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10.4 File System Conventions

While autoconf and friends will usually be run on some Unix variety, it can and will be used on other systems, most notably DOS variants. This impacts several assumptions regarding file and path names.

For example, the following code:

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

will fail to properly detect absolute paths on those systems, because they can use a drivespec, and will usually use a backslash as directory separator. The canonical way to check for absolute paths is:

 
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 section 10.8 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, use the PATH_SEPARATOR output variable instead. configure sets this to the appropriate value (`:' or `;') when it starts up.

File names need extra care as well. While DOS-based environments that are Unixy enough to run autoconf (such as DJGPP) will usually be 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 boxes under Windows, while LFN identifies problems that exist even under Windows.

No multiple dots (SFN)
DOS cannot handle multiple dots in filenames. 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 Unices:

 
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 filenames that start with a dot. This is usually not a very important issue 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' will do 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 filename 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 filename (`FOOBAR-P.C'). The same goes for `foo.bar' and `foo.bartender'.

Note: This is not usually a problem under Windows, as it uses numeric tails in the short version of filenames 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
Some characters are invalid in DOS filenames, and should therefore be avoided. In a LFN environment, these are `/', `\', `?', `*', `:', `<', `>', `|' and `"'. In a SFN environment, other characters are also invalid. These include `+', `,', `[' and `]'.


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