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READLINK(2) Linux Programmer's Manual READLINK(2)
NAME
readlink, readlinkat - read value of a symbolic link
SYNOPSIS
#include <unistd.h>
ssize_t readlink(const char *pathname, char *buf, size_t bufsiz);
#include <fcntl.h> /* Definition of AT_* constants */
#include <unistd.h>
ssize_t readlinkat(int dirfd, const char *pathname,
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
readlinkat():
Since glibc 2.10:
_XOPEN_SOURCE >= 700 || _POSIX_C_SOURCE >= 200809L
Before glibc 2.10:
_ATFILE_SOURCE
DESCRIPTION
readlink() places the contents of the symbolic link pathname 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 con-
tents.
readlinkat()
The readlinkat() system call operates in exactly the same way as read-
link(), 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 file referred to by dirfd (which may have been
obtained using the open(2) O_PATH flag). In this case, dirfd can refer
to any type of file, not just a directory.
See openat(2) for an explanation of the need for readlinkat().
RETURN VALUE
On success, these calls return 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.
The following additional errors can occur for readlinkat():
EBADF dirfd is not a valid file descriptor.
ENOTDIR
pathname is relative and dirfd is a file descriptor referring to
a file other than a directory.
VERSIONS
readlinkat() was added to Linux in kernel 2.6.16; library support was
added to glibc in version 2.4.
CONFORMING TO
readlink(): 4.4BSD (readlink() first appeared in 4.2BSD), POSIX.1-2001,
POSIX.1-2008.
readlinkat(): POSIX.1-2008.
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() and read-
linkat() 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 porta-
bility problem when using PATH_MAX for the buffer size, as this con-
stant 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("readlink");
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 ALSO
readlink(1), lstat(2), stat(2), symlink(2), path_resolution(7), sym-
link(7)
COLOPHON
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Linux 2014-05-10 READLINK(2)
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