mirror of https://github.com/mkerrisk/man-pages
353 lines
9.6 KiB
Groff
353 lines
9.6 KiB
Groff
.\" Copyright (c) 1983, 1991 The Regents of the University of California.
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.\" And Copyright (C) 2011 Guillem Jover <guillem@hadrons.org>
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.\" And Copyright (C) 2006, 2014 Michael Kerrisk
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.\" All rights reserved.
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.\"
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.\" %%%LICENSE_START(BSD_4_CLAUSE_UCB)
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\" 3. All advertising materials mentioning features or use of this software
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.\" must display the following acknowledgement:
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.\" This product includes software developed by the University of
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.\" California, Berkeley and its contributors.
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.\" 4. Neither the name of the University nor the names of its contributors
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.\" may be used to endorse or promote products derived from this software
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.\" without specific prior written permission.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
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.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
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.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
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.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
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.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
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.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
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.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
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.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
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.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
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.\" SUCH DAMAGE.
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.\" %%%LICENSE_END
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.\"
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.\" @(#)readlink.2 6.8 (Berkeley) 3/10/91
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.\"
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.\" Modified Sat Jul 24 00:10:21 1993 by Rik Faith (faith@cs.unc.edu)
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.\" Modified Tue Jul 9 23:55:17 1996 by aeb
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.\" Modified Fri Jan 24 00:26:00 1997 by aeb
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.\" 2011-09-20, Guillem Jover <guillem@hadrons.org>:
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.\" Added text on dynamically allocating buffer + example program
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.\"
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.TH READLINK 2 2021-03-22 "Linux" "Linux Programmer's Manual"
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.SH NAME
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readlink, readlinkat \- read value of a symbolic link
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.SH SYNOPSIS
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.nf
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.B #include <unistd.h>
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.PP
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.BI "ssize_t readlink(const char *restrict " pathname ", char *restrict " buf ,
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.BI " size_t " bufsiz );
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.PP
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.BR "#include <fcntl.h> " "/* Definition of AT_* constants */"
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.B #include <unistd.h>
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.PP
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.BI "ssize_t readlinkat(int " dirfd ", const char *restrict " pathname ,
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.BI " char *restrict " buf ", size_t " bufsiz );
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.PP
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.fi
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.RS -4
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Feature Test Macro Requirements for glibc (see
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.BR feature_test_macros (7)):
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.RE
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.PP
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.BR readlink ():
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.nf
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_XOPEN_SOURCE >= 500 || _POSIX_C_SOURCE >= 200112L
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.\" || _XOPEN_SOURCE && _XOPEN_SOURCE_EXTENDED
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|| /* Glibc <= 2.19: */ _BSD_SOURCE
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.fi
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.PP
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.BR readlinkat ():
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.nf
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Since glibc 2.10:
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_POSIX_C_SOURCE >= 200809L
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Before glibc 2.10:
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_ATFILE_SOURCE
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.fi
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.SH DESCRIPTION
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.BR readlink ()
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places the contents of the symbolic link
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.I pathname
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in the buffer
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.IR buf ,
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which has size
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.IR bufsiz .
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.BR readlink ()
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does not append a terminating null byte to
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.IR buf .
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It will (silently) truncate the contents (to a length of
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.I bufsiz
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characters), in case the buffer is too small to hold all of the contents.
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.SS readlinkat()
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The
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.BR readlinkat ()
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system call operates in exactly the same way as
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.BR readlink (),
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except for the differences described here.
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.PP
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If the pathname given in
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.I pathname
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is relative, then it is interpreted relative to the directory
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referred to by the file descriptor
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.I dirfd
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(rather than relative to the current working directory of
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the calling process, as is done by
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.BR readlink ()
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for a relative pathname).
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.PP
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If
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.I pathname
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is relative and
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.I dirfd
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is the special value
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.BR AT_FDCWD ,
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then
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.I pathname
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is interpreted relative to the current working
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directory of the calling process (like
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.BR readlink ()).
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.PP
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If
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.I pathname
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is absolute, then
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.I dirfd
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is ignored.
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.PP
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Since Linux 2.6.39,
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.\" commit 65cfc6722361570bfe255698d9cd4dccaf47570d
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.I pathname
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can be an empty string,
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in which case the call operates on the symbolic link referred to by
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.IR dirfd
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(which should have been obtained using
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.BR open (2)
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with the
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.B O_PATH
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and
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.B O_NOFOLLOW
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flags).
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.PP
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See
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.BR openat (2)
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for an explanation of the need for
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.BR readlinkat ().
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.SH RETURN VALUE
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On success, these calls return the number of bytes placed in
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.IR buf .
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(If the returned value equals
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.IR bufsiz ,
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then truncation may have occurred.)
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On error, \-1 is returned and
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.I errno
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is set to indicate the error.
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.SH ERRORS
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.TP
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.B EACCES
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Search permission is denied for a component of the path prefix.
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(See also
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.BR path_resolution (7).)
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.TP
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.B EFAULT
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.I buf
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extends outside the process's allocated address space.
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.TP
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.B EINVAL
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.I bufsiz
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is not positive.
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.\" At the glibc level, bufsiz is unsigned, so this error can only occur
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.\" if bufsiz==0. However, the in the kernel syscall, bufsiz is signed,
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.\" and this error can also occur if bufsiz < 0.
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.\" See: http://thread.gmane.org/gmane.linux.man/380
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.\" Subject: [patch 0/3] [RFC] kernel/glibc mismatch of "readlink" syscall?
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.TP
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.B EINVAL
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The named file (i.e., the final filename component of
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.IR pathname )
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is not a symbolic link.
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.TP
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.B EIO
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An I/O error occurred while reading from the filesystem.
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.TP
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.B ELOOP
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Too many symbolic links were encountered in translating the pathname.
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.TP
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.B ENAMETOOLONG
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A pathname, or a component of a pathname, was too long.
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.TP
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.B ENOENT
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The named file does not exist.
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.TP
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.B ENOMEM
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Insufficient kernel memory was available.
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.TP
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.B ENOTDIR
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A component of the path prefix is not a directory.
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.PP
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The following additional errors can occur for
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.BR readlinkat ():
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.TP
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.B EBADF
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.I dirfd
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is not a valid file descriptor.
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.TP
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.B ENOTDIR
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.I pathname
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is relative and
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.I dirfd
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is a file descriptor referring to a file other than a directory.
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.SH VERSIONS
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.BR readlinkat ()
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was added to Linux in kernel 2.6.16;
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library support was added to glibc in version 2.4.
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.SH CONFORMING TO
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.BR readlink ():
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4.4BSD
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.RB ( readlink ()
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first appeared in 4.2BSD),
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POSIX.1-2001, POSIX.1-2008.
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.PP
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.BR readlinkat ():
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POSIX.1-2008.
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.SH NOTES
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In versions of glibc up to and including glibc 2.4, the return type of
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.BR readlink ()
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was declared as
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.IR int .
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Nowadays, the return type is declared as
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.IR ssize_t ,
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as (newly) required in POSIX.1-2001.
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.PP
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Using a statically sized buffer might not provide enough room for the
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symbolic link contents.
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The required size for the buffer can be obtained from the
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.I stat.st_size
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value returned by a call to
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.BR lstat (2)
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on the link.
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However, the number of bytes written by
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.BR readlink ()
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and
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.BR readlinkat ()
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should be checked to make sure that the size of the
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symbolic link did not increase between the calls.
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Dynamically allocating the buffer for
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.BR readlink ()
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and
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.BR readlinkat ()
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also addresses a common portability problem when using
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.B PATH_MAX
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for the buffer size,
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as this constant is not guaranteed to be defined per POSIX
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if the system does not have such limit.
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.SS Glibc notes
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On older kernels where
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.BR readlinkat ()
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is unavailable, the glibc wrapper function falls back to the use of
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.BR readlink ().
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When
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.I pathname
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is a relative pathname,
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glibc constructs a pathname based on the symbolic link in
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.IR /proc/self/fd
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that corresponds to the
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.IR dirfd
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argument.
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.SH EXAMPLES
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The following program allocates the buffer needed by
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.BR readlink ()
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dynamically from the information provided by
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.BR lstat (2),
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falling back to a buffer of size
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.BR PATH_MAX
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in cases where
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.BR lstat (2)
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reports a size of zero.
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.PP
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.EX
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#include <sys/types.h>
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#include <sys/stat.h>
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#include <limits.h>
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#include <stdio.h>
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#include <stdlib.h>
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#include <unistd.h>
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int
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main(int argc, char *argv[])
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{
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struct stat sb;
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char *buf;
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ssize_t nbytes, bufsiz;
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if (argc != 2) {
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fprintf(stderr, "Usage: %s <pathname>\en", argv[0]);
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exit(EXIT_FAILURE);
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}
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if (lstat(argv[1], &sb) == \-1) {
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perror("lstat");
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exit(EXIT_FAILURE);
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}
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/* Add one to the link size, so that we can determine whether
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the buffer returned by readlink() was truncated. */
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bufsiz = sb.st_size + 1;
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/* Some magic symlinks under (for example) /proc and /sys
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report \(aqst_size\(aq as zero. In that case, take PATH_MAX as
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a "good enough" estimate. */
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if (sb.st_size == 0)
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bufsiz = PATH_MAX;
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buf = malloc(bufsiz);
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if (buf == NULL) {
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perror("malloc");
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exit(EXIT_FAILURE);
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}
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nbytes = readlink(argv[1], buf, bufsiz);
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if (nbytes == \-1) {
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perror("readlink");
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exit(EXIT_FAILURE);
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}
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/* Print only \(aqnbytes\(aq of \(aqbuf\(aq, as it doesn't contain a terminating
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null byte (\(aq\e0\(aq). */
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printf("\(aq%s\(aq points to \(aq%.*s\(aq\en", argv[1], (int) nbytes, buf);
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/* If the return value was equal to the buffer size, then the
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the link target was larger than expected (perhaps because the
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target was changed between the call to lstat() and the call to
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readlink()). Warn the user that the returned target may have
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been truncated. */
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if (nbytes == bufsiz)
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printf("(Returned buffer may have been truncated)\en");
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free(buf);
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exit(EXIT_SUCCESS);
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}
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.EE
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.SH SEE ALSO
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.BR readlink (1),
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.BR lstat (2),
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.BR stat (2),
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.BR symlink (2),
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.BR realpath (3),
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.BR path_resolution (7),
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.BR symlink (7)
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