readlinkat
==========

POSIX specification:
 https://pubs.opengroup.org/onlinepubs/9699919799/functions/readlinkat.html

Gnulib module: readlinkat

Portability problems fixed by Gnulib:

  * This function is missing on some platforms: glibc 2.3.6, Mac OS X 10.9, FreeBSD 6.0, NetBSD 5.0, OpenBSD 3.8, Minix 3.1.8, AIX 5.1, HP-UX 11, IRIX 6.5, Solaris 10, Cygwin 1.5.x, mingw, MSVC 14, Android 4.4. But the replacement function is not safe to be used in libraries and is not multithread-safe.

  * Some platforms mistakenly succeed on file names ending in /: OS X 10.10.

  * On some platforms, this function returns int instead of ssize_t: AIX 7.1.

Portability problems mostly fixed by Gnulib:

  * On some platforms, this function fails and sets errno to ERANGE rather than returning truncated contents: AIX 7.2. The Gnulib replacement normally works as POSIX requires by returning the truncated contents.  However, if the full link contents are unreasonably large (more than 4000 bytes) the replacement clears the entire buffer and returns the buffer size; although this is not a complete fix, it suffices for typical callers, which ignore the buffer contents anyway.

Portability problems not fixed by Gnulib:

  * This function always fails on platforms that don't support symlinks: mingw, MSVC 14.

  * When this function is called on a directory: In the case of NFS mounted directories, Cygwin sets errno to ENOENT or EIO instead of EINVAL.  To avoid this problem, check for a directory before calling this function.

  * When this function is called on a file that is not a symbolic link: IRIX may set errno to ENXIO instead of EINVAL.  Cygwin may set errno to EACCES instead of EINVAL.

  * When this function fails because it is called on an existing non-directory's name concatenated to /, it sets errno to EINVAL: AIX 7.2.

  * When this function is called on an empty file name, it fails with error EBADF instead of ENOENT: Cygwin 3.4.6.

  * Symlink contents do not always have a trailing null byte, and there is no indication if symlink contents were truncated if the return value matches the length.  Furthermore, Linux sets errno to EINVAL if the requested length is zero.  Use the gnulib module areadlink for improved ability to read symlink contents.