readlink, readlinkat - read value of a symbolic link
#include <unistd.h>
ssize_t readlink(const char *restrict pathname, char *restrict buf,
size_t bufsiz);
#include <fcntl.h> /* Definition of AT_* constants */
#include <unistd.h>
ssize_t readlinkat(int dirfd, const char *restrict pathname,
char *restrict buf, size_t bufsiz);Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
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.
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) with the O_PATH and
O_NOFOLLOW flags).
See openat(2) for an explanation of the need for readlinkat().
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 is set to indicate the error.
The following program allocates the buffer needed by readlink() dynamically from the information provided by lstat(2), falling back to a buffer of size PATH_MAX in cases where lstat(2) reports a size of zero.
#include <limits.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/stat.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);
}