readlink(2) - Linux manual page (original) (raw)
readlink(2) System Calls Manual readlink(2)
NAME top
readlink, readlinkat - read value of a symbolic linkLIBRARY top
Standard C library (_libc_, _-lc_)SYNOPSIS top
**#include <unistd.h>**
**ssize_t readlink(const char *restrict** _pathname_**,**
**char** _buf_**[restrict .**_bufsiz_**], size_t** _bufsiz_**);**
**#include <fcntl.h>** /* Definition of **AT_*** constants */
**#include <unistd.h>**
**ssize_t readlinkat(int** _dirfd_**, const char *restrict** _pathname_**,**
**char** _buf_**[restrict .**_bufsiz_**], size_t** _bufsiz_**);**Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
**readlink**():
_XOPEN_SOURCE >= 500 || _POSIX_C_SOURCE >= 200112L
|| /* glibc <= 2.19: */ _BSD_SOURCE
**readlinkat**():
Since glibc 2.10:
_POSIX_C_SOURCE >= 200809L
Before glibc 2.10:
_ATFILE_SOURCEDESCRIPTION top
**readlink**() places the contents of the symbolic link _pathname_ in
the buffer _buf_, which has size _bufsiz_. **readlink**() does not append
a terminating null byte to _buf_. It will (silently) truncate the
contents (to a length of _bufsiz_ characters), in case the buffer is
too small to hold all of the contents.readlinkat() The readlinkat() system call operates in exactly the same way as readlink(), except for the differences described here.
If the pathname given in _pathname_ is relative, then it is
interpreted relative to the directory referred to by the file
descriptor _dirfd_ (rather than relative to the current working
directory of the calling process, as is done by **readlink**() for a
relative pathname).
If _pathname_ is relative and _dirfd_ is the special value **AT_FDCWD**,
then _pathname_ is interpreted relative to the current working
directory of the calling process (like **readlink**()).
If _pathname_ is absolute, then _dirfd_ is ignored.
Since Linux 2.6.39, _pathname_ can be an empty string, in which case
the call operates on the symbolic link referred to by _dirfd_ (which
should have been obtained using [open(2)](../man2/open.2.html) with the **O_PATH** and
**O_NOFOLLOW** flags).
See [openat(2)](../man2/openat.2.html) for an explanation of the need for **readlinkat**().RETURN VALUE top
On success, these calls return the number of bytes placed in _buf_.
(If the returned value equals _bufsiz_, then truncation may have
occurred.) On error, -1 is returned and _[errno](../man3/errno.3.html)_ is set to indicate
the error.ERRORS top
**EACCES** Search permission is denied for a component of the path
prefix. (See also [path_resolution(7)](../man7/path%5Fresolution.7.html).)
**EBADF** (**readlinkat**()) _pathname_ is relative but _dirfd_ is neither
**AT_FDCWD** nor a valid file descriptor.
**EFAULT** _buf_ extends outside the process's allocated address space.
**EINVAL** _bufsiz_ is not positive.
**EINVAL** The named file (i.e., the final filename component of
_pathname_) is not a symbolic link.
**EIO** An I/O error occurred while reading from the filesystem.
**ELOOP** Too many symbolic links were encountered in translating the
pathname.
**ENAMETOOLONG**
A pathname, or a component of a pathname, was too long.
**ENOENT** The named file does not exist.
**ENOMEM** Insufficient kernel memory was available.
**ENOTDIR**
A component of the path prefix is not a directory.
**ENOTDIR**
(**readlinkat**()) _pathname_ is relative and _dirfd_ is a file
descriptor referring to a file other than a directory.STANDARDS top
POSIX.1-2008.HISTORY top
**readlink**()
4.4BSD (first appeared in 4.2BSD), POSIX.1-2001,
POSIX.1-2008.
**readlinkat**()
POSIX.1-2008. Linux 2.6.16, glibc 2.4.
Up to and including glibc 2.4, the return type of **readlink**() was
declared as _int_. Nowadays, the return type is declared as
_ssizet_, as (newly) required in POSIX.1-2001.glibc On older kernels where readlinkat() is unavailable, the glibc wrapper function falls back to the use of readlink(). When pathname is a relative pathname, glibc constructs a pathname based on the symbolic link in /proc/self/fd that corresponds to the dirfd argument.
NOTES top
Using a statically sized buffer might not provide enough room for
the symbolic link contents. The required size for the buffer can
be obtained from the _stat.stsize_ value returned by a call to
[lstat(2)](../man2/lstat.2.html) on the link. However, the number of bytes written by
**readlink**() and **readlinkat**() should be checked to make sure that
the size of the symbolic link did not increase between the calls.
Dynamically allocating the buffer for **readlink**() and **readlinkat**()
also addresses a common portability problem when using **PATH_MAX**
for the buffer size, as this constant is not guaranteed to be
defined per POSIX if the system does not have such limit.EXAMPLES top
The following program allocates the buffer needed by **readlink**()
dynamically from the information provided by [lstat(2)](../man2/lstat.2.html), falling
back to a buffer of size **PATH_MAX** in cases where [lstat(2)](../man2/lstat.2.html) reports
a size of zero.
#include <limits.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <unistd.h>
int
main(int argc, char *argv[])
{
char *buf;
ssize_t nbytes, bufsiz;
struct stat sb;
if (argc != 2) {
fprintf(stderr, "Usage: %s <pathname>\n", argv[0]);
exit(EXIT_FAILURE);
}
if (lstat(argv[1], &sb) == -1) {
perror("lstat");
exit(EXIT_FAILURE);
}
/* Add one to the link size, so that we can determine whether
the buffer returned by readlink() was truncated. */
bufsiz = sb.st_size + 1;
/* Some magic symlinks under (for example) /proc and /sys
report 'st_size' as zero. In that case, take PATH_MAX as
a "good enough" estimate. */
if (sb.st_size == 0)
bufsiz = PATH_MAX;
buf = malloc(bufsiz);
if (buf == NULL) {
perror("malloc");
exit(EXIT_FAILURE);
}
nbytes = readlink(argv[1], buf, bufsiz);
if (nbytes == -1) {
perror("readlink");
exit(EXIT_FAILURE);
}
/* Print only 'nbytes' of 'buf', as it doesn't contain a terminating
null byte ('\0'). */
printf("'%s' points to '%.*s'\n", argv[1], (int) nbytes, buf);
/* If the return value was equal to the buffer size, then
the link target was larger than expected (perhaps because the
target was changed between the call to lstat() and the call to
readlink()). Warn the user that the returned target may have
been truncated. */
if (nbytes == bufsiz)
printf("(Returned buffer may have been truncated)\n");
free(buf);
exit(EXIT_SUCCESS);
}SEE ALSO top
[readlink(1)](../man1/readlink.1.html), [lstat(2)](../man2/lstat.2.html), [stat(2)](../man2/stat.2.html), [symlink(2)](../man2/symlink.2.html), [realpath(3)](../man3/realpath.3.html),
[path_resolution(7)](../man7/path%5Fresolution.7.html), [symlink(7)](../man7/symlink.7.html)COLOPHON top
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man-pages@man7.orgLinux man-pages 6.10 2024-11-28 readlink(2)
Pages that refer to this page:readlink(1), realpath(1), mount(2), open(2), open_by_handle_at(2), ptrace(2), read(2), stat(2), statx(2), symlink(2), syscalls(2), canonicalize_file_name(3), handle(3), realpath(3), size_t(3type), proc(5), proc_pid_cwd(5), proc_pid_exe(5), proc_pid_fd(5), proc_pid_root(5), namespaces(7), path_resolution(7), pid_namespaces(7), signal-safety(7), symlink(7), lsof(8), mount(8), umount(8)