Simple Library Tests

The macros AC_CHECK_LIB, AC_SEARCH_LIBS from GNU Autoconf check for the presence of certain C, C++, or Fortran library archive files. The libraries are looked up in the default linker path—a system dependent list of directories, that usually contains the /usr/lib directory—and those directories given by -L options in the LDFLAGS variable.

Locating Libraries

The following macros, defined in the Gnulib module havelib, search for the location of certain C, C++, or Fortran library archive files and make the found location available to the compilation process and to further Autoconf tests.

Macro: AC_LIB_LINKFLAGS(name, [dependencies]) ¶

Searches for lib<name> and the libraries corresponding to explicit and implicit dependencies. Sets and AC_SUBSTs the LIB<NAME> and LTLIB<NAME> variables (with <NAME> in upper case) and augments the CPPFLAGS variable by -I options.

This macro should be used when lib<name> is expected to be found.

Macro: AC_LIB_HAVE_LINKFLAGS(name, [dependencies], [includes], [testcode], [missing-message]) ¶

Searches for lib<name> and the libraries corresponding to explicit and implicit dependencies, together with the specified include files and the ability to compile and link the specified testcode. The missing-message defaults to no and may contain additional hints for the user. If found, it sets and AC_SUBSTs HAVE_LIB<NAME>=yes and the LIB<NAME> and LTLIB<NAME> variables (with <NAME> in upper case) and augments the CPPFLAGS variable by -I options, and #defines HAVE_LIB<NAME> to 1. Otherwise, it sets and AC_SUBSTs HAVE_LIB<NAME>=no and LIB<NAME> and LTLIB<NAME> to empty.

These macros assume that when a library is installed in some_directory/lib, its include files are installed in some_directory/include.

The complexities that AC_LIB_LINKFLAGS and AC_LIB_HAVE_LINKFLAGS deal with are the following:

• The library is not necessarily already in the search path (CPPFLAGS for the include file search path, LDFLAGS for the library search path). The macro provides a ‘--with-lib<name>’ option. The user of the ‘configure’ script can use this option to indicate the location of the library and its include files. If not provided, the --prefix directory is searched as well.
• The library is not necessarily already in the run time library search path. To avoid the need for setting an environment variable like LD_LIBRARY_PATH, the macro adds the appropriate run time search path options to the LIB<NAME> variable. This works on most systems. It can also be inhibited: The user of ‘configure’ can use the --disable-rpath option to force an installation that doesn’t contain hardcoded library search paths but instead may require the use of an environment variable like LD_LIBRARY_PATH.

The macros also set a variable LTLIB<NAME>, that should be used when linking with libtool. Both LTLIB<NAME> and LIB<NAME> contain essentially the same option, but where LIB<NAME> contains platform dependent flags like ‘-Wl,-rpath’, LTLIB<NAME> contains platform independent flags like ‘-R’.

If you, by mistake, use LIB<NAME> instead of LTLIB<NAME> when linking with libtool, you will observe that the binaries created in the build dir will prefer the shared libraries in the installation directories over the shared libraries in the build dir; this can lead to all sorts of build failures, test failures, or crashes!

If you, on the other hand, by mistake, use LTLIB<NAME> instead of LIB<NAME> when linking without libtool, you will observe build failures, because the ‘-R’ options contained in LTLIB<NAME> are not valid options to compilers such as GCC.

• Example of using AC_LIB_LINKFLAGS
• Dependencies
• Static vs. shared
• CPPFLAGS vs. LDFLAGS
• Bi-arch systems

AC_LIB_LINKFLAGS([foox])
AC_LIB_LINKFLAGS([fooy], [foox])
AC_LIB_LINKFLAGS([fooz], [foox fooy])

AC_LIB_LINKFLAGS([fooy])

  #define LIBNAME_VERSION 10203
  extern int libname_version; /* initialized to LIBNAME_VERSION */

  AC_TRY_RUN([int main () { return libname_version != LIBNAME_VERSION; }])