10.69.70 readlinkat
¶
POSIX specification:
https://pubs.opengroup.org/onlinepubs/9799919799/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, 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 /:
macOS 14.
- On some platforms, this function returns
int
instead of
ssize_t
:
AIX 7.1.
- For the file name /dev/null, this function returns \Device\Null,
which is unusable, on some platforms:
Cygwin 3.3.6.
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:
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.