library is installed actually agrees with the -rpath flag you used to build it.

Then, running ‘libtool -n finish libdir’ can give you further hints on what to do (see Finish mode):

burger# libtool -n finish /usr/local/lib
PATH="$PATH:/sbin" ldconfig -m /usr/local/lib
-----------------------------------------------------------------
Libraries have been installed in:
   /usr/local/lib

To link against installed libraries in a given directory, LIBDIR,
you must use the '-LLIBDIR' flag during linking.

 You will also need to do one of the following:
   - add LIBDIR to the 'LD_LIBRARY_PATH' environment variable
     during execution
   - add LIBDIR to the 'LD_RUN_PATH' environment variable
     during linking
   - use the '-RLIBDIR' linker flag

See any operating system documentation about shared libraries for
more information, such as the ld and ld.so manual pages.
-----------------------------------------------------------------
burger#

After you have completed these steps, you can go on to begin using the installed libraries. You may also install any executables that depend on libraries you created.

3.6 Installing executables ¶

If you used libtool to link any executables against uninstalled libtool libraries (see Linking executables), you need to use libtool to install the executables after the libraries have been installed (see Installing libraries).

So, for our Ultrix example, we would run:

a23# libtool --mode=install install -c hell /usr/local/bin/hell
install -c hell /usr/local/bin/hell
a23#

On shared library systems that require wrapper scripts, libtool just ignores the wrapper script and installs the correct binary:

burger# libtool --mode=install install -c hell /usr/local/bin/hell
install -c .libs/hell /usr/local/bin/hell
burger#

3.7 Linking static libraries ¶

Why return to ar and ranlib silliness when you’ve had a taste of libtool? Well, sometimes it is desirable to create a static archive that can never be shared. The most frequent case is when you have a set of object files that you use to build several different libraries. You can create a “convenience library” out of those objects, and link against that with the other libraries, instead of listing all the object files every time.

If you just want to link this convenience library into programs, then you could just ignore libtool entirely, and use the old ar and ranlib commands (or the corresponding GNU Automake ‘_LIBRARIES’ rules). You can even install a convenience library using GNU Libtool, though you probably don’t want to and hence GNU Automake doesn’t allow you to do so.

burger$ libtool --mode=install ./install-sh -c libhello.a \
                /local/lib/libhello.a
./install-sh -c libhello.a /local/lib/libhello.a
ranlib /local/lib/libhello.a
burger$

Using libtool for static library installation protects your library from being accidentally stripped (if the installer used the -s flag), as well as automatically running the correct ranlib command.

But libtool libraries are more than just collections of object files: they can also carry library dependency information, which old archives do not. If you want to create a libtool static convenience library, you can omit the -rpath flag and use -static to indicate that you’re only interested in a static library. When you link a program with such a library, libtool will actually link all object files and dependency libraries into the program.

If you omit both -rpath and -static, libtool will create a convenience library that can be used to create other libtool libraries, even shared ones. Just like in the static case, the library behaves as an alias to a set of object files and dependency libraries, but in this case the object files are suitable for inclusion in shared libraries. But be careful not to link a single convenience library, directly or indirectly, into a single program or library, otherwise you may get errors about symbol redefinitions.

The key is remembering that a convenience library contains PIC objects, and can be linked where a list of PIC objects makes sense; i.e. into a shared library. A static convenience library contains non-PIC objects, so can be linked into an old static library, or a program.

When GNU Automake is used, you should use noinst_LTLIBRARIES instead of lib_LTLIBRARIES for convenience libraries, so that the -rpath option is not passed when they are linked.

As a rule of thumb, link a libtool convenience library into at most one libtool library, and never into a program, and link libtool static convenience libraries only into programs, and only if you need to carry library dependency information to the user of the static convenience library.

Another common situation where static linking is desirable is in creating a standalone binary. Use libtool to do the linking and add the -all-static flag.

4.1 Compile mode ¶

For compile mode, mode-args is a compiler command to be used in creating a “standard” object file. These arguments should begin with the name of the C compiler, and contain the -c compiler flag so that only an object file is created.

Libtool determines the name of the output file by removing the directory component from the source file name, then substituting the source code suffix (e.g. ‘.c’ for C source code) with the library object suffix, ‘.lo’.

If shared libraries are being built, any necessary PIC generation flags are substituted into the compilation command.

The following components of mode-args are treated specially:

-o

Note that the -o option is now fully supported. It is emulated on the platforms that don’t support it (by locking and moving the objects), so it is really easy to use libtool, just with minor modifications to your Makefiles. Typing for example

libtool --mode=compile gcc -c foo/x.c -o foo/x.lo

will do what you expect.

Note, however, that, if the compiler does not support -c and -o, it is impossible to compile foo/x.c without overwriting an existing ./x.o. Therefore, if you do have a source file ./x.c, make sure you introduce dependencies in your Makefile to make sure ./x.o (or ./x.lo) is re-created after any sub-directory’s x.lo:

x.o x.lo: foo/x.lo bar/x.lo

This will also ensure that make won’t try to use a temporarily corrupted x.o to create a program or library. It may cause needless recompilation on platforms that support -c and -o together, but it’s the only way to make it safe for those that don’t.

-no-suppress

If both PIC and non-PIC objects are being built, libtool will normally suppress the compiler output for the PIC object compilation to save showing very similar, if not identical duplicate output for each object. If the -no-suppress option is given in compile mode, libtool will show the compiler output for both objects.

-prefer-pic

Libtool will try to build only PIC objects.

-prefer-non-pic

Libtool will try to build only non-PIC objects.

-shared

Even if Libtool was configured with --enable-static, the object file Libtool builds will not be suitable for static linking. Libtool will signal an error if it was configured with --disable-shared, or if the host does not support shared libraries.

-static

Even if libtool was configured with --disable-static, the object file Libtool builds will be suitable for static linking.

-Wc,flag

-Xcompiler flag

Pass a flag directly to the compiler. With -Wc,, multiple flags may be separated by commas, whereas -Xcompiler passes through commas unchanged.

4.2 Link mode ¶

Link mode links together object files (including library objects) to form another library or to create an executable program.

mode-args consist of a command using the C compiler to create an output file (with the -o flag) from several object files.

The following components of mode-args are treated specially:

-all-static ¶

If output-file is a program, then do not link it against any shared libraries at all. If output-file is a library, then only create a static library. In general, this flag cannot be used together with ‘disable-static’ (see The LT_INIT macro).

-avoid-version

Tries to avoid versioning (see Library interface versions) for libraries and modules, i.e. no version information is stored and no symbolic links are created. If the platform requires versioning, this option has no effect.

-bindir

Pass the absolute name of the directory for installing executable programs (see Directory Variables in The GNU Coding Standards). libtool may use this value to install shared libraries there on systems that do not provide for any library hardcoding and use the directory of a program and the PATH variable as library search path. This is typically used for DLLs on Windows or other systems using the PE (Portable Executable) format. On other systems, -bindir is ignored. The default value used is libdir/../bin for libraries installed to libdir. You should not use -bindir for modules.

-dlopen file

Same as -dlpreopen file, if native dlopening is not supported on the host platform (see Dlopened modules) or if the program is linked with -static, -static-libtool-libs, or -all-static. Otherwise, no effect. If file is self Libtool will make sure that the program can dlopen itself, either by enabling -export-dynamic or by falling back to -dlpreopen self.

-dlpreopen file

Link file into the output program, and add its symbols to the list of preloaded symbols (see Dlpreopening). If file is self, the symbols of the program itself will be added to preloaded symbol lists. If file is force Libtool will make sure that a preloaded symbol list is always defined, regardless of whether it’s empty or not.

-export-dynamic

Allow symbols from output-file to be resolved with dlsym (see Dlopened modules).

-export-symbols symfile

Tells the linker to export only the symbols listed in symfile. The symbol file should end in .sym and must contain the name of one symbol per line. This option has no effect:

• on static libraries, and
• on shared libraries on some platforms, such as AIX and Haiku.

By default all symbols are exported.

-export-symbols-regex regex

Same as -export-symbols, except that only symbols matching the regular expression regex are exported. By default all symbols are exported.

-Llibdir

Search libdir for required libraries that have already been installed.

-lname

output-file requires the installed library libname. This option is required even when output-file is not an executable.

-module

Creates a library that can be dlopened (see Dlopened modules). This option doesn’t work for programs. Module names don’t need to be prefixed with ‘lib’. In order to prevent name clashes, however, libname and name must not be used at the same time in your package.

-no-fast-install

Disable fast-install mode for the executable output-file. Useful if the program won’t be necessarily installed.

-no-install

Link an executable output-file that can’t be installed and therefore doesn’t need a wrapper script on systems that allow hardcoding of library paths. Useful if the program is only used in the build tree, e.g., for testing or generating other files.

-no-undefined

Declare that output-file does not depend on any libraries other than the ones listed on the command line, i.e., after linking, it will not have unresolved symbols. Some platforms require all symbols in shared libraries to be resolved at library creation (see Inter-library dependencies), and using this parameter allows libtool to assume that this will not happen.

-o output-file

Create output-file from the specified objects and libraries.

-objectlist file

Use a list of object files found in file to specify objects.

-os2dllname name

Use this to change the DLL base name on OS/2 to name, to keep within the 8 character base name limit on this system.

-precious-files-regex regex

Prevents removal of files from the temporary output directory whose names match this regular expression. You might specify ‘\.bbg?$’ to keep those files created with gcc -ftest-coverage for example.

-release release

Specify that the library was generated by release release of your package, so that users can easily tell what versions are newer than others. Be warned that no two releases of your package will be binary compatible if you use this flag. If you want binary compatibility, use the -version-info flag instead (see Library interface versions).

-rpath libdir

If output-file is a library, it will eventually be installed in libdir. If output-file is a program, add libdir to the run-time path of the program. On platforms that don’t support hardcoding library paths into executables and only search PATH for shared libraries, such as when output-file is a Windows (or other PE platform) DLL, the .la control file will be installed in libdir, but see -bindir above for the eventual destination of the .dll or other library file itself.

-R libdir

If output-file is a program, add libdir to its run-time path. If output-file is a library, add -Rlibdir to its dependency_libs, so that, whenever the library is linked into a program, libdir will be added to its run-time path.

-shared

If output-file is a program, then link it against any uninstalled shared libtool libraries (this is the default behavior). If output-file is a library, then only create a shared library. In the later case, libtool will signal an error if it was configured with --disable-shared, or if the host does not support shared libraries.

-shrext suffix

If output-file is a libtool library, replace the system’s standard file name extension for shared libraries with suffix (most systems use .so here). This option is helpful in certain cases where an application requires that shared libraries (typically modules) have an extension other than the default one. Please note you must supply the full file name extension including any leading dot.

-static

If output-file is a program, then do not link it against any uninstalled shared libtool libraries. If output-file is a library, then only create a static library.

-static-libtool-libs

If output-file is a program, then do not link it against any shared libtool libraries. If output-file is a library, then only create a static library.

-version-info current[:revision[:age]]

If output-file is a libtool library, use interface version information current, revision, and age to build it (see Library interface versions). Do not use this flag to specify package release information, rather see the -release flag.

-version-number major[:minor[:revision]]

If output-file is a libtool library, compute interface version information so that the resulting library uses the specified major, minor and revision numbers. This is designed to permit libtool to be used with existing projects where identical version numbers are already used across operating systems. New projects should use the -version-info flag instead.

-weak libname

if output-file is a libtool library, declare that it provides a weak libname interface. This is a hint to libtool that there is no need to append libname to the list of dependency libraries of output-file, because linking against output-file already supplies the same interface (see Linking with dlopened modules).

-Wc,flag

-Xcompiler flag

Pass a linker-specific flag directly to the compiler. With -Wc,, multiple flags may be separated by commas, whereas -Xcompiler passes through commas unchanged.

-Wa,flag

-Xassembler flag

Pass a linker-specific flag directly to the assembler. With -Wa,, multiple flags may be separated by commas, whereas -Xassembler passes through commas unchanged.

-Wl,flag

-Xlinker flag

Pass a linker-specific flag directly to the linker.

-XCClinker flag

Pass a link-specific flag to the compiler driver (CC) during linking.

If the output-file ends in .la, then a libtool library is created, which must be built only from library objects (.lo files). The -rpath option is required. In the current implementation, libtool libraries may not depend on other uninstalled libtool libraries (see Inter-library dependencies).

If the output-file ends in .a, then a standard library is created using ar and possibly ranlib.

If output-file ends in .o or .lo, then a reloadable object file is created from the input files (generally using ‘ld -r’). This method is often called partial linking.

Otherwise, an executable program is created.

4.3 Execute mode ¶

For execute mode, the library path is automatically set, then a program is executed.

The first of the mode-args is treated as a program name, with the rest as arguments to that program.

The following components of mode-args are treated specially:

-dlopen file

Add the directory containing file to the library path.

This mode sets the library path environment variable according to any -dlopen flags.

If any of the args are libtool executable wrappers, then they are translated into the name of their corresponding uninstalled binary, and any of their required library directories are added to the library path.

4.4 Install mode ¶

In install mode, libtool interprets most of the elements of mode-args as an installation command beginning with cp, or a BSD-compatible install program.

The following components of mode-args are treated specially:

-inst-prefix-dir inst-prefix-dir

When installing into a temporary staging area, rather than the final prefix, this argument is used to reflect the temporary path, in much the same way automake uses DESTDIR. For instance, if prefix is /usr/local, but inst-prefix-dir is /tmp, then the object will be installed under /tmp/usr/local/. If the installed object is a libtool library, then the internal fields of that library will reflect only prefix, not inst-prefix-dir:

# Directory that this library needs to be installed in:
libdir='/usr/local/lib'

not

# Directory that this library needs to be installed in:
libdir='/tmp/usr/local/lib'

inst-prefix is also used to ensure that if the installed object must be relinked upon installation, that it is relinked against the libraries in inst-prefix-dir/prefix, not prefix.

In truth, this option is not really intended for use when calling libtool directly; it is automatically used when libtool --mode=install calls libtool --mode=relink. Libtool does this by analyzing the destination path given in the original libtool --mode=install command and comparing it to the expected installation path established during libtool --mode=link.

Thus, end-users need change nothing, and automake-style make install DESTDIR=/tmp will Just Work(tm) most of the time. For systems where fast installation cannot be turned on, relinking may be needed. In this case, a ‘DESTDIR’ install will fail.

Currently it is not generally possible to install into a temporary staging area that contains needed third-party libraries that are not yet visible at their final location.

The rest of the mode-args are interpreted as arguments to the cp or install command.

The command is run, and any necessary unprivileged post-installation commands are also completed.

4.5 Finish mode ¶

Finish mode has two functions. One is to help system administrators install libtool libraries so that they can be located and linked into user programs. To invoke this functionality, pass the name of a library directory as mode-arg. Running this command may require superuser privileges, and the --dry-run option may be useful.

The second is to facilitate transferring libtool libraries to a native compilation environment after they were built in a cross-compilation environment. Cross-compilation environments may rely on recent libtool features, and running libtool in finish mode will make it easier to work with older versions of libtool. This task is performed whenever the mode-arg is a .la file.

4.6 Uninstall mode ¶

Uninstall mode deletes installed libraries, executables and objects.

The first mode-arg is the name of the program to use to delete files (typically /bin/rm).

The remaining mode-args are either flags for the deletion program (beginning with a ‘-’), or the names of files to delete.

4.7 Clean mode ¶

Clean mode deletes uninstalled libraries, executables, objects and libtool’s temporary files associated with them.

The first mode-arg is the name of the program to use to delete files (typically /bin/rm).

The remaining mode-args are either flags for the deletion program (beginning with a ‘-’), or the names of files to delete.

5.1 Autoconf macros exported by libtool ¶

Libtool uses a number of macros to interrogate the host system when it is being built, and you can use some of them yourself too. Although there are a great many other macros in the libtool installed m4 files, these do not form part of the published interface, and are subject to change between releases.

Macros in the ‘LT_CMD_’ namespace check for various shell commands:

Macro: LT_CMD_MAX_LEN ¶

Finds the longest command line that can be safely passed to ‘$SHELL’ without being truncated, and store in the shell variable ‘$max_cmd_len’. It is only an approximate value, but command lines of this length or shorter are guaranteed not to be truncated.

Macros in the ‘LT_FUNC_’ namespace check characteristics of library functions:

Macro: LT_FUNC_DLSYM_USCORE ¶

‘AC_DEFINE’ the preprocessor symbol ‘DLSYM_USCORE’ if we have to add an underscore to symbol-names passed in to ‘dlsym’.

Macros in the ‘LT_LIB_’ namespace check characteristics of system libraries:

Macro: LT_LIB_M ¶

Set ‘LIBM’ to the math library or libraries required on this machine, if any.

Macro: LT_LIB_DLLOAD ¶

This is the macro used by ‘libltdl’ to determine what dlloaders to use on this machine, if any. Several shell variables are set (and ‘AC_SUBST’ed) depending on the dlload interfaces are available on this machine. ‘LT_DLLOADERS’ contains a list of libtool libraries that can be used, and if necessary also sets ‘LIBADD_DLOPEN’ if additional system libraries are required by the ‘dlopen’ loader, and ‘LIBADD_SHL_LOAD’ if additional system libraries are required by the ‘shl_load’ loader, respectively. Finally some symbols are set in config.h depending on the loaders that are found to work: ‘HAVE_LIBDL’, ‘HAVE_SHL_LOAD’, ‘HAVE_DYLD’, ‘HAVE_DLD’.

Macros in the ‘LT_PATH_’ namespace search the system for the full path to particular system commands:

Macro: LT_PATH_LD ¶

Add a --with-gnu-ld option to configure. Try to find the path to the linker used by ‘$CC’, and whether it is the GNU linker. The result is stored in the shell variable ‘$LD’, which is AC_SUBSTed.

Macro: LT_PATH_NM ¶

Try to find a BSD-compatible nm or a MS-compatible dumpbin command on this machine. The result is stored in the shell variable ‘$NM’, which is AC_SUBSTed.

Macros in the ‘LT_SYS_’ namespace probe for system characteristics:

Macro: LT_SYS_DLOPEN_SELF ¶

Tests whether a program can dlopen itself, and then also whether the same program can still dlopen itself when statically linked. Results are stored in the shell variables ‘$enable_dlopen_self’ and ‘enable_dlopen_self_static’ respectively.

Macro: LT_SYS_DLOPEN_DEPLIBS ¶

Define the preprocessor symbol ‘LTDL_DLOPEN_DEPLIBS’ if the OS needs help to load dependent libraries for ‘dlopen’ (or equivalent).

Macro: LT_SYS_DLSEARCH_PATH ¶

Define the preprocessor symbol ‘LT_DLSEARCH_PATH’ to the system default library search path.

Macro: LT_SYS_MODULE_EXT ¶

Define the preprocessor symbol ‘LT_MODULE_EXT’ to the extension used for runtime loadable modules. If you use libltdl to open modules, then you can simply use the libtool library extension, .la.

Macro: LT_SYS_MODULE_PATH ¶

Define the preprocessor symbol ‘LT_MODULE_PATH_VAR’ to the name of the shell environment variable that determines the run-time module search path.

Macro: LT_SYS_SYMBOL_USCORE ¶

Set the shell variable ‘sys_symbol_underscore’ to ‘no’ unless the compiler prefixes global symbols with an underscore.

5.2 Writing Makefile rules for libtool ¶

Libtool is fully integrated with Automake (see Introduction in The Automake Manual), starting with Automake version 1.2.

If you want to use libtool in a regular Makefile (or Makefile.in), you are on your own. If you’re not using Automake, and you don’t know how to incorporate libtool into your package you need to do one of the following:

1. Download the latest Automake distribution from your nearest GNU mirror, install it, and start using it.
2. Learn how to write Makefile rules by hand. They’re sometimes complex, but if you’re clever enough to write rules for compiling your old libraries, then you should be able to figure out new rules for libtool libraries (hint: examine the Makefile.in in the tests/testsuite.dir/027 subdirectory, generated from the Autotest labeled ’link against a preloaded static library’ in tests/demo.at, of the libtool distribution; note especially that it was automatically generated from the Makefile.am by Automake).

5.3 Using Automake with libtool ¶

Libtool library support is implemented under the ‘LTLIBRARIES’ primary.

Here are some samples from the Automake Makefile.am in the libtool distribution’s tests/demo.at.

First, to link a program against a libtool library, just use the ‘program_LDADD’⁶ variable:

bin_PROGRAMS = hell hell_static

# Build hell from main.c and libhello.la
hell_SOURCES = main.c
hell_LDADD = libhello.la

# Create a statically linked version of hell.
hell_static_SOURCES = main.c
hell_static_LDADD = libhello.la
hell_static_LDFLAGS = -static

You may use the ‘program_LDFLAGS’ variable to stuff in any flags you want to pass to libtool while linking program (such as -static to avoid linking uninstalled shared libtool libraries).

Building a libtool library is almost as trivial… note the use of ‘libhello_la_LDFLAGS’ to pass the -version-info (see Library interface versions) option to libtool:

# Build a libtool library, libhello.la for installation in libdir.
lib_LTLIBRARIES = libhello.la
libhello_la_SOURCES = hello.c foo.c
libhello_la_LDFLAGS = -version-info 3:12:1

The -rpath option is passed automatically by Automake (except for libraries listed as noinst_LTLIBRARIES), so you should not specify it.

See The Automake Manual in The Automake Manual, for more information.

When building libtool archives which depend on built sources (for example a generated header file), you may find it necessary to manually record these dependencies. Because libtool archives generate object file names manually recording these dependencies is not as straightforward as the examples in Automake’s manual describe. This affects header files in particular, because simply listing them as ‘nodist_libfoo_la_SOURCES’ will not cause Automake to establish a dependent relationship for the object files of libfoo.la. A useful trick (although somewhat imprecise) is to manually record built sources used by a libtool archive as dependencies of all the objects for that library as shown below (as opposed to a particular object file):

# Build a libtool library, libhello.la which depends on a generated header.
hello.h:
	echo '#define HELLO_MESSAGE  "Hello, World!"' > $@
BUILT_SOURCES = hello.h
CLEANFILES = hello.h
nodist_libhello_la_SOURCES = hello.h
libhello_la_SOURCES = hello.c foo.h foo.c bar.h bar.c
# Manually record hello.h as a prerequisite for all objects in libhello.la
$(libhello_la_OBJECTS): hello.h

See The Automake Manual in The Automake Manual, for more information.

5.4 Configuring libtool ¶

Libtool requires intimate knowledge of your compiler suite and operating system to be able to create shared libraries and link against them properly. When you install the libtool distribution, a system-specific libtool script is installed into your binary directory.

However, when you distribute libtool with your own packages (see Including libtool in your package), you do not always know the compiler suite and operating system that are used to compile your package.

For this reason, libtool must be configured before it can be used. This idea should be familiar to anybody who has used a GNU configure script. configure runs a number of tests for system features, then generates the Makefiles (and possibly a config.h header file), after which you can run make and build the package.

Libtool adds its own tests to your configure script to generate a libtool script for the installer’s host machine.

• The LT_INIT macro
• Platform-specific configuration notes

5.5 Including libtool in your package ¶

In order to use libtool, you need to include the following files with your package:

config.guess ¶

Attempt to guess a canonical system name.

config.sub ¶

Canonical system name validation subroutine script.

install-sh ¶

BSD-compatible install replacement script.

ltmain.sh ¶

A generic script implementing basic libtool functionality.

Note that the libtool script itself should not be included with your package. See Configuring libtool.

You should use the libtoolize program, rather than manually copying these files into your package.

• Invoking libtoolize
• Autoconf and LTLIBOBJS

libtoolize [option]...

trick$ LIBTOOLIZE_OPTIONS=--no-warn,--quiet autoreconf --install

ACLOCAL_AMFLAGS = -I m4

trick$ aclocal -I m4

LTLIBOBJS=`echo "$LIBOBJS" | sed 's/\.[^.]* /.lo /g;s/\.[^.]*$/.lo/'`
AC_SUBST([LTLIBOBJS])

5.6 Static-only libraries ¶

When you are developing a package, it is often worthwhile to configure your package with the --disable-shared flag, or to override the defaults for LT_INIT by using the disable-shared option (see The LT_INIT macro). This prevents libtool from building shared libraries, which has several advantages:

• compilation is twice as fast, which can speed up your development cycle,
• debugging is easier because you don’t need to deal with any complexities added by shared libraries, and
• you can see how libtool behaves on static-only platforms.

You may want to put a small note in your package README to let other developers know that --disable-shared can save them time. The following example note is taken from the GIMP⁸ distribution README:

The GIMP uses GNU Libtool to build shared libraries on a
variety of systems.  While this is very nice for making usable
binaries, it can be a pain when trying to debug a program.  For that
reason, compilation of shared libraries can be turned off by
specifying the --disable-shared option to configure.

6.1 Writing libraries for C++ ¶

Creating libraries of C++ code should be a fairly straightforward process, because its object files differ from C ones in only three ways:

1. Because of name mangling, C++ libraries are only usable by the C++ compiler that created them. This decision was made by the designers of C++ to protect users from conflicting implementations of features such as constructors, exception handling, and RTTI.
2. On some systems, the C++ compiler must take special actions for the dynamic linker to run dynamic (i.e., run-time) initializers. This means that we should not call ld directly to link such libraries, and we should use the C++ compiler instead.
3. C++ compilers will link some Standard C++ library in by default, but libtool does not know what these libraries are, so it cannot even run the inter-library dependence analyzer to check how to link it in. Therefore, running ld to link a C++ program or library is deemed to fail.

Because of these three issues, Libtool has been designed to always use the C++ compiler to compile and link C++ programs and libraries. In some instances the main() function of a program must also be compiled with the C++ compiler for static C++ objects to be properly initialized.

6.2 Tags ¶

Libtool supports multiple languages through the use of tags. Technically a tag corresponds to a set of configuration variables associated with a language. These variables tell libtool how it should create objects and libraries for each language.

Tags are defined at configure-time for each language activated in the package (see LT_LANG in The LT_INIT macro). Here is the correspondence between language names and tags names.

libtool tries to automatically infer what tag to use from the compiler command being used to compile or link. If it can’t infer a tag, then it defaults to the configuration for the C language.

The tag can also be specified using libtool’s --tag=tag option (see Invoking libtool). It is a good idea to do so in Makefile rules, because that will allow users to substitute the compiler without relying on libtool inference heuristics. When no tag is specified, libtool will default to CC; this tag always exists.

Finally, the set of tags available in a particular project can be retrieved by tracing for the LT_SUPPORTED_TAG macro (see Libtool’s trace interface).

7.1 What are library interfaces? ¶

Interfaces for libraries may be any of the following (and more):

• global variables: both names and types
• global functions: argument types and number, return types, and function names
• standard input, standard output, standard error, and file formats
• sockets, pipes, and other inter-process communication protocol formats

Note that static functions do not count as interfaces, because they are not directly available to the user of the library.

7.2 Libtool’s versioning system ¶

Libtool has its own formal versioning system. It is not as flexible as some, but it is definitely the simplest of the more powerful versioning systems.

Think of a library as exporting several sets of interfaces, arbitrarily represented by integers. When a program is linked against a library, it may use any subset of those interfaces.

Libtool’s description of the interfaces that a program uses is simple: it encodes the least and the greatest interface numbers in the resulting binary (first-interface, last-interface).

The dynamic linker is guaranteed that if a library supports every interface number between first-interface and last-interface, then the program can be relinked against that library.

Note that this can cause problems because libtool’s compatibility requirements are actually stricter than is necessary.

Say libhello supports interfaces 5, 16, 17, 18, and 19, and that libtool is used to link test against libhello.

Libtool encodes the numbers 5 and 19 in test, and the dynamic linker will only link test against libraries that support every interface between 5 and 19. So, the dynamic linker refuses to link test against libhello!

In order to eliminate this problem, libtool only allows libraries to declare consecutive interface numbers. So, libhello can declare at most that it supports interfaces 16 through 19. Then, the dynamic linker will link test against libhello.

So, libtool library versions are described by three integers:

current

The most recent interface number that this library implements.

revision

The implementation number of the current interface.

age

The difference between the newest and oldest interfaces that this library implements. In other words, the library implements all the interface numbers in the range from number current - age to current.

If two libraries have identical current and age numbers, then the dynamic linker chooses the library with the greater revision number.

7.3 Updating library version information ¶

If you want to use libtool’s versioning system, then you must specify the version information to libtool using the -version-info flag during link mode (see Link mode).

This flag accepts an argument of the form ‘current[:revision[:age]]’. So, passing -version-info 3:12:1 sets current to 3, revision to 12, and age to 1.

If either revision or age are omitted, they default to 0. Also note that age must be less than or equal to the current interface number.

Here are a set of rules to help you update your library version information:

1. Start with version information of ‘0:0:0’ for each libtool library.
2. Update the version information only immediately before a public release of your software. More frequent updates are unnecessary, and only guarantee that the current interface number gets larger faster.
3. If the library source code has changed at all since the last update, then increment revision (‘c:r:a’ becomes ‘c:r+1:a’).
4. If any interfaces have been added, removed, or changed since the last update, increment current, and set revision to 0.
5. If any interfaces have been added since the last public release, then increment age.
6. If any interfaces have been removed or changed since the last public release, then set age to 0.

Never try to set the interface numbers so that they correspond to the release number of your package. This is an abuse that only fosters misunderstanding of the purpose of library versions. Instead, use the -release flag (see Managing release information), but be warned that every release of your package will not be binary compatible with any other release.

The following explanation may help to understand the above rules a bit better: consider that there are three possible kinds of reactions from users of your library to changes in a shared library:

1. Programs using the previous version may use the new version as drop-in replacement, and programs using the new version can also work with the previous one. In other words, no recompiling nor relinking is needed. In this case, bump revision only, don’t touch current nor age.
2. Programs using the previous version may use the new version as drop-in replacement, but programs using the new version may use APIs not present in the previous one. In other words, a program linking against the new version may fail with “unresolved symbols” if linking against the old version at runtime: set revision to 0, bump current and age.
3. Programs may need to be changed, recompiled, and relinked in order to use the new version. Bump current, set revision and age to 0.

In the above description, programs using the library in question may also be replaced by other libraries using it.

7.4 Managing release information ¶

Often, people want to encode the name of the package release into the shared library so that it is obvious to the user what package their programs are linked against. This convention is used especially on GNU/Linux:

trick$ ls /usr/lib/libbfd*
/usr/lib/libbfd.a           /usr/lib/libbfd.so.2.7.0.2
/usr/lib/libbfd.so
trick$

On ‘trick’, /usr/lib/libbfd.so is a symbolic link to libbfd.so.2.7.0.2, which was distributed as a part of ‘binutils-2.7.0.2’.

Unfortunately, this convention conflicts directly with libtool’s idea of library interface versions, because the library interface rarely changes at the same time that the release number does, and the library suffix is never the same across all platforms.

So, to accommodate both views, you can use the -release flag to set release information for libraries for which you do not want to use -version-info. For the libbfd example, the next release that uses libtool should be built with ‘-release 2.9.0’, which will produce the following files on GNU/Linux:

trick$ ls /usr/lib/libbfd*
/usr/lib/libbfd-2.9.0.so     /usr/lib/libbfd.a
/usr/lib/libbfd.so
trick$

In this case, /usr/lib/libbfd.so is a symbolic link to libbfd-2.9.0.so. This makes it obvious that the user is dealing with ‘binutils-2.9.0’, without compromising libtool’s idea of interface versions.

Note that this option causes a modification of the library name, so do not use it unless you want to break binary compatibility with any past library releases. In general, you should only use -release for package-internal libraries or for ones whose interfaces change very frequently.

8.1 Writing C header files ¶

Writing portable C header files can be difficult, since they may be read by different types of compilers:

C++ compilers

C++ compilers require that functions be declared with full prototypes, since C++ is more strongly typed than C. C functions and variables also need to be declared with the extern "C" directive, so that the names aren’t mangled. See Writing libraries for C++, for other issues relevant to using C++ with libtool.

ANSI C compilers

ANSI C compilers are not as strict as C++ compilers, but functions should be prototyped to avoid unnecessary warnings when the header file is #included.

non-ANSI C compilers

Non-ANSI compilers will report errors if functions are prototyped.

These complications mean that your library interface headers must use some C preprocessor magic to be usable by each of the above compilers.

foo.h, defined in the tests/demo.at Autotest of the libtool distribution, serves as an example for how to write a header file that can be safely installed in a system directory.

Here are the relevant portions of that file:

#ifndef FOO_H
#define FOO_H

#ifdef __cplusplus
extern "C" {
#endif

int foo (void);
int hello (void);

#ifdef __cplusplus
}
#endif

#endif /* !FOO_H */

This can also be achieved by utilizing macros:

/* BEGIN_C_DECLS should be used at the beginning of your declarations,
   so that C++ compilers don't mangle their names.  Use END_C_DECLS at
   the end of C declarations. */
#undef BEGIN_C_DECLS
#undef END_C_DECLS
#ifdef __cplusplus
# define BEGIN_C_DECLS extern "C" {
# define END_C_DECLS }
#else
# define BEGIN_C_DECLS /* empty */
# define END_C_DECLS /* empty */
#endif

/* PARAMS is a macro used to wrap function prototypes, so that
   compilers that don't understand ANSI C prototypes still work,
   and ANSI C compilers can issue warnings about type mismatches. */
#undef PARAMS
#if defined __STDC__ || defined _AIX \
        || (defined __mips && defined _SYSTYPE_SVR4) \
        || defined WIN32 || defined __cplusplus
# define PARAMS(protos) protos
#else
# define PARAMS(protos) ()
#endif

These macros can be used in foo.h as follows:

#ifndef FOO_H
#define FOO_H

/* The above macro definitions. */
#include "..."

BEGIN_C_DECLS

int foo PARAMS((void));
int hello PARAMS((void));

END_C_DECLS

#endif /* !FOO_H */

Note that the #ifndef FOO_H prevents the body of foo.h from being read more than once in a given compilation.

Also the only thing that must go outside the BEGIN_C_DECLS/END_C_DECLS pair are #include lines. Strictly speaking it is only C symbol names that need to be protected, but your header files will be more maintainable if you have a single pair of these macros around the majority of the header contents.

You should use these definitions of PARAMS, BEGIN_C_DECLS, and END_C_DECLS into your own headers. Then, you may use them to create header files that are valid for C++, ANSI, and non-ANSI compilers⁹.

Do not be naive about writing portable code. Following the tips given above will help you miss the most obvious problems, but there are definitely other subtle portability issues. You may need to cope with some of the following issues:

• Pre-ANSI compilers do not always support the void * generic pointer type, and so need to use char * in its place.
• The const, inline and signed keywords are not supported by some compilers, especially pre-ANSI compilers.
• The long double type is not supported by many compilers.

10.1 Building modules to dlopen ¶

On some operating systems, a program symbol must be specially declared in order to be dynamically resolved with the dlsym (or equivalent) function. Libtool provides the -export-dynamic and -module link flags (see Link mode), for you to make that declaration. You need to use these flags if you are linking an application program that dlopens other modules or a libtool library that will also be dlopened.

For example, if we wanted to build a shared library, hello, that would later be dlopened by an application, we would add -module to the other link flags:

burger$ libtool --mode=link gcc -module -o hello.la foo.lo \
                hello.lo -rpath /usr/local/lib -lm
burger$

If symbols from your executable are needed to satisfy unresolved references in a library you want to dlopen you will have to use the flag -export-dynamic. You should use -export-dynamic while linking the executable that calls dlopen:

burger$ libtool --mode=link gcc -export-dynamic -o helldl main.o
burger$

10.2 Dlpreopening ¶

Libtool provides special support for dlopening libtool object and libtool library files, so that their symbols can be resolved even on platforms without any dlopen and dlsym functions.

Consider the following alternative ways of loading code into your program, in order of increasing “laziness”:

1. Linking against object files that become part of the program executable, whether or not they are referenced. If an object file cannot be found, then the compile time linker refuses to create the executable.
2. Declaring a static library to the linker, so that it is searched at link time to satisfy any undefined references in the above object files. If the static library cannot be found, then the compile time linker refuses to create the executable.
3. Declaring a shared library to the runtime linker, so that it is searched at runtime to satisfy any undefined references in the above files. If the shared library cannot be found, then the dynamic linker aborts the program before it runs.
4. Dlopening a module, so that the application can resolve its own, dynamically-computed references. If there is an error opening the module, or the module is not found, then the application can recover without crashing.

Libtool emulates -dlopen on static platforms by linking objects into the program at compile time, and creating data structures that represent the program’s symbol table. In order to use this feature, you must declare the objects you want your application to dlopen by using the -dlopen or -dlpreopen flags when you link your program (see Link mode).

Data Type: lt_dlsymlist typedef struct { const char *name; void *address; } lt_dlsymlist ¶

The name attribute is a null-terminated character string of the symbol name, such as "fprintf". The address attribute is generic pointer to the appropriate object, such as &fprintf.

Variable: const lt_dlsymlist lt_preloaded_symbols[] ¶

An array of lt_dlsymlist structures, representing all the preloaded symbols linked into the program proper. For each module -dlpreopened by the Libtool linked program there is an element with the name of the module and an address of 0, followed by all symbols exported from this file. For the executable itself the special name ‘@PROGRAM@’ is used. The last element of all has a name and address of 0.

To facilitate inclusion of symbol lists into libraries, lt_preloaded_symbols is ‘#define’d to a suitably unique name in ltdl.h.

This variable may not be declared const on some systems due to relocation issues.

Some compilers may allow identifiers that are not valid in ANSI C, such as dollar signs. Libtool only recognizes valid ANSI C symbols (an initial ASCII letter or underscore, followed by zero or more ASCII letters, digits, and underscores), so non-ANSI symbols will not appear in lt_preloaded_symbols.

Function: int lt_dlpreload (const lt_dlsymlist *preloaded) ¶

Register the list of preloaded modules preloaded. If preloaded is NULL, then all previously registered symbol lists, except the list set by lt_dlpreload_default, are deleted. Return 0 on success.

Function: int lt_dlpreload_default (const lt_dlsymlist *preloaded) ¶

Set the default list of preloaded modules to preloaded, which won’t be deleted by lt_dlpreload. Note that this function does not require libltdl to be initialized using lt_dlinit and can be used in the program to register the default preloaded modules. Instead of calling this function directly, most programs will use the macro LTDL_SET_PRELOADED_SYMBOLS.

Return 0 on success.

Macro: LTDL_SET_PRELOADED_SYMBOLS ¶

Set the default list of preloaded symbols. Should be used in your program to initialize libltdl’s list of preloaded modules.

#include <ltdl.h>

int main() {
  /* ... */
  LTDL_SET_PRELOADED_SYMBOLS();
  /* ... */
}

Function Type: int lt_dlpreload_callback_func (lt_dlhandle handle) ¶

Functions of this type can be passed to lt_dlpreload_open, which in turn will call back into a function thus passed for each preloaded module that it opens.

Function: int lt_dlpreload_open (const char *originator, lt_dlpreload_callback_func *func) ¶

Load all of the preloaded modules for originator. For every module opened in this way, call func.

To open all of the modules preloaded into libhell.la (presumably from within the libhell.a initialisation code):

#define preloaded_symbols lt_libhell_LTX_preloaded_symbols

static int hell_preload_callback (lt_dlhandle handle);

int
hell_init (void)
{
  ...
  if (lt_dlpreload (&preloaded_symbols) == 0)
    {
      lt_dlpreload_open ("libhell", preload_callback);
    }
  ...
}

Note that to prevent clashes between multiple preloaded modules, the preloaded symbols are accessed via a mangled symbol name: to get the symbols preloaded into ‘libhell’, you must prefix ‘preloaded_symbols’ with ‘lt_’; the originator name, ‘libhell’ in this case; and ‘_LTX_’. That is, ‘lt_libhell_LTX_preloaded_symbols’ here.

10.3 Linking with dlopened modules ¶

When, say, an interpreter application uses dlopened modules to extend the list of methods it provides, an obvious abstraction for the maintainers of the interpreter is to have all methods (including the built in ones supplied with the interpreter) accessed through dlopen. For one thing, the dlopening functionality will be tested even during routine invocations. For another, only one subsystem has to be written for getting methods into the interpreter.

The downside of this abstraction is, of course, that environments that provide only static linkage can’t even load the intrinsic interpreter methods. Not so! We can statically link those methods by dlpreopening them.

Unfortunately, since platforms such as AIX and cygwin require that all library symbols must be resolved at compile time, the interpreter maintainers will need to provide a library to both its own dlpreopened modules, and third-party modules loaded by dlopen. In itself, that is not so bad, except that the interpreter too must provide those same symbols otherwise it will be impossible to resolve all the symbols required by the modules as they are loaded. Things are even worse if the code that loads the modules for the interpreter is itself in a library – and that is usually the case for any non-trivial application. Modern platforms take care of this by automatically loading all of a module’s dependency libraries as the module is loaded (libltdl can do this even on platforms that can’t do it by themselves). In the end, this leads to problems with duplicated symbols and prevents modules from loading, and prevents the application from compiling when modules are preloaded.

,-------------.    ,------------------.    ,-----------------.
| Interpreter |---->     Module------------>   Third-party   |
`-------------'    |     Loader       |    |Dlopened Modules |
                   |        |         |    `-----------------'
                   |,-------v--------.|             |
                   ||  Dlpreopened   ||             |
                   ||    Modules     ||             |
                   |`----------------'|             |
                   |        |         |             |
                   |,-------v--------.|    ,--------v--------.
                   ||Module Interface||    |Module Interface |
                   ||    Library     ||    |     Library     |
                   |`----------------'|    `-----------------'
                   `------------------'

Libtool has the concept of weak library interfaces to circumvent this problem. Recall that the code that dlopens method-provider modules for the interpreter application resides in a library: All of the modules and the dlopener library itself should be linked against the common library that resolves the module symbols at compile time. To guard against duplicate symbol definitions, and for dlpreopened modules to work at all in this scenario, the dlopener library must declare that it provides a weak library interface to the common symbols in the library it shares with the modules. That way, when libtool links the Module Loader library with some Dlpreopened Modules that were in turn linked against the Module Interface Library, it knows that the Module Loader provides an already loaded Module Interface Library to resolve symbols for the Dlpreopened Modules, and doesn’t ask the compiler driver to link an identical Module Interface Library dependency library too.

In conjunction with Automake, the Makefile.am for the Module Loader might look like this:

lib_LTLIBRARIES = libinterface.la libloader.la

libinterface_la_SOURCES = interface.c interface.h
libinterface_la_LDFLAGS = -version-info 3:2:1

libloader_la_SOURCES    = loader.c
libloader_la_LDFLAGS    = -weak libinterface.la \
                          -version-info 3:2:1 \
                          -dlpreopen ../modules/intrinsics.la
libloader_la_LIBADD     = $(libinterface_la_OBJECTS)

And the Makefile.am for the intrinsics.la module in a sibling modules directory might look like this:

AM_CPPFLAGS             = -I$(srcdir)/../libloader
AM_LDFLAGS              = -no-undefined -module -avoid-version \
                          -export-dynamic

noinst_LTLIBRARIES      = intrinsics.la

intrinsics_la_LIBADD    = ../libloader/libinterface.la

../libloader/libinterface.la:
        cd ../libloader && $(MAKE) $(AM_MAKEFLAGS) libinterface.la

For a more complex example, see the sources of libltdl in the Libtool distribution, which is built with the help of the -weak option.

10.4 Finding the correct name to dlopen ¶

After a library has been linked with -module, it can be dlopened. Unfortunately, because of the variation in library names, your package needs to determine the correct file to dlopen.

The most straightforward and flexible implementation is to determine the name at runtime, by finding the installed .la file, and searching it for the following lines:

# The name that we can dlopen.
dlname='dlname'

If dlname is empty, then the library cannot be dlopened. Otherwise, it gives the dlname of the library. So, if the library was installed as /usr/local/lib/libhello.la, and the dlname was libhello.so.3, then /usr/local/lib/libhello.so.3 should be dlopened.

If your program uses this approach, then it should search the directories listed in the LD_LIBRARY_PATH¹⁰ environment variable, as well as the directory where libraries will eventually be installed. Searching this variable (or equivalent) will guarantee that your program can find its dlopened modules, even before installation, provided you have linked them using libtool.

10.5 Unresolved dlopen issues ¶

The following problems are not solved by using libtool’s dlopen support:

• Dlopen functions are generally only available on shared library platforms. If you want your package to be portable to static platforms, you have to use either libltdl (see Using libltdl) or develop your own alternatives to dlopening dynamic code. Most reasonable solutions involve writing wrapper functions for the dlopen family, which do package-specific tricks when dlopening is unsupported or not available on a given platform.
• There are major differences in implementations of the dlopen family of functions. Some platforms do not even use the same function names (notably HP-UX, with its shl_load family).
• The application developer must write a custom search function to discover the correct module filename to supply to dlopen.

11.1 How to use libltdl in your programs ¶

The libltdl API is similar to the POSIX dlopen interface, which is very simple but powerful.

To use libltdl in your program you have to include the header file ltdl.h:

#include <ltdl.h>

The early releases of libltdl used some symbols that violated the POSIX namespace conventions. These symbols are now deprecated, and have been replaced by those described here. If you have code that relies on the old deprecated symbol names, defining ‘LT_NON_POSIX_NAMESPACE’ before you include ltdl.h provides conversion macros. Whichever set of symbols you use, the new API is not binary compatible with the last, so you will need to recompile your application to use this version of libltdl.

Note that libltdl is not well tested in a multithreaded environment, though the intention is that it should work (see Using libltdl in a multi threaded environment). If there are any issues, working around them is left as an exercise for the reader; contributions are certainly welcome.

The following macros are defined by including ltdl.h:

Macro: LT_PATHSEP_CHAR ¶

LT_PATHSEP_CHAR is the system-dependent path separator, that is, ‘;’ on Windows and ‘:’ everywhere else.

Macro: LT_DIRSEP_CHAR ¶

If LT_DIRSEP_CHAR is defined, it can be used as directory separator in addition to ‘/’. On Windows, this contains ‘\’.

The following types are defined in ltdl.h:

Type: lt_dlhandle ¶

lt_dlhandle is a module “handle”. Every lt_dlopened module has a handle associated with it.

Type: lt_dladvise ¶

lt_dladvise is used to control optional module loading modes. If it is not used, the default mode of the underlying system module loader is used.

Type: lt_dlsymlist ¶

lt_dlsymlist is a symbol list for dlpreopened modules (see Dlpreopening).

libltdl provides the following functions:

Function: int lt_dlinit (void) ¶

Initialize libltdl. This function must be called before using libltdl and may be called several times. Return 0 on success, otherwise the number of errors.

Function: int lt_dlexit (void) ¶

Shut down libltdl and close all modules. This function will only then shut down libltdl when it was called as many times as lt_dlinit has been successfully called. Return 0 on success, otherwise the number of errors.

Function: lt_dlhandle lt_dlopen (const char *filename) ¶

Open the module with the file name filename and return a handle for it. lt_dlopen is able to open libtool dynamic modules, preloaded static modules, the program itself and native dynamic modules¹¹.

Unresolved symbols in the module are resolved using its dependency libraries and, on some platforms, previously dlopened modules. If the executable using this module was linked with the -export-dynamic flag, then the global symbols in the executable will also be used to resolve references in the module.

If filename is NULL and the program was linked with -export-dynamic or -dlopen self, lt_dlopen will return a handle for the program itself, which can be used to access its symbols.

If libltdl cannot find the library and the file name filename does not have a directory component it will additionally look in the following search paths for the module (in the following order):

1. user-defined search path: This search path can be changed by the program using the functions lt_dlsetsearchpath, lt_dladdsearchdir and lt_dlinsertsearchdir.
2. libltdl’s search path: This search path is the value of the environment variable LTDL_LIBRARY_PATH.
3. system library search path: The system dependent library search path (e.g. on GNU/Linux it is LD_LIBRARY_PATH).

Each search path must be a list of absolute directories separated by LT_PATHSEP_CHAR, for example, "/usr/lib/mypkg:/lib/foo". The directory names may not contain the path separator.

If the same module is loaded several times, the same handle is returned. If lt_dlopen fails for any reason, it returns NULL.

Function: lt_dlhandle lt_dlopenext (const char *filename) ¶

The same as lt_dlopen, except that it tries to append different file name extensions to the file name. If the file with the file name filename cannot be found libltdl tries to append the following extensions:

1. the libtool archive extension .la
2. the extension used for native dynamically loadable modules on the host platform, e.g., .so, .sl, etc.

This lookup strategy was designed to allow programs that don’t have knowledge about native dynamic libraries naming conventions to be able to dlopen such libraries as well as libtool modules transparently.

Function: lt_dlhandle lt_dlopenadvise (const char *filename, lt_dladvise advise) ¶

The same as lt_dlopen, except that it also requires an additional argument that may contain additional hints to the underlying system module loader. The advise parameter is opaque and can only be accessed with the functions documented below.

Note that this function does not change the content of advise, so unlike the other calls in this API takes a direct lt_dladvise type, and not a pointer to the same.

Function: int lt_dladvise_init (lt_dladvise *advise) ¶

The advise parameter can be used to pass hints to the module loader when using lt_dlopenadvise to perform the loading. The advise parameter needs to be initialised by this function before it can be used. Any memory used by advise needs to be recycled with lt_dladvise_destroy when it is no longer needed.

On failure, lt_dladvise_init returns non-zero and sets an error message that can be retrieved with lt_dlerror.

Function: int lt_dladvise_destroy (lt_dladvise *advise) ¶

Recycle the memory used by advise. For an example, see the documentation for lt_dladvise_ext.

On failure, lt_dladvise_destroy returns non-zero and sets an error message that can be retrieved with lt_dlerror.

Function: int lt_dladvise_ext (lt_dladvise *advise) ¶

Set the ext hint on advise. Passing an advise parameter to lt_dlopenadvise with this hint set causes it to try to append different file name extensions like lt_dlopenext.

The following example is equivalent to calling lt_dlopenext (filename):

lt_dlhandle
my_dlopenext (const char *filename)
{
  lt_dlhandle handle = 0;
  lt_dladvise advise;

  if (!lt_dladvise_init (&advise) && !lt_dladvise_ext (&advise))
    handle = lt_dlopenadvise (filename, advise);

  lt_dladvise_destroy (&advise);

  return handle;
}

On failure, lt_dladvise_ext returns non-zero and sets an error message that can be retrieved with lt_dlerror.

Function: int lt_dladvise_global (lt_dladvise *advise) ¶

Set the symglobal hint on advise. Passing an advise parameter to lt_dlopenadvise with this hint set causes it to try to make the loaded module’s symbols globally available for resolving unresolved symbols in subsequently loaded modules.

If neither the symglobal nor the symlocal hints are set, or if a module is loaded without using the lt_dlopenadvise call in any case, then the visibility of the module’s symbols will be as per the default for the underlying module loader and OS. Even if a suitable hint is passed, not all loaders are able to act upon it in which case lt_dlgetinfo will reveal whether the hint was actually followed.

On failure, lt_dladvise_global returns non-zero and sets an error message that can be retrieved with lt_dlerror.

Function: int lt_dladvise_local (lt_dladvise *advise) ¶

Set the symlocal hint on advise. Passing an advise parameter to lt_dlopenadvise with this hint set causes it to try to keep the loaded module’s symbols hidden so that they are not visible to subsequently loaded modules.

If neither the symglobal nor the symlocal hints are set, or if a module is loaded without using the lt_dlopenadvise call in any case, then the visibility of the module’s symbols will be as per the default for the underlying module loader and OS. Even if a suitable hint is passed, not all loaders are able to act upon it in which case lt_dlgetinfo will reveal whether the hint was actually followed.

On failure, lt_dladvise_local returns non-zero and sets an error message that can be retrieved with lt_dlerror.

Function: int lt_dladvise_resident (lt_dladvise *advise) ¶

Set the resident hint on advise. Passing an advise parameter to lt_dlopenadvise with this hint set causes it to try to make the loaded module resident in memory, so that it cannot be unloaded with a later call to lt_dlclose.

On failure, lt_dladvise_resident returns non-zero and sets an error message that can be retrieved with lt_dlerror.

Function: int lt_dladvise_preload (lt_dladvise *advise) ¶

Set the preload hint on advise. Passing an advise parameter to lt_dlopenadvise with this hint set causes it to load only preloaded modules, so that if a suitable preloaded module is not found, lt_dlopenadvise will return NULL.

Function: int lt_dlclose (lt_dlhandle handle) ¶

Decrement the reference count on the module handle. If it drops to zero and no other module depends on this module, then the module is unloaded. Return 0 on success.

Function: void * lt_dlsym (lt_dlhandle handle, const char *name) ¶

Return the address in the module handle, where the symbol given by the null-terminated string name is loaded. If the symbol cannot be found, NULL is returned.

Function: const char * lt_dlerror (void) ¶

Return a human readable string describing the most recent error that occurred from any of libltdl’s functions. Return NULL if no errors have occurred since initialization or since it was last called.

Function: int lt_dladdsearchdir (const char *search_dir) ¶

Append the search directory search_dir to the current user-defined library search path. Return 0 on success.

Function: int lt_dlinsertsearchdir (const char *before, const char *search_dir) ¶

Insert the search directory search_dir into the user-defined library search path, immediately before the element starting at address before. If before is ‘NULL’, then search_dir is appending as if lt_dladdsearchdir had been called. Return 0 on success.

Function: int lt_dlsetsearchpath (const char *search_path) ¶

Replace the current user-defined library search path with search_path, which must be a list of absolute directories separated by LT_PATHSEP_CHAR. Return 0 on success.

Function: const char * lt_dlgetsearchpath (void) ¶

Return the current user-defined library search path.

Function: int lt_dlforeachfile (const char *search_path, int (*func) (const char *filename, void * data), void * data) ¶

In some applications you may not want to load individual modules with known names, but rather find all of the modules in a set of directories and load them all during initialisation. With this function you can have libltdl scan the LT_PATHSEP_CHAR-delimited directory list in search_path for candidates, and pass them, along with data to your own callback function, func. If search_path is ‘NULL’, then search all of the standard locations that lt_dlopen would examine. This function will continue to make calls to func for each file that it discovers in search_path until one of these calls returns non-zero, or until the files are exhausted. ‘lt_dlforeachfile’ returns the value returned by the last call made to func.

For example you could define func to build an ordered argv-like vector of files using data to hold the address of the start of the vector.

Function: int lt_dlmakeresident (lt_dlhandle handle) ¶

Mark a module so that it cannot be ‘lt_dlclose’d. This can be useful if a module implements some core functionality in your project that would cause your code to crash if removed. Return 0 on success.

If you use ‘lt_dlopen (NULL)’ to get a handle for the running binary, that handle will always be marked as resident, and consequently cannot be successfully ‘lt_dlclose’d.

Function: int lt_dlisresident (lt_dlhandle handle) ¶

Check whether a particular module has been marked as resident, returning 1 if it has or 0 otherwise. If there is an error while executing this function, return -1 and set an error message for retrieval with lt_dlerror.

11.2 Creating modules that can be dlopened ¶

Libtool modules are created like normal libtool libraries with a few exceptions:

You have to link the module with libtool’s -module switch, and you should link any program that is intended to dlopen the module with -dlopen modulename.la where possible, so that libtool can dlpreopen the module on platforms that do not support dlopening. If the module depends on any other libraries, make sure you specify them either when you link the module or when you link programs that dlopen it. If you want to disable versioning (see Library interface versions) for a specific module you should link it with the -avoid-version switch. Note that libtool modules don’t need to have a "lib" prefix. However, Automake 1.4 or higher is required to build such modules.

Usually a set of modules provide the same interface, i.e. exports the same symbols, so that a program can dlopen them without having to know more about their internals: In order to avoid symbol conflicts all exported symbols must be prefixed with "modulename_LTX_" (modulename is the name of the module). Internal symbols must be named in such a way that they won’t conflict with other modules, for example, by prefixing them with "_modulename_". Although some platforms support having the same symbols defined more than once it is generally not portable and it makes it impossible to dlpreopen such modules.

libltdl will automatically cut the prefix off to get the real name of the symbol. Additionally, it supports modules that do not use a prefix so that you can also dlopen non-libtool modules.

foo1.c gives an example of a portable libtool module. Exported symbols are prefixed with "foo1_LTX_", internal symbols with "_foo1_". Aliases are defined at the beginning so that the code is more readable.

/* aliases for the exported symbols */
#define foo  foo1_LTX_foo
#define bar  foo1_LTX_bar

/* a global variable definition */
int bar = 1;

/* a private function */
int _foo1_helper() {
  return bar;
}

/* an exported function */
int foo() {
  return _foo1_helper();
}

The Makefile.am contains the necessary rules to build the module foo1.la:

...
lib_LTLIBRARIES = foo1.la

foo1_la_SOURCES = foo1.c
foo1_la_LDFLAGS = -module
...

11.3 Using libltdl in a multi threaded environment ¶

Libltdl provides a wrapper around whatever dynamic run-time object loading mechanisms are provided by the host system, many of which are themselves not thread safe. Consequently libltdl cannot itself be consistently thread safe.

If you wish to use libltdl in a multithreaded environment, then you must mutex lock around libltdl calls, since they may in turn be calling non-thread-safe system calls on some target hosts.

Some old releases of libtool provided a mutex locking API that was unusable with POSIX threads, so callers were forced to lock around all libltdl API calls anyway. That mutex locking API was next to useless, and is not present in current releases.

Some future release of libtool may provide a new POSIX thread compliant mutex locking API.

11.4 Data associated with loaded modules ¶

Some of the internal information about each loaded module that is maintained by libltdl is available to the user, in the form of this structure:

Type: struct lt_dlinfo { char *filename; char *name; int ref_count; int is_resident; int is_symglobal; int is_symlocal;} ¶

lt_dlinfo is used to store information about a module. The filename attribute is a null-terminated character string of the real module file name. If the module is a libtool module then name is its module name (e.g. "libfoo" for "dir/libfoo.la"), otherwise it is set to NULL. The ref_count attribute is a reference counter that describes how often the same module is currently loaded. The remaining fields can be compared to any hints that were passed to lt_dlopenadvise to determine whether the underlying loader was able to follow them.

The following function will return a pointer to libltdl’s internal copy of this structure for the given handle:

Function: const lt_dlinfo * lt_dlgetinfo (lt_dlhandle handle) ¶

Return a pointer to a struct that contains some information about the module handle. The contents of the struct must not be modified. Return NULL on failure.

Furthermore, to save you from having to keep a list of the handles of all the modules you have loaded, these functions allow you to iterate over libltdl’s list of loaded modules:

Type: lt_dlinterface_id ¶

The opaque type used to hold the module interface details for each registered libltdl client.

Type: int lt_dlhandle_interface (lt_dlhandle handle, const char *id_string) ¶

Functions of this type are called to check that a handle conforms to a library’s expected module interface when iterating over the global handle list. You should be careful to write a callback function of this type that can correctly identify modules that belong to this client, both to prevent other clients from accidentally finding your loaded modules with the iterator functions below, and vice versa. The best way to do this is to check that module handle conforms to the interface specification of your loader using lt_dlsym.

The callback may be given every module loaded by all the libltdl module clients in the current address space, including any modules loaded by other libraries such as libltdl itself, and should return non-zero if that module does not fulfill the interface requirements of your loader.

int
my_interface_cb (lt_dlhandle handle, const char *id_string)
{
  char *(*module_id) (void) = NULL;

  /* A valid my_module must provide all of these symbols.  */
  if (!((module_id = (char*(*)(void)) lt_dlsym ("module_version"))
        && lt_dlsym ("my_module_entrypoint")))
      return 1;

  if (strcmp (id_string, module_id()) != 0)
      return 1;

  return 0;
}

Function: lt_dlinterface_id lt_dlinterface_register (const char *id_string, lt_dlhandle_interface *iface) ¶

Use this function to register your interface validator with libltdl, and in return obtain a unique key to store and retrieve per-module data. You supply an id_string and iface so that the resulting lt_dlinterface_id can be used to filter the module handles returned by the iteration functions below. If iface is NULL, all modules will be matched.

Function: void lt_dlinterface_free (lt_dlinterface_id iface) ¶

Release the data associated with iface.

Function: int lt_dlhandle_map (lt_dlinterface_id iface, int (*func) (lt_dlhandle handle, void * data), void * data) ¶

For each module that matches iface, call the function func. When writing the func callback function, the argument handle is the handle of a loaded module, and data is the last argument passed to lt_dlhandle_map. As soon as func returns a non-zero value for one of the handles, lt_dlhandle_map will stop calling func and immediately return that non-zero value. Otherwise 0 is eventually returned when func has been successfully called for all matching modules.

Function: lt_dlhandle lt_dlhandle_iterate (lt_dlinterface_id iface, lt_dlhandle place) ¶

Iterate over the module handles loaded by iface, returning the first matching handle in the list if place is NULL, and the next one on subsequent calls. If place is the last element in the list of eligible modules, this function returns NULL.

lt_dlhandle handle = 0;
lt_dlinterface_id iface = my_interface_id;

while ((handle = lt_dlhandle_iterate (iface, handle)))
  {
    ...
  }

Function: lt_dlhandle lt_dlhandle_fetch (lt_dlinterface_id iface, const char *module_name) ¶

Search through the module handles loaded by iface for a module named module_name, returning its handle if found or else NULL if no such named module has been loaded by iface.

However, you might still need to maintain your own list of loaded module handles (in parallel with the list maintained inside libltdl) if there were any other data that your application wanted to associate with each open module. Instead, you can use the following API calls to do that for you. You must first obtain a unique interface id from libltdl as described above, and subsequently always use it to retrieve the data you stored earlier. This allows different libraries to each store their own data against loaded modules, without interfering with one another.

Function: void * lt_dlcaller_set_data (lt_dlinterface_id key, lt_dlhandle handle, void * data) ¶

Set data as the set of data uniquely associated with key and handle for later retrieval. This function returns the data previously associated with key and handle if any. A result of 0, may indicate that a diagnostic for the last error (if any) is available from lt_dlerror().

For example, to correctly remove some associated data:

void *stale = lt_dlcaller_set_data (key, handle, 0);
if (stale != NULL)
  {
    free (stale);
  }
else
  {
    char *error_msg = lt_dlerror ();

    if (error_msg != NULL)
      {
        my_error_handler (error_msg);
        return STATUS_FAILED;
      }
  }

Function: void * lt_dlcaller_get_data (lt_dlinterface_id key, lt_dlhandle handle) ¶

Return the address of the data associated with key and handle, or else NULL if there is none.

Old versions of libltdl also provided a simpler, but similar, API based around lt_dlcaller_id. Unfortunately, it had no provision for detecting whether a module belonged to a particular interface as libltdl didn’t support multiple loaders in the same address space at that time. Those APIs are no longer supported as there would be no way to stop clients of the old APIs from seeing (and accidentally altering) modules loaded by other libraries.

11.5 How to create and register new module loaders ¶

Sometimes libltdl’s many ways of gaining access to modules are not sufficient for the purposes of a project. You can write your own loader, and register it with libltdl so that lt_dlopen will be able to use it.

Writing a loader involves writing at least three functions that can be called by lt_dlopen, lt_dlsym and lt_dlclose. Optionally, you can provide a finalisation function to perform any cleanup operations when lt_dlexit executes, and a symbol prefix string that will be prepended to any symbols passed to lt_dlsym. These functions must match the function pointer types below, after which they can be allocated to an instance of lt_user_dlloader and registered.

Registering the loader requires that you choose a name for it, so that it can be recognised by lt_dlloader_find and removed with lt_dlloader_remove. The name you choose must be unique, and not already in use by libltdl’s builtin loaders:

"dlopen"

The system dynamic library loader, if one exists.

"dld"

The GNU dld loader, if libdld was installed when libltdl was built.

"dlpreload"

The loader for lt_dlopening of preloaded static modules.

The prefix "dl" is reserved for loaders supplied with future versions of libltdl, so you should not use that for your own loader names.

The following types are defined in ltdl.h:

Type: lt_module ¶

lt_module is a dlloader dependent module. The dynamic module loader extensions communicate using these low level types.

Type: lt_dlloader ¶

lt_dlloader is a handle for module loader types.

Type: lt_user_data ¶

lt_user_data is used for specifying loader instance data.

Type: struct lt_user_dlloader {const char *sym_prefix; lt_module_open *module_open; lt_module_close *module_close; lt_find_sym *find_sym; lt_dlloader_exit *dlloader_exit; } ¶

If you want to define a new way to open dynamic modules, and have the lt_dlopen API use it, you need to instantiate one of these structures and pass it to lt_dlloader_add. You can pass whatever you like in the dlloader_data field, and it will be passed back as the value of the first parameter to each of the functions specified in the function pointer fields.

Type: lt_module lt_module_open (const char *filename) ¶

The type of the loader function for an lt_dlloader module loader. The value set in the dlloader_data field of the struct lt_user_dlloader structure will be passed into this function in the loader_data parameter. Implementation of such a function should attempt to load the named module, and return an lt_module suitable for passing in to the associated lt_module_close and lt_sym_find function pointers. If the function fails it should return NULL, and set the error message with lt_dlseterror.

Type: int lt_module_close (lt_user_data loader_data, lt_module module) ¶

The type of the unloader function for a user defined module loader. Implementation of such a function should attempt to release any resources tied up by the module module, and then unload it from memory. If the function fails for some reason, set the error message with lt_dlseterror and return non-zero.

Type: void * lt_find_sym (lt_module module, const char *symbol) ¶

The type of the symbol lookup function for a user defined module loader. Implementation of such a function should return the address of the named symbol in the module module, or else set the error message with lt_dlseterror and return NULL if lookup fails.

Type: int lt_dlloader_exit (lt_user_data loader_data) ¶

The type of the finalisation function for a user defined module loader. Implementation of such a function should free any resources associated with the loader, including any user specified data in the dlloader_data field of the lt_user_dlloader. If non-NULL, the function will be called by lt_dlexit, and lt_dlloader_remove.

For example:

int
register_myloader (void)
{
  lt_user_dlloader dlloader;

  /* User modules are responsible for their own initialisation. */
  if (myloader_init () != 0)
    return MYLOADER_INIT_ERROR;

  dlloader.sym_prefix    = NULL;
  dlloader.module_open   = myloader_open;
  dlloader.module_close  = myloader_close;
  dlloader.find_sym      = myloader_find_sym;
  dlloader.dlloader_exit = myloader_exit;
  dlloader.dlloader_data = (lt_user_data)myloader_function;

  /* Add my loader as the default module loader. */
  if (lt_dlloader_add (lt_dlloader_next (NULL), &dlloader,
                       "myloader") != 0)
    return ERROR;

  return OK;
}

Note that if there is any initialisation required for the loader, it must be performed manually before the loader is registered – libltdl doesn’t handle user loader initialisation.

Finalisation is handled by libltdl however, and it is important to ensure the dlloader_exit callback releases any resources claimed during the initialisation phase.

libltdl provides the following functions for writing your own module loaders:

Function: int lt_dlloader_add (lt_dlloader *place, lt_user_dlloader *dlloader, const char *loader_name) ¶

Add a new module loader to the list of all loaders, either as the last loader (if place is NULL), else immediately before the loader passed as place. loader_name will be returned by lt_dlloader_name if it is subsequently passed a newly registered loader. These loader_names must be unique, or lt_dlloader_remove and lt_dlloader_find cannot work. Returns 0 for success.

/* Make myloader be the last one. */
if (lt_dlloader_add (NULL, myloader) != 0)
  perror (lt_dlerror ());

Function: int lt_dlloader_remove (const char *loader_name) ¶

Remove the loader identified by the unique name, loader_name. Before this can succeed, all modules opened by the named loader must have been closed. Returns 0 for success, otherwise an error message can be obtained from lt_dlerror.

/* Remove myloader. */
if (lt_dlloader_remove ("myloader") != 0)
  perror (lt_dlerror ());

Function: lt_dlloader * lt_dlloader_next (lt_dlloader *place) ¶

Iterate over the module loaders, returning the first loader if place is NULL, and the next one on subsequent calls. The handle is for use with lt_dlloader_add.

/* Make myloader be the first one. */
if (lt_dlloader_add (lt_dlloader_next (NULL), myloader) != 0)
  return ERROR;

Function: lt_dlloader * lt_dlloader_find (const char *loader_name) ¶

Return the first loader with a matching loader_name identifier, or else NULL, if the identifier is not found.

The identifiers that may be used by libltdl itself, if the host architecture supports them are dlopen¹², dld and dlpreload.

/* Add a user loader as the next module loader to be tried if
   the standard dlopen loader were to fail when lt_dlopening. */
if (lt_dlloader_add (lt_dlloader_find ("dlopen"), myloader) != 0)
  return ERROR;

Function: const char * lt_dlloader_name (lt_dlloader *place) ¶

Return the identifying name of place, as obtained from lt_dlloader_next or lt_dlloader_find. If this function fails, it will return NULL and set an error for retrieval with lt_dlerror.

Function: lt_user_data * lt_dlloader_data (lt_dlloader *place) ¶

Return the address of the dlloader_data of place, as obtained from lt_dlloader_next or lt_dlloader_find. If this function fails, it will return NULL and set an error for retrieval with lt_dlerror.

• Error handling within user module loaders

11.6 How to distribute libltdl with your package ¶

Even though libltdl is installed together with libtool, you may wish to include libltdl in the distribution of your package, for the convenience of users of your package that don’t have libtool or libltdl installed, or if you are using features of a very new version of libltdl that you don’t expect your users to have yet. In such cases, you must decide what flavor of libltdl you want to use: a convenience library or an installable libtool library.

The most simplistic way to add libltdl to your package is to copy all the libltdl source files to a subdirectory within your package and to build and link them along with the rest of your sources. To help you do this, the m4 macros for Autoconf are available in ltdl.m4. You must ensure that they are available in aclocal.m4 before you run Autoconf¹³. Having made the macros available, you must add a call to the ‘LTDL_INIT’ macro (after the call to ‘LT_INIT’) to your package’s configure.ac to perform the configure time checks required to build the library correctly. Unfortunately, this method has problems if you then try to link the package binaries with an installed libltdl, or a library that depends on libltdl, because of the duplicate symbol definitions. For example, ultimately linking against two different versions of libltdl, or against both a local convenience library and an installed libltdl is bad. Ensuring that only one copy of the libltdl sources are linked into any program is left as an exercise for the reader.

Macro: LT_CONFIG_LTDL_DIR (directory) ¶

Declare directory to be the location of the libltdl source files, for libtoolize --ltdl to place them. See Invoking libtoolize, for more details. Provided that you add an appropriate LT_CONFIG_LTDL_DIR call in your configure.ac before calling libtoolize, the appropriate libltdl files will be installed automatically.

Macro: LTDL_INIT (options) ¶

Macro: LT_WITH_LTDL ¶

Macro: AC_WITH_LTDL ¶

AC_WITH_LTDL and LT_WITH_LTDL are deprecated names for older versions of this macro; autoupdate will update your configure.ac file.

This macro adds the following options to the configure script:

--with-ltdl-include installed-ltdl-header-dir

The LTDL_INIT macro will look in the standard header file locations to find the installed libltdl headers. If LTDL_INIT can’t find them by itself, the person who builds your package can use this option to tell configure where the installed libltdl headers are.

--with-ltdl-lib installed-ltdl-library-dir

Similarly, the person building your package can use this option to help configure find the installed libltdl.la.

--with-included-ltdl

If there is no installed libltdl, or in any case if the person building your package would rather use the libltdl sources shipped with the package in the subdirectory named by LT_CONFIG_LTDL_DIR, they should pass this option to configure.

If the --with-included-ltdl is not passed at configure time, and an installed libltdl is not found¹⁴, then configure will exit immediately with an error that asks the user to either specify the location of an installed libltdl using the --with-ltdl-include and --with-ltdl-lib options, or to build with the libltdl sources shipped with the package by passing --with-included-ltdl.

If an installed libltdl is found, then LIBLTDL is set to the link flags needed to use it, and LTDLINCL to the preprocessor flags needed to find the installed headers, and LTDLDEPS will be empty. Note, however, that no version checking is performed. You should manually check for the libltdl features you need in configure.ac:

LT_INIT([dlopen])
LTDL_INIT

# The lt_dladvise_init symbol was added with libtool-2.2
if test yes != "$with_included_ltdl"; then
  save_CFLAGS=$CFLAGS
  save_LDFLAGS=$LDFLAGS
  CFLAGS="$CFLAGS $LTDLINCL"
  LDFLAGS="$LDFLAGS $LIBLTDL"
  AC_CHECK_LIB([ltdl], [lt_dladvise_init],
                [],
        [AC_MSG_ERROR([installed libltdl is too old])])
  LDFLAGS=$save_LDFLAGS
  CFLAGS=$save_CFLAGS
fi

options may include no more than one of the following build modes depending on how you want your project to build libltdl: ‘nonrecursive’, ‘recursive’, or ‘subproject’. In order for libtoolize to detect this option correctly, if you supply one of these arguments, they must be given literally (i.e., macros or shell variables that expand to the correct ltdl mode will not work).

‘nonrecursive’

This is how the Libtool project distribution builds the libltdl we ship and install. If you wish to use Automake to build libltdl without invoking a recursive make to descend into the libltdl subdirectory, then use this option. You will need to set your configuration up carefully to make this work properly, and you will need releases of Autoconf and Automake that support subdir-objects and LIBOBJDIR properly. In your configure.ac, add:

AM_INIT_AUTOMAKE([subdir-objects])
AC_CONFIG_HEADERS([config.h])
LT_CONFIG_LTDL_DIR([libltdl])
LT_INIT([dlopen])
LTDL_INIT([nonrecursive])

You have to use a config header, but it may have a name different than config.h.

Also, add the following near the top of your Makefile.am:

AM_CPPFLAGS =
AM_LDFLAGS =

BUILT_SOURCES =
EXTRA_DIST =
CLEANFILES =
MOSTLYCLEANFILES =

include_HEADERS =
noinst_LTLIBRARIES =
lib_LTLIBRARIES =
EXTRA_LTLIBRARIES =

include libltdl/ltdl.mk

Unless you build no other libraries from this Makefile.am, you will also need to change lib_LTLIBRARIES to assign with ‘+=’ so that the libltdl targets declared in ltdl.mk are not overwritten.

‘recursive’

This build mode still requires that you use Automake, but (in contrast with ‘nonrecursive’) uses the more usual device of starting another make process in the libltdl subdirectory. To use this mode, you should add to your configure.ac:

AM_INIT_AUTOMAKE
AC_CONFIG_HEADERS([config.h])
LT_CONFIG_LTDL_DIR([libltdl])
LT_INIT([dlopen])
LTDL_INIT([recursive])
AC_CONFIG_FILES([libltdl/Makefile])

Again, you have to use a config header, but it may have a name different than config.h if you like.

Also, add this to your Makefile.am:

SUBDIRS = libltdl

‘subproject’

This mode is the default unless you explicitly add recursive or nonrecursive to your LTDL_INIT options; subproject is the only mode supported by previous releases of libltdl. Even if you do not use Autoconf in the parent project, then, in ‘subproject’ mode, still libltdl contains all the necessary files to configure and build itself – you just need to arrange for your build system to call libltdl/configure with appropriate options, and then run make in the libltdl subdirectory.

If you are using Autoconf and Automake, then you will need to add the following to your configure.ac:

LT_CONFIG_LTDL_DIR([libltdl])
LTDL_INIT

and to Makefile.am:

SUBDIRS = libltdl

Aside from setting the libltdl build mode, there are other keywords that you can pass to LTDL_INIT to modify its behavior when --with-included-ltdl has been given:

‘convenience’

This is the default unless you explicitly add installable to your LTDL_INIT options.

This keyword will cause options to be passed to the configure script in the subdirectory named by LT_CONFIG_LTDL_DIR to cause it to be built as a convenience library. If you’re not using automake, you will need to define top_build_prefix, top_builddir, and top_srcdir in your makefile so that LIBLTDL, LTDLDEPS, and LTDLINCL expand correctly.

One advantage of the convenience library is that it is not installed, so the fact that you use libltdl will not be apparent to the user, and it won’t overwrite a pre-installed version of libltdl the system might already have in the installation directory. On the other hand, if you want to upgrade libltdl for any reason (e.g. a bugfix) you’ll have to recompile your package instead of just replacing the shared installed version of libltdl. However, if your programs or libraries are linked with other libraries that use such a pre-installed version of libltdl, you may get linker errors or run-time crashes. Another problem is that you cannot link the convenience library into more than one libtool library, then link a single program with those libraries, because you may get duplicate symbols. In general you can safely use the convenience library in programs that don’t depend on other libraries that might use libltdl too.

‘installable’

This keyword will pass options to the configure script in the subdirectory named by LT_CONFIG_LTDL_DIR to cause it to be built as an installable library. If you’re not using automake, you will need to define top_build_prefix, top_builddir and top_srcdir in your makefile so that LIBLTDL, LTDLDEPS, and LTDLINCL are expanded properly.

Be aware that you could overwrite another libltdl already installed to the same directory if you use this option.

Whatever method you use, ‘LTDL_INIT’ will define the shell variable LIBLTDL to the link flag that you should use to link with libltdl, the shell variable LTDLDEPS to the files that can be used as a dependency in Makefile rules, and the shell variable LTDLINCL to the preprocessor flag that you should use to compile programs that include ltdl.h. So, when you want to link a program with libltdl, be it a convenience, installed or installable library, just use ‘$(LTDLINCL)’ for preprocessing and compilation, and ‘$(LIBLTDL)’ for linking.

• If your package is built using an installed version of libltdl, LIBLTDL will be set to the compiler flags needed to link against the installed library, LTDLDEPS will be empty, and LTDLINCL will be set to the compiler flags needed to find the libltdl header files.
• If your package is built using the convenience libltdl, LIBLTDL and LTDLDEPS will be the pathname for the convenience version of libltdl (starting with ‘${top_builddir}/’ or ‘${top_build_prefix}’) and LTDLINCL will be -I followed by the directory that contains ltdl.h (starting with ‘${top_srcdir}/’).
• If an installable version of the included libltdl is being built, its pathname starting with ‘${top_builddir}/’ or ‘${top_build_prefix}’, will be stored in LIBLTDL and LTDLDEPS, and LTDLINCL will be set just like in the case of convenience library.

You should probably also use the ‘dlopen’ option to LT_INIT in your configure.ac, otherwise libtool will assume no dlopening mechanism is supported, and revert to dlpreopening, which is probably not what you want. Avoid using the -static, -static-libtool-libs, or -all-static switches when linking programs with libltdl. This will not work on all platforms, because the dlopening functions may not be available for static linking.

The following example shows you how to embed an installable libltdl in your package. In order to use the convenience variant, just replace the LTDL_INIT option ‘installable’ with ‘convenience’. We assume that libltdl was embedded using ‘libtoolize --ltdl’.

configure.ac:

...
# Name the subdirectory that contains libltdl sources
LT_CONFIG_LTDL_DIR([libltdl])

# Configure libtool with dlopen support if possible
LT_INIT([dlopen])

# Enable building of the installable libltdl library
LTDL_INIT([installable])
...

Makefile.am:

...
SUBDIRS = libltdl

AM_CPPFLAGS = $(LTDLINCL)

myprog_LDFLAGS = -export-dynamic
myprog_LDADD = $(LIBLTDL) -dlopen self -dlopen foo1.la
myprog_DEPENDENCIES = $(LTDLDEPS) foo1.la
...

Macro: LTDL_INSTALLABLE ¶

Macro: AC_LIBLTDL_INSTALLABLE ¶

These macros are deprecated, the ‘installable’ option to LTDL_INIT should be used instead.

Macro: LTDL_CONVENIENCE ¶

Macro: AC_LIBLTDL_CONVENIENCE ¶

These macros are deprecated, the ‘convenience’ option to LTDL_INIT should be used instead.

13.1 Why does libtool strip link flags when creating a library? ¶

When creating a shared library, but not when compiling or creating a program, libtool drops some flags from the command line provided by the user. This is done because flags unknown to libtool may interfere with library creation or require additional support from libtool, and because omitting flags is usually the conservative choice for a successful build.

If you encounter flags that you think are useful to pass, as a work-around you can prepend flags with -Wc, or -Xcompiler to allow them to be passed through to the compiler driver (see Link mode). Another possibility is to add flags already to the compiler command at configure run time:

./configure CC='gcc -m64'

If you think libtool should let some flag through by default, here’s how you can test such an inclusion: grab the Libtool development tree, edit the ltmain.in file in the libltdl/config subdirectory to pass through the flag (search for ‘Flags to be passed through’), re-bootstrap and build with the flags in question added to LDFLAGS, CFLAGS, CXXFLAGS, etc. on the configure command line as appropriate. Run the testsuite as described in the README file and report results to the Libtool bug reporting address bug-libtool@gnu.org.

14.1 The libtool test suite ¶

Libtool comes with an integrated set of tests to check that your build is sane, that test its capabilities, and report obvious bugs in the libtool program. The test suite is based on Autotest from Autoconf (see Generating Test Suites with Autotest in The Autoconf Manual). These tests, too, are constantly evolving, based on past problems with libtool, and known deficiencies in other operating systems.

As described in the README file, you may run make -k check after you have built libtool (possibly before you install it) to make sure that it meets basic functional requirements.

• Description of test suite
• When tests fail

14.2 Reporting bugs ¶

If you think you have discovered a bug in libtool, you should think twice: the libtool maintainer is notorious for passing the buck (or maybe that should be “passing the bug”). Libtool was invented to fix known deficiencies in shared library implementations, so, in a way, most of the bugs in libtool are actually bugs in other operating systems. However, the libtool maintainer would definitely be happy to add support for somebody else’s buggy operating system. [I wish there was a good way to do winking smiley-faces in Texinfo.]

Genuine bugs in libtool include problems with shell script portability, documentation errors, and failures in the test suite (see The libtool test suite).

First, check the documentation and help screens to make sure that the behaviour you think is a problem is not already mentioned as a feature.

Then, you should read the Emacs guide to reporting bugs (see Reporting Bugs in The Emacs Manual). Some of the details listed there are specific to Emacs, but the principle behind them is a general one.

Finally, send a bug report to the Libtool bug reporting address bug-libtool@gnu.org with any appropriate facts, such as test suite output (see When tests fail), all the details needed to reproduce the bug, and a brief description of why you think the behaviour is a bug. Be sure to include the word “libtool” in the subject line, as well as the version number you are using (which can be found by typing libtool --version).

15.1 Porting libtool to new systems ¶

Before you embark on porting libtool to an unsupported system, it is worthwhile to send e-mail to the Libtool mailing list libtool@gnu.org, to make sure that you are not duplicating existing work.

If you find that any porting documentation is missing, please complain! Complaints with patches and improvements to the documentation, or to libtool itself, are more than welcome.

• Information sources
• Porting inter-library dependencies support

15.2 Tested platforms ¶

This table describes when libtool was last known to be tested on platforms where it claims to support shared libraries:

-------------------------------------------------------
canonical host name          compiler  libtool results
  (tools versions)                     release
-------------------------------------------------------
alpha-dec-osf5.1		cc	 1.3e	  ok (1.910)
alpha-dec-osf4.0f               gcc      1.3e     ok (1.910)
alpha-dec-osf4.0f               cc       1.3e     ok (1.910)
alpha-dec-osf3.2                gcc      0.8      ok
alpha-dec-osf3.2                cc       0.8      ok
alpha-dec-osf2.1                gcc      1.2f     NS
alpha*-unknown-linux-gnu        gcc      1.3b     ok
  (egcs-1.1.2, GNU ld 2.9.1.0.23)
hppa2.0w-hp-hpux11.00           cc       1.2f     ok
hppa2.0-hp-hpux10.20            cc       1.3.2    ok
hppa1.1-hp-hpux10.20            gcc      1.2f     ok
hppa1.1-hp-hpux10.20            cc       1.3c     ok (1.821)
hppa1.1-hp-hpux10.10            gcc      1.2f     ok
hppa1.1-hp-hpux10.10            cc       1.2f     ok
hppa1.1-hp-hpux9.07             gcc      1.2f     ok
hppa1.1-hp-hpux9.07             cc       1.2f     ok
hppa1.1-hp-hpux9.05             gcc      1.2f     ok
hppa1.1-hp-hpux9.05             cc       1.2f     ok
hppa1.1-hp-hpux9.01             gcc      1.2f     ok
hppa1.1-hp-hpux9.01             cc       1.2f     ok
i*86-*-beos                     gcc      1.2f     ok
i*86-*-bsdi4.0.1                gcc      1.3c     ok
  (gcc-2.7.2.1)
i*86-*-bsdi4.0                  gcc      1.2f     ok
i*86-*-bsdi3.1                  gcc      1.2e     NS
i*86-*-bsdi3.0                  gcc      1.2e     NS
i*86-*-bsdi2.1                  gcc      1.2e     NS
i*86-pc-cygwin                  gcc      1.3b     NS
  (egcs-1.1 stock b20.1 compiler)
i*86-*-dguxR4.20MU01            gcc      1.2      ok
i*86-*-freebsd4.3		gcc      1.3e     ok (1.912)
i*86-*-freebsdelf4.0            gcc      1.3c     ok
  (egcs-1.1.2)
i*86-*-freebsdelf3.2            gcc      1.3c     ok
  (gcc-2.7.2.1)
i*86-*-freebsdelf3.1            gcc      1.3c     ok
  (gcc-2.7.2.1)
i*86-*-freebsdelf3.0            gcc      1.3c     ok
i*86-*-freebsd3.0               gcc      1.2e     ok
i*86-*-freebsd2.2.8             gcc      1.3c     ok
  (gcc-2.7.2.1)
i*86-*-freebsd2.2.6             gcc      1.3b     ok
  (egcs-1.1 & gcc-2.7.2.1, native ld)
i*86-*-freebsd2.1.5             gcc      0.5      ok
i*86-*-netbsd1.5                gcc      1.3e     ok (1.901)
  (egcs-1.1.2)
i*86-*-netbsd1.4                gcc      1.3c     ok
  (egcs-1.1.1)
i*86-*-netbsd1.4.3A             gcc      1.3e     ok (1.901)
i*86-*-netbsd1.3.3              gcc      1.3c     ok
  (gcc-2.7.2.2+myc2)
i*86-*-netbsd1.3.2              gcc      1.2e     ok
i*86-*-netbsd1.3I               gcc      1.2e     ok
  (egcs 1.1?)
i*86-*-netbsd1.2                gcc      0.9g     ok
i*86-*-linux-gnu		gcc	 1.3e	  ok (1.901)
  (Red Hat 7.0, gcc "2.96")
i*86-*-linux-gnu		gcc	 1.3e	  ok (1.911)
  (SuSE 7.0, gcc 2.95.2)
i*86-*-linux-gnulibc1           gcc      1.2f     ok
i*86-*-openbsd2.5               gcc      1.3c     ok
  (gcc-2.8.1)
i*86-*-openbsd2.4               gcc      1.3c     ok
  (gcc-2.8.1)
i*86-*-solaris2.7               gcc      1.3b     ok
  (egcs-1.1.2, native ld)
i*86-*-solaris2.6               gcc      1.2f     ok
i*86-*-solaris2.5.1             gcc      1.2f     ok
i*86-ncr-sysv4.3.03             gcc      1.2f     ok
i*86-ncr-sysv4.3.03             cc       1.2e     ok
  (cc -Hnocopyr)
i*86-pc-sco3.2v5.0.5		cc	 1.3c	  ok
i*86-pc-sco3.2v5.0.5		gcc	 1.3c	  ok
  (gcc 95q4c)
i*86-pc-sco3.2v5.0.5		gcc	 1.3c	  ok
  (egcs-1.1.2)
i*86-sco-sysv5uw7.1.1		gcc	 1.3e	  ok (1.901)
  (gcc-2.95.2, SCO linker)
i*86-UnixWare7.1.0-sysv5	cc	 1.3c	  ok
i*86-UnixWare7.1.0-sysv5	gcc	 1.3c	  ok
  (egcs-1.1.1)
m68k-next-nextstep3             gcc      1.2f     NS
m68k-sun-sunos4.1.1             gcc      1.2f     NS
  (gcc-2.5.7)
m88k-dg-dguxR4.12TMU01          gcc      1.2      ok
m88k-motorola-sysv4             gcc      1.3      ok
  (egcs-1.1.2)
mips-sgi-irix6.5                gcc      1.2f     ok
  (gcc-2.8.1)
mips-sgi-irix6.4                gcc      1.2f     ok
mips-sgi-irix6.3                gcc      1.3b     ok
  (egcs-1.1.2, native ld)
mips-sgi-irix6.3                cc       1.3b     ok
  (cc 7.0)
mips-sgi-irix6.2                gcc      1.2f     ok
mips-sgi-irix6.2                cc       0.9      ok
mips-sgi-irix5.3                gcc      1.2f     ok
  (egcs-1.1.1)
mips-sgi-irix5.3                gcc      1.2f     NS
  (gcc-2.6.3)
mips-sgi-irix5.3                cc       0.8      ok
mips-sgi-irix5.2                gcc      1.3b     ok
  (egcs-1.1.2, native ld)
mips-sgi-irix5.2                cc       1.3b     ok
  (cc 3.18)
mips-sni-sysv4			cc       1.3.5    ok
  (Siemens C-compiler)
mips-sni-sysv4			gcc      1.3.5    ok
  (gcc-2.7.2.3, GNU assembler 2.8.1, native ld)
mipsel-unknown-openbsd2.1       gcc      1.0      ok
powerpc-apple-darwin6.4         gcc      1.5      ok
(apple dev tools released 12/2002)
powerpc-ibm-aix4.3.1.0          gcc      1.2f     ok
  (egcs-1.1.1)
powerpc-ibm-aix4.2.1.0          gcc      1.2f     ok
  (egcs-1.1.1)
powerpc-ibm-aix4.1.5.0          gcc      1.2f     ok
  (egcs-1.1.1)
powerpc-ibm-aix4.1.5.0          gcc      1.2f     NS
  (gcc-2.8.1)
powerpc-ibm-aix4.1.4.0          gcc      1.0      ok
powerpc-ibm-aix4.1.4.0          xlc      1.0i     ok
rs6000-ibm-aix4.1.5.0           gcc      1.2f     ok
  (gcc-2.7.2)
rs6000-ibm-aix4.1.4.0           gcc      1.2f     ok
  (gcc-2.7.2)
rs6000-ibm-aix3.2.5             gcc      1.0i     ok
rs6000-ibm-aix3.2.5             xlc      1.0i     ok
sparc-sun-solaris2.8		gcc	 1.3e	  ok (1.913)
  (gcc-2.95.3 & native ld)
sparc-sun-solaris2.7            gcc      1.3e     ok (1.913)
  (gcc-2.95.3 & native ld)
sparc-sun-solaris2.6            gcc      1.3e     ok (1.913)
  (gcc-2.95.3 & native ld)
sparc-sun-solaris2.5.1          gcc      1.3e     ok (1.911)
sparc-sun-solaris2.5            gcc      1.3b     ok
  (egcs-1.1.2, GNU ld 2.9.1 & native ld)
sparc-sun-solaris2.5            cc       1.3b     ok
  (SC 3.0.1)
sparc-sun-solaris2.4            gcc      1.0a     ok
sparc-sun-solaris2.4            cc       1.0a     ok
sparc-sun-solaris2.3            gcc      1.2f     ok
sparc-sun-sunos4.1.4            gcc      1.2f     ok
sparc-sun-sunos4.1.4            cc       1.0f     ok
sparc-sun-sunos4.1.3_U1         gcc      1.2f     ok
sparc-sun-sunos4.1.3C           gcc      1.2f     ok
sparc-sun-sunos4.1.3            gcc      1.3b     ok
  (egcs-1.1.2, GNU ld 2.9.1 & native ld)
sparc-sun-sunos4.1.3            cc       1.3b     ok
sparc-unknown-bsdi4.0           gcc      1.2c     ok
sparc-unknown-linux-gnulibc1    gcc      1.2f     ok
sparc-unknown-linux-gnu         gcc      1.3b     ok
  (egcs-1.1.2, GNU ld 2.9.1.0.23)
sparc64-unknown-linux-gnu       gcc      1.2f     ok

Notes:
- "ok" means "all tests passed".
- "NS" means "Not Shared", but OK for static libraries

Note: The vendor-distributed HP-UX sed(1) programs are horribly broken, and cannot handle libtool’s requirements, so users may report unusual problems. There is no workaround except to install a working sed (such as GNU sed) on these systems.

Note: The vendor-distributed NCR MP-RAS cc programs emits copyright on standard error that confuse tests on size of conftest.err. The workaround is to specify CC when run configure with CC='cc -Hnocopyr'.

15.3 Platform quirks ¶

This section is dedicated to the sanity of the libtool maintainers. It describes the programs that libtool uses, how they vary from system to system, and how to test for them.

Because libtool is a shell script, it can be difficult to understand just by reading it from top to bottom. This section helps show why libtool does things a certain way. Combined with the scripts themselves, you should have a better sense of how to improve libtool, or write your own.

• Compilers
• Reloadable objects
• Multiple dependencies
• Archivers
• Cross compiling
• File name conversion
• Windows DLLs

/var/tmp/foo-1.2.3/app/

(application source code)

/var/tmp/foo-1.2.3/lib/

(library source code)

/var/tmp/BUILD/app/

(application build objects here)

/var/tmp/BUILD/lib/

(library build objects here)

Z:\var\tmp\BUILD\lib

Could not determine the host file name corresponding to
  `... a file name ...'
Continuing, but uninstalled executables may not work.

Could not determine the host path corresponding to
  `... a path ...'
Continuing, but uninstalled executables may not work.

export PATH="/c/MinGW/bin:${PATH}"
configure --build=i686-pc-cygwin \
	--host=mingw32 \
	NM=/c/MinGW/bin/nm.exe

export lt_cv_to_tool_file_cmd=func_convert_file_cygwin_to_w32

export PATH="/c/MinGW/bin:${PATH}"
configure --build=i686-pc-mingw32 \
	--host=i686-pc-mingw32 \
	--disable-dependency-tracking

export lt_cv_to_host_file_cmd=func_convert_file_cygwin_to_w32
export lt_cv_to_tool_file_cmd=func_convert_file_cygwin_to_w32

Makefile:1: *** target pattern contains no `%'.  Stop.

# cygwin-root/etc/fstab
D:/foo    /foo     some_fs binary 0 0
D:/bar    /bar     some_fs binary 0 0
E:/grill  /grill   some_fs binary 0 0

#ifndef FOO_H
#define FOO_H

int one (void);
int two (void);
extern int three;

#endif /* FOO_H */

#include "foo.h"

int one (void)
{
  return 1;
}

int two (void)
{
  return three - one ();
}

int three = 3;

#ifndef FOO_H
#define FOO_H

#if defined _WIN32 && !defined __GNUC__
# ifdef LIBFOO_BUILD
#  ifdef DLL_EXPORT
#   define LIBFOO_SCOPE            __declspec (dllexport)
#   define LIBFOO_SCOPE_VAR extern __declspec (dllexport)
#  endif
# elif defined _MSC_VER
#  define LIBFOO_SCOPE
#  define LIBFOO_SCOPE_VAR  extern __declspec (dllimport)
# elif defined DLL_EXPORT
#  define LIBFOO_SCOPE             __declspec (dllimport)
#  define LIBFOO_SCOPE_VAR  extern __declspec (dllimport)
# endif
#endif
#ifndef LIBFOO_SCOPE
# define LIBFOO_SCOPE
# define LIBFOO_SCOPE_VAR extern
#endif

LIBFOO_SCOPE     int one (void);
LIBFOO_SCOPE     int two (void);
LIBFOO_SCOPE_VAR int three;

#endif /* FOO_H */

#ifndef FOO_H
#define FOO_H

#if defined _WIN32 && !defined __GNUC__ && !defined LIBFOO_BUILD
# define LIBFOO_SCOPE_VAR extern __declspec (dllimport)
#else
# define LIBFOO_SCOPE_VAR extern
#endif

int one (void);
int two (void);
LIBFOO_SCOPE_VAR int three;

#endif /* FOO_H */

15.4 libtool script contents ¶

Since version 1.4, the libtool script is generated by configure (see Configuring libtool). In earlier versions, configure achieved this by calling a helper script called ltconfig. From libtool version 0.7 to 1.0, this script simply set shell variables, then sourced the libtool backend, ltmain.sh. ltconfig from libtool version 1.1 through 1.3 inlined the contents of ltmain.sh into the generated libtool, which improved performance on many systems. The tests that ltconfig used to perform are now kept in libtool.m4 where they can be written using Autoconf. This has the runtime performance benefits of inlined ltmain.sh, and improves the build time a little while considerably easing the amount of raw shell code that used to need maintaining.

The convention used for naming variables that hold shell commands for delayed evaluation, is to use the suffix _cmd where a single line of valid shell script is needed, and the suffix _cmds where multiple lines of shell script may be delayed for later evaluation. By convention, _cmds variables delimit the evaluation units with the ~ character where necessary.

Here is a listing of each of the configuration variables, and how they are used within ltmain.sh (see Configuring libtool):

Variable: AR ¶

The name of the system library archiver.

Variable: CC ¶

The name of the compiler used to configure libtool. This will always contain the compiler for the current language (see Tags).

Variable: ECHO ¶

An echo program that does not interpret backslashes as an escape character. It may be given only one argument, so due quoting is necessary.

Variable: LD ¶

The name of the linker that libtool should use internally for reloadable linking and possibly shared libraries.

Variable: LTCC ¶

Variable: LTCFLAGS ¶

The name of the C compiler and C compiler flags used to configure libtool.

Variable: NM ¶

The name of a BSD- or MS-compatible program that produces listings of global symbols. For BSD nm, the symbols should be in one the following formats:

address C global-variable-name
address D global-variable-name
address T global-function-name

For MS dumpbin, the symbols should be in one of the following formats:

counter size    UNDEF    notype       External     | global-var
counter address section  notype       External     | global-var
counter address section  notype ()    External     | global-func

The size of the global variables are not zero and the section of the global functions are not "UNDEF". Symbols in "pick any" sections ("pick any" appears in the section header) are not global either.

Variable: RANLIB ¶

Set to the name of the ranlib program, if any.

Variable: allow_undefined_flag ¶

The flag that is used by ‘archive_cmds’ to declare that there will be unresolved symbols in the resulting shared library. Empty, if no such flag is required. Set to ‘unsupported’ if there is no way to generate a shared library with references to symbols that aren’t defined in that library.

Variable: always_export_symbols ¶

Whether libtool should automatically generate a list of exported symbols using export_symbols_cmds before linking an archive. Set to ‘yes’ or ‘no’. Default is ‘no’.

Variable: archive_cmds ¶

Variable: archive_expsym_cmds ¶

Variable: old_archive_cmds ¶

Commands used to create shared libraries, shared libraries with -export-symbols and static libraries, respectively.

Variable: archiver_list_spec ¶

Specify filename containing input files for AR.

Variable: old_archive_from_new_cmds ¶

If the shared library depends on a static library, ‘old_archive_from_new_cmds’ contains the commands used to create that static library. If this variable is not empty, ‘old_archive_cmds’ is not used.

Variable: old_archive_from_expsyms_cmds ¶

If a static library must be created from the export symbol list to correctly link with a shared library, ‘old_archive_from_expsyms_cmds’ contains the commands needed to create that static library. When these commands are executed, the variable soname contains the name of the shared library in question, and the ‘$objdir/$newlib’ contains the path of the static library these commands should build. After executing these commands, libtool will proceed to link against ‘$objdir/$newlib’ instead of soname.

Variable: lock_old_archive_extraction ¶

Set to ‘yes’ if the extraction of a static library requires locking the library file. This is required on Darwin.

Variable: build ¶

Variable: build_alias ¶

Variable: build_os ¶

Set to the specified and canonical names of the system that libtool was built on.

Variable: build_libtool_libs ¶

Whether libtool should build shared libraries on this system. Set to ‘yes’ or ‘no’.

Variable: build_old_libs ¶

Whether libtool should build static libraries on this system. Set to ‘yes’ or ‘no’.

Variable: compiler_c_o ¶

Whether the compiler supports the -c and -o options simultaneously. Set to ‘yes’ or ‘no’.

Variable: compiler_needs_object ¶

Whether the compiler has to see an object listed on the command line in order to successfully invoke the linker. If ‘no’, then a set of convenience archives or a set of object file names can be passed via linker-specific options or linker scripts.

Variable: dlopen_support ¶

Whether dlopen is supported on the platform. Set to ‘yes’ or ‘no’.

Variable: dlopen_self ¶

Whether it is possible to dlopen the executable itself. Set to ‘yes’ or ‘no’.

Variable: dlopen_self_static ¶

Whether it is possible to dlopen the executable itself, when it is linked statically (-all-static). Set to ‘yes’ or ‘no’.

Variable: exclude_expsyms ¶

List of symbols that should not be listed in the preloaded symbols.

Variable: export_dynamic_flag_spec ¶

Compiler link flag that allows a dlopened shared library to reference symbols that are defined in the program.

Variable: export_symbols_cmds ¶

Commands to extract exported symbols from libobjs to the file export_symbols.

Variable: extract_expsyms_cmds ¶

Commands to extract the exported symbols list from a shared library. These commands are executed if there is no file ‘$objdir/$soname-def’, and should write the names of the exported symbols to that file, for the use of ‘old_archive_from_expsyms_cmds’.

Variable: fast_install ¶

Determines whether libtool will privilege the installer or the developer. The assumption is that installers will seldom run programs in the build tree, and the developer will seldom install. This is only meaningful on platforms where shlibpath_overrides_runpath is not ‘yes’, so fast_install will be set to ‘needless’ in this case. If fast_install set to ‘yes’, libtool will create programs that search for installed libraries, and, if a program is run in the build tree, a new copy will be linked on-demand to use the yet-to-be-installed libraries. If set to ‘no’, libtool will create programs that use the yet-to-be-installed libraries, and will link a new copy of the program at install time. The default value is ‘yes’ or ‘needless’, depending on platform and configuration flags, and it can be turned from ‘yes’ to ‘no’ with the configure flag --disable-fast-install.

On some systems, the linker always hardcodes paths to dependent libraries into the output. In this case, fast_install is never set to ‘yes’, and relinking at install time is triggered. This also means that DESTDIR installation does not work as expected.

Variable: file_magic_glob ¶

How to find potential files when deplibs_check_method is ‘file_magic’. file_magic_glob is a sed expression, and the sed instance is fed potential file names that are transformed by the file_magic_glob expression. Useful when the shell does not support the shell option nocaseglob, making want_nocaseglob inappropriate. Normally disabled (i.e. file_magic_glob is empty).

Variable: finish_cmds ¶

Commands to tell the dynamic linker how to find shared libraries in a specific directory. These commands can be disabled during testing local changes to shared libraries with --no-finish.

Variable: finish_eval ¶

Same as finish_cmds, except the commands are not displayed.

Variable: global_symbol_pipe ¶

A pipeline that takes the output of NM, and produces a listing of raw symbols followed by their C names. For example:

$ eval "$NM progname | $global_symbol_pipe"
D symbol1 C-symbol1
T symbol2 C-symbol2
C symbol3 C-symbol3
...
$

The first column contains the symbol type (used to tell data from code) but its meaning is system dependent.

Variable: global_symbol_to_cdecl ¶

A pipeline that translates the output of global_symbol_pipe into proper C declarations. Since some platforms, such as HP/UX, have linkers that differentiate code from data, data symbols are declared as data, and code symbols are declared as functions.

Variable: hardcode_action ¶

Either ‘immediate’ or ‘relink’, depending on whether shared library paths can be hardcoded into executables before they are installed, or if they need to be relinked.

Variable: hardcode_direct ¶

Set to ‘yes’ or ‘no’, depending on whether the linker hardcodes directories if a library is directly specified on the command line (such as ‘dir/libname.a’) when hardcode_libdir_flag_spec is specified.

Variable: hardcode_direct_absolute ¶

Some architectures hardcode "absolute" library directories that cannot be overridden by shlibpath_var when hardcode_direct is ‘yes’. In that case set hardcode_direct_absolute to ‘yes’, or otherwise ‘no’.

Variable: hardcode_into_libs ¶

Whether the platform supports hardcoding of run-paths into libraries. If enabled, linking of programs will be much simpler but libraries will need to be relinked during installation. Set to ‘yes’ or ‘no’.

Variable: hardcode_libdir_flag_spec ¶

Flag to hardcode a libdir variable into a binary, so that the dynamic linker searches libdir for shared libraries at runtime. If it is empty, libtool will try to use some other hardcoding mechanism.

Variable: hardcode_libdir_separator ¶

If the compiler only accepts a single hardcode_libdir_flag, then this variable contains the string that should separate multiple arguments to that flag.

Variable: hardcode_minus_L ¶

Set to ‘yes’ or ‘no’, depending on whether the linker hardcodes directories specified by -L flags into the resulting executable when hardcode_libdir_flag_spec is specified.

Variable: hardcode_shlibpath_var ¶

Set to ‘yes’ or ‘no’, depending on whether the linker hardcodes directories by writing the contents of ‘$shlibpath_var’ into the resulting executable when hardcode_libdir_flag_spec is specified. Set to ‘unsupported’ if directories specified by ‘$shlibpath_var’ are searched at run time, but not at link time.

Variable: host ¶

Variable: host_alias ¶

Variable: host_os ¶

Set to the specified and canonical names of the system that libtool was configured for.

Variable: include_expsyms ¶

List of symbols that must always be exported when using export_symbols.

Variable: inherit_rpath ¶

Whether the linker adds runtime paths of dependency libraries to the runtime path list, requiring libtool to relink the output when installing. Set to ‘yes’ or ‘no’. Default is ‘no’.

Variable: install_override_mode ¶

Permission mode override for installation of shared libraries. If the runtime linker fails to load libraries with wrong permissions, then it may fail to execute programs that are needed during installation, because these need the library that has just been installed. In this case, it is necessary to pass the mode to install with -m install_override_mode.

Variable: libext ¶

The standard old archive suffix (normally ‘a’).

Variable: libname_spec ¶

The format of a library name prefix. On all Unix systems, static libraries are called ‘libname.a’, but on some systems (such as OS/2 or MS-DOS), the library is just called ‘name.a’.

Variable: library_names_spec ¶

A list of shared library names. The first is the name of the file, the rest are symbolic links to the file. The name in the list is the file name that the linker finds when given -lname.

Variable: link_all_deplibs ¶

Whether libtool must link a program against all its dependency libraries. Set to ‘yes’ or ‘no’. Default is ‘unknown’, which is a synonym for ‘yes’.

Variable: link_static_flag ¶

Linker flag (passed through the C compiler) used to prevent dynamic linking.

Variable: macro_version ¶

Variable: macro_revision ¶

The release and revision from which the libtool.m4 macros were taken. This is used to ensure that macros and ltmain.sh correspond to the same Libtool version.

Variable: max_cmd_len ¶

The approximate longest command line that can be passed to ‘$SHELL’ without being truncated, as computed by ‘LT_CMD_MAX_LEN’.

Variable: need_lib_prefix ¶

Whether we can dlopen modules without a ‘lib’ prefix. Set to ‘yes’ or ‘no’. By default, it is ‘unknown’, which means the same as ‘yes’, but documents that we are not really sure about it. ‘no’ means that it is possible to dlopen a module without the ‘lib’ prefix.

Variable: need_version ¶

Whether versioning is required for libraries, i.e. whether the dynamic linker requires a version suffix for all libraries. Set to ‘yes’ or ‘no’. By default, it is ‘unknown’, which means the same as ‘yes’, but documents that we are not really sure about it.

Variable: need_locks ¶

Whether files must be locked to prevent conflicts when compiling simultaneously. Set to ‘yes’ or ‘no’.

Variable: nm_file_list_spec ¶

Specify filename containing input files for NM.

Variable: no_builtin_flag ¶

Compiler flag to disable builtin functions that conflict with declaring external global symbols as char.

Variable: no_undefined_flag ¶

The flag that is used by ‘archive_cmds’ to declare that there will be no unresolved symbols in the resulting shared library. Empty, if no such flag is required.

Variable: objdir ¶

The name of the directory that contains temporary libtool files.

Variable: objext ¶

The standard object file suffix (normally ‘o’).

Variable: pic_flag ¶

Any additional compiler flags for building library object files.

Variable: postinstall_cmds ¶

Variable: old_postinstall_cmds ¶

Commands run after installing a shared or static library, respectively.

Variable: postuninstall_cmds ¶

Variable: old_postuninstall_cmds ¶

Commands run after uninstalling a shared or static library, respectively.

Variable: postlink_cmds ¶

Commands necessary for finishing linking programs. postlink_cmds are executed immediately after the program is linked. Any occurrence of the string @OUTPUT@ in postlink_cmds is replaced by the name of the created executable (i.e. not the wrapper, if a wrapper is generated) prior to execution. Similarly, @TOOL_OUTPUT@ is replaced by the toolchain format of @OUTPUT@. Normally disabled (i.e. postlink_cmds empty).

Variable: reload_cmds ¶

Variable: reload_flag ¶

Commands to create a reloadable object. Set reload_cmds to ‘false’ on systems that cannot create reloadable objects.

Variable: runpath_var ¶

The environment variable that tells the linker what directories to hardcode in the resulting executable.

Variable: shlibpath_overrides_runpath ¶

Indicates whether it is possible to override the hard-coded library search path of a program with an environment variable. If this is set to no, libtool may have to create two copies of a program in the build tree, one to be installed and one to be run in the build tree only. When each of these copies is created depends on the value of fast_install. The default value is ‘unknown’, which is equivalent to ‘no’.

Variable: shlibpath_var ¶

The environment variable that tells the dynamic linker where to find shared libraries.

Variable: soname_spec ¶

The name coded into shared libraries, if different from the real name of the file.

Variable: striplib ¶

Variable: old_striplib ¶

Command to strip a shared (striplib) or static (old_striplib) library, respectively. If these variables are empty, the strip flag in the install mode will be ignored for libraries (see Install mode).

Variable: sys_lib_dlsearch_path_spec ¶

Expression to get the run-time system library search path. Directories that appear in this list are never hard-coded into executables.

Variable: sys_lib_search_path_spec ¶

Expression to get the compile-time system library search path. This variable is used by libtool when it has to test whether a certain library is shared or static. The directories listed in shlibpath_var are automatically appended to this list, every time libtool runs (i.e., not at configuration time), because some linkers use this variable to extend the library search path. Linker switches such as -L also augment the search path.

Variable: thread_safe_flag_spec ¶

Linker flag (passed through the C compiler) used to generate thread-safe libraries.

Variable: to_host_file_cmd ¶

If the toolchain is not native to the build platform (e.g. if you are using MSYS to drive the scripting, but are using the MinGW native Windows compiler) this variable describes how to convert file names from the format used by the build platform to the format used by host platform. Normally set to ‘func_convert_file_noop’, libtool will autodetect most cases where other values should be used. On rare occasions, it may be necessary to override the autodetected value (see Cygwin to MinGW Cross).

Variable: to_tool_file_cmd ¶

If the toolchain is not native to the build platform (e.g. if you are using some Unix to drive the scripting together with a Windows toolchain running in Wine) this variable describes how to convert file names from the format used by the build platform to the format used by the toolchain. Normally set to ‘func_convert_file_noop’.

Variable: version_type ¶

The library version numbering type. One of ‘libtool’, ‘freebsd-aout’, ‘freebsd-elf’, ‘irix’, ‘linux’, ‘osf’, ‘sunos’, ‘windows’, or ‘none’.

Variable: want_nocaseglob ¶

Find potential files using the shell option nocaseglob, when deplibs_check_method is ‘file_magic’. Normally set to ‘no’. Set to ‘yes’ to enable the nocaseglob shell option when looking for potential file names in a case-insensitive manner.

Variable: whole_archive_flag_spec ¶

Compiler flag to generate shared objects from convenience archives.

Variable: wl ¶

The C compiler flag that allows libtool to pass a flag directly to the linker. Used as: ${wl}some-flag.

Variables ending in ‘_cmds’ or ‘_eval’ contain a ‘~’-separated list of commands that are evaled one after another. If any of the commands return a nonzero exit status, libtool generally exits with an error message.

Variables ending in ‘_spec’ are evaled before being used by libtool.

15.5 Cheap tricks ¶

Here are a few tricks that you can use to make maintainership easier:

• When people report bugs, ask them to use the --config, --debug, or --features flags, if you think they will help you. These flags are there to help you get information directly, rather than having to trust second-hand observation.
• Rather than reconfiguring libtool every time I make a change to ltmain.in, I keep a permanent libtool script in my PATH, which sources ltmain.in directly.

  The following steps describe how to create such a script, where /home/src/libtool is the directory containing the libtool source tree, /home/src/libtool/libtool is a libtool script that has been configured for your platform, and ~/bin is a directory in your PATH:

  trick$ cd ~/bin
  trick$ sed 's%^\(macro_version=\).*$%\1@VERSION@%;
              s%^\(macro_revision=\).*$%\1@package_revision@%;
              /^# ltmain\.sh/q' /home/src/libtool/libtool > libtool
  trick$ echo '. /home/src/libtool/ltmain.in' >> libtool
  trick$ chmod +x libtool
  trick$ libtool --version
  ltmain.sh (GNU @PACKAGE@@TIMESTAMP@) @VERSION@
  
  Copyright (C) 2014 Free Software Foundation, Inc.
  This is free software; see the source for copying conditions.  There is NO
  warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
  trick$
  

The output of the final ‘libtool --version’ command shows that the ltmain.in script is being used directly. Now, modify ~/bin/libtool or /home/src/libtool/ltmain.in directly in order to test new changes without having to rerun configure.