11.6 File System Conventions

Autoconf uses shell-script processing extensively, so the file names that it processes should not contain characters that are special to the shell. Special characters include space, tab, newline, NUL, and the following:

" # $ & ' ( ) * ; < = > ? [ \ ` |

Also, file names should not begin with ‘~’ or ‘-’, and should contain neither ‘-’ immediately after ‘/’ nor ‘~’ immediately after ‘:’. On Posix-like platforms, directory names should not contain ‘:’, as this runs afoul of ‘:’ used as the path separator.

These restrictions apply not only to the files that you distribute, but also to the absolute file names of your source, build, and destination directories.

On some Posix-like platforms, ‘!’ and ‘^’ are special too, so they should be avoided.

Posix lets implementations treat leading // specially, but requires leading /// and beyond to be equivalent to /. Most Unix variants treat // like /. However, some treat // as a “super-root” that can provide access to files that are not otherwise reachable from /. The super-root tradition began with Apollo Domain/OS, which died out long ago, but unfortunately Cygwin has revived it.

While autoconf and friends are usually run on some Posix variety, they can be used on other systems, most notably DOS variants. This impacts several assumptions regarding file names.

For example, the following code:

case $foo_dir in
  /*) # Absolute
     ;;
  *)
     foo_dir=$dots$foo_dir ;;
esac

fails to properly detect absolute file names on those systems, because they can use a drivespec, and usually use a backslash as directory separator. If you want to be portable to DOS variants (at the price of rejecting valid but oddball Posix file names like a:\b), you can check for absolute file names like this:

case $foo_dir in
  [\\/]* | ?:[\\/]* ) # Absolute
     ;;
  *)
     foo_dir=$dots$foo_dir ;;
esac

Make sure you quote the brackets if appropriate and keep the backslash as first character. See Limitations of Shell Builtins.

Also, because the colon is used as part of a drivespec, these systems don’t use it as path separator. When creating or accessing paths, you can use the PATH_SEPARATOR output variable instead. configure sets this to the appropriate value for the build system (‘:’ or ‘;’) when it starts up.

File names need extra care as well. While DOS variants that are Posixy enough to run autoconf (such as DJGPP) are usually able to handle long file names properly, there are still limitations that can seriously break packages. Several of these issues can be easily detected by the doschk package.

A short overview follows; problems are marked with SFN/LFN to indicate where they apply: SFN means the issues are only relevant to plain DOS, not to DOS under Microsoft Windows variants, while LFN identifies problems that exist even under Microsoft Windows variants.

No multiple dots (SFN)

DOS cannot handle multiple dots in file names. This is an especially important thing to remember when building a portable configure script, as autoconf uses a .in suffix for template files.

This is perfectly OK on Posix variants:

AC_CONFIG_HEADERS([config.h])
AC_CONFIG_FILES([source.c foo.bar])
AC_OUTPUT

but it causes problems on DOS, as it requires ‘config.h.in’, ‘source.c.in’ and ‘foo.bar.in’. To make your package more portable to DOS-based environments, you should use this instead:

AC_CONFIG_HEADERS([config.h:config.hin])
AC_CONFIG_FILES([source.c:source.cin foo.bar:foobar.in])
AC_OUTPUT

No leading dot (SFN)

DOS cannot handle file names that start with a dot. This is usually not important for autoconf.

Case insensitivity (LFN)

DOS is case insensitive, so you cannot, for example, have both a file called ‘INSTALL’ and a directory called ‘install’. This also affects make; if there’s a file called ‘INSTALL’ in the directory, ‘make install’ does nothing (unless the ‘install’ target is marked as PHONY).

The 8+3 limit (SFN)

Because the DOS file system only stores the first 8 characters of the file name and the first 3 of the extension, those must be unique. That means that foobar-part1.c, foobar-part2.c and foobar-prettybird.c all resolve to the same file name (FOOBAR-P.C). The same goes for foo.bar and foo.bartender.

The 8+3 limit is not usually a problem under Microsoft Windows, as it uses numeric tails in the short version of file names to make them unique. However, a registry setting can turn this behavior off. While this makes it possible to share file trees containing long file names between SFN and LFN environments, it also means the above problem applies there as well.

Invalid characters (LFN)

Some characters are invalid in DOS file names, and should therefore be avoided. In a LFN environment, these are ‘/’, ‘\’, ‘?’, ‘*’, ‘:’, ‘<’, ‘>’, ‘|’ and ‘"’. In a SFN environment, other characters are also invalid. These include ‘+’, ‘,’, ‘[’ and ‘]’.

Invalid names (LFN)

Some DOS file names are reserved, and cause problems if you try to use files with those names. These names include CON, AUX, COM1, COM2, COM3, COM4, LPT1, LPT2, LPT3, NUL, and PRN. File names are case insensitive, so even names like aux/config.guess are disallowed.