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)):

    _XOPEN_SOURCE >= 500 || _POSIX_C_SOURCE >= 200112L
        || /* glibc <= 2.19: */ _BSD_SOURCE

    Since glibc 2.10:
        _POSIX_C_SOURCE >= 200809L
    Before glibc 2.10:
        _ATFILE_SOURCE

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) 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);
}

readlink(1), lstat(2), stat(2), symlink(2), realpath(3), path_resolution(7), symlink(7)