feature_test_macros.7: Clarify that _XOPEN_SOURCE_EXTENDED is obsolete

[thirdparty/man-pages.git] / man2 / readlink.2

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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 2014-10-15 "Linux" "Linux Programmer's Manual"
.SH NAME
readlink, readlinkat \- read value of a symbolic link
.SH SYNOPSIS
.nf
.B #include <unistd.h>
.sp
.BI "ssize_t readlink(const char *" pathname ", char *" buf \
", size_t " bufsiz );
.sp
.BR "#include <fcntl.h>           " "/* Definition of AT_* constants */"
.B #include <unistd.h>
.sp
.BI "ssize_t readlinkat(int " dirfd ", const char *" pathname ,
.BI "                   char *" buf ", size_t " bufsiz );
.sp
.fi
.in -4n
Feature Test Macro Requirements for glibc (see
.BR feature_test_macros (7)):
.in
.sp
.ad l
.BR readlink ():
.RS 4
_BSD_SOURCE || _XOPEN_SOURCE\ >=\ 500 ||
_XOPEN_SOURCE\ &&\ _XOPEN_SOURCE_EXTENDED || _POSIX_C_SOURCE\ >=\ 200112L
.RE
.sp
.BR readlinkat ():
.PD 0
.ad l
.RS 4
.TP 4
Since glibc 2.10:
_XOPEN_SOURCE\ >=\ 700 || _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 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.

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).

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 ()).

If
.I pathname
is absolute, then
.I dirfd
is ignored.

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 .
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.

.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.

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 (),
making sure there's no race condition between the calls.
.nf

#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] = \(aq\\0\(aq;

    printf("\(aq%s\(aq points to \(aq%s\(aq\\n", argv[1], linkname);

    free(linkname);

    exit(EXIT_SUCCESS);
}
.fi
.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)