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.\"

.\"     @(#)readlink.2	6.8 (Berkeley) 3/10/91

.\"

.\" Modified Sat Jul 24 00:10:21 1993 by Rik Faith (faith@cs.unc.edu)

.\" Modified Tue Jul  9 23:55:17 1996 by aeb

.\" Modified Fri Jan 24 00:26:00 1997 by aeb

.\" 2011-09-20, Guillem Jover <guillem@hadrons.org>:

.\"     Added text on dynamically allocating buffer + example program

.\"

.TH READLINK 2 2017-09-15 "Linux" "Linux Programmer's Manual"

.SH NAME

readlink, readlinkat \- read value of a symbolic link

.SH SYNOPSIS

.nf

.B #include <unistd.h>

.PP

.BI "ssize_t readlink(const char *" pathname ", char *" buf \

", size_t " bufsiz );

.BR "#include <fcntl.h>           " "/* Definition of AT_* constants */"

.B #include <unistd.h>

.PP

.BI "ssize_t readlinkat(int " dirfd ", const char *" pathname ,

.BI "                   char *" buf ", size_t " bufsiz );

.PP

.fi

.in -4n

Feature Test Macro Requirements for glibc (see

.BR feature_test_macros (7)):

.in

.PP

.ad l

.BR readlink ():

.RS 4

_XOPEN_SOURCE\ >=\ 500 || _POSIX_C_SOURCE\ >=\ 200112L

.\"    || _XOPEN_SOURCE\ &&\ _XOPEN_SOURCE_EXTENDED

    || /* Glibc versions <= 2.19: */ _BSD_SOURCE

.RE

.PP

.BR readlinkat ():

.PD 0

.ad l

.RS 4

.TP 4

Since glibc 2.10:

_POSIX_C_SOURCE\ >=\ 200809L

.TP

Before glibc 2.10:

_ATFILE_SOURCE

.RE

.ad b

.PD

.SH DESCRIPTION

.BR readlink ()

places the contents of the symbolic link

.I pathname

in the buffer

.IR buf ,

which has size

.IR bufsiz .

.BR readlink ()

does not append a null byte to

.IR buf .

It will (silently) truncate the contents (to a length of

.I bufsiz

characters), in case the buffer is too small to hold all of the contents.

.SS readlinkat()

The

.BR readlinkat ()

system call operates in exactly the same way as

.BR readlink (),

except for the differences described here.

.PP

If the pathname given in

.I pathname

is relative, then it is interpreted relative to the directory

referred to by the file descriptor

.I dirfd

(rather than relative to the current working directory of

the calling process, as is done by

.BR readlink ()

for a relative pathname).

.PP

If

.I pathname

is relative and

.I dirfd

is the special value

.BR AT_FDCWD ,

then

.I pathname

is interpreted relative to the current working

directory of the calling process (like

.BR readlink ()).

.PP

If

.I pathname

is absolute, then

.I dirfd

is ignored.

.PP

Since Linux 2.6.39,

.\" commit 65cfc6722361570bfe255698d9cd4dccaf47570d

.I pathname

can be an empty string,

in which case the call operates on the symbolic link referred to by

.IR dirfd

(which should have been obtained using

.BR open (2)

with the

.B O_PATH

and

.B O_NOFOLLOW

flags).

.PP

See

.BR openat (2)

for an explanation of the need for

.BR readlinkat ().

.SH RETURN VALUE

On success, these calls return the number of bytes placed in

.IR buf .

(If the returned value equals

.IR bufsiz ,

then truncation may have occurred.)

On error, \-1 is returned and

.I errno

is set to indicate the error.

.SH ERRORS

.TP

.B EACCES

Search permission is denied for a component of the path prefix.

(See also

.BR path_resolution (7).)

.TP

.B EFAULT

.I buf

extends outside the process's allocated address space.

.TP

.B EINVAL

.I bufsiz

is not positive.

.\" At the glibc level, bufsiz is unsigned, so this error can only occur

.\" if bufsiz==0.  However, the in the kernel syscall, bufsiz is signed,

.\" and this error can also occur if bufsiz < 0.

.\" See: http://thread.gmane.org/gmane.linux.man/380

.\" Subject: [patch 0/3] [RFC] kernel/glibc mismatch of "readlink" syscall?

.TP

.B EINVAL

The named file (i.e., the final filename component of

.IR pathname )

is not a symbolic link.

.TP

.B EIO

An I/O error occurred while reading from the filesystem.

.TP

.B ELOOP

Too many symbolic links were encountered in translating the pathname.

.TP

.B ENAMETOOLONG

A pathname, or a component of a pathname, was too long.

.TP

.B ENOENT

The named file does not exist.

.TP

.B ENOMEM

Insufficient kernel memory was available.

.TP

.B ENOTDIR

A component of the path prefix is not a directory.

.PP

The following additional errors can occur for

.BR readlinkat ():

.TP

.B EBADF

.I dirfd

is not a valid file descriptor.

.TP

.B ENOTDIR

.I pathname

is relative and

.I dirfd

is a file descriptor referring to a file other than a directory.

.SH VERSIONS

.BR readlinkat ()

was added to Linux in kernel 2.6.16;

library support was added to glibc in version 2.4.

.SH CONFORMING TO

.BR readlink ():

4.4BSD

.RB ( readlink ()

first appeared in 4.2BSD),

POSIX.1-2001, POSIX.1-2008.

.PP

.BR readlinkat ():

POSIX.1-2008.

.SH NOTES

In versions of glibc up to and including glibc 2.4, the return type of

.BR readlink ()

was declared as

.IR int .

Nowadays, the return type is declared as

.IR ssize_t ,

as (newly) required in POSIX.1-2001.

.PP

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

.I stat.st_size

value returned by a call to

.BR lstat (2)

on the link.

However, the number of bytes written by

.BR readlink ()

and

.BR 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

.BR readlink ()

and

.BR readlinkat ()

also addresses a common portability problem when using

.I 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.

.SS Glibc notes

On older kernels where

.BR readlinkat ()

is unavailable, the glibc wrapper function falls back to the use of

.BR readlink ().

When

.I pathname

is a relative pathname,

glibc constructs a pathname based on the symbolic link in

.IR /proc/self/fd

that corresponds to the

.IR dirfd

argument.

.SH EXAMPLE

The following program allocates the buffer needed by

.BR readlink ()

dynamically from the information provided by

.BR lstat (2),

falling back to a buffer of size

.BR PATH_MAX

in cases where

.BR lstat (2)

reports a size of zero.

.PP

.EX

#include <sys/types.h>

#include <sys/stat.h>

#include <limits.h>

#include <stdio.h>

#include <stdlib.h>

#include <unistd.h>

int

main(int argc, char *argv[])

{

    struct stat sb;

    char *buf;

    ssize_t nbytes, bufsiz;

    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 \(aqst_size\(aq 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);

    }

    printf("\(aq%s\(aq points to \(aq%.*s\(aq\\n", argv[1], (int) nbytes, buf);

    /* If the return value was equal to the buffer size, then the

       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);

}

.EE

.SH SEE ALSO

.BR readlink (1),

.BR lstat (2),

.BR stat (2),

.BR symlink (2),

.BR realpath (3),

.BR path_resolution (7),

.BR symlink (7)

.SH COLOPHON

This page is part of release 4.15 of the Linux

.I man-pages

project.

A description of the project,

information about reporting bugs,

and the latest version of this page,

can be found at

\%https://www.kernel.org/doc/man\-pages/.