READLINK(2) Linux Programmer's Manual READLINK(2)NAMEreadlink - read value of a symbolic link
SYNOPSIS
#include <unistd.h>
ssize_t readlink(const char *path, char *buf, size_t bufsiz);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
readlink():
_BSD_SOURCE || _XOPEN_SOURCE >= 500 ||
_XOPEN_SOURCE && _XOPEN_SOURCE_EXTENDED ||
_POSIX_C_SOURCE >= 200112L
DESCRIPTIONreadlink() places the contents of the symbolic link path in the buffer
buf, which has size bufsiz. readlink() does not append a null byte to
buf. It will truncate the contents (to a length of bufsiz characters),
in case the buffer is too small to hold all of the contents.
RETURN VALUE
On success, readlink() returns the number of bytes placed in buf. On
error, -1 is returned and errno is set to indicate the error.
ERRORS
EACCES Search permission is denied for a component of the path prefix.
(See also path_resolution(7).)
EFAULT buf extends outside the process's allocated address space.
EINVAL bufsiz is not positive.
EINVAL The named file 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.
CONFORMING TO
4.4BSD (readlink() first appeared in 4.2BSD), POSIX.1-2001.
NOTES
In versions of glibc up to and including glibc 2.4, the return type of
readlink() was declared as int. Nowadays, the return type is declared
as ssize_t, as (newly) required in POSIX.1-2001.
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.st_size value returned by a call to lstat(2) on
the link. However, the number of bytes written by readlink() 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() 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.
EXAMPLE
The following program allocates the buffer needed by readlink() dynami‐
cally from the information provided by lstat(), making sure there's no
race condition between the calls.
#include <sys/types.h>
#include <sys/stat.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
int
main(int argc, char *argv[])
{
struct stat sb;
char *linkname;
ssize_t r;
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);
}
linkname = malloc(sb.st_size + 1);
if (linkname == NULL) {
fprintf(stderr, "insufficient memory\n");
exit(EXIT_FAILURE);
}
r = readlink(argv[1], linkname, sb.st_size + 1);
if (r == -1) {
perror("lstat");
exit(EXIT_FAILURE);
}
if (r > sb.st_size) {
fprintf(stderr, "symlink increased in size "
"between lstat() and readlink()\n");
exit(EXIT_FAILURE);
}
linkname[r] = '\0';
printf("'%s' points to '%s'\n", argv[1], linkname);
exit(EXIT_SUCCESS);
}
SEE ALSOreadlink(1), lstat(2), readlinkat(2), stat(2), symlink(2), path_resolu‐
tion(7), symlink(7)COLOPHON
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Linux 2013-07-18 READLINK(2)
