.\" Copyright (c) 1983, 1991 The Regents of the University of California. .\" And Copyright (C) 2011 Guillem Jover .\" And Copyright (C) 2006, 2014 Michael Kerrisk .\" All rights reserved. .\" .\" %%%LICENSE_START(BSD_4_CLAUSE_UCB) .\" Redistribution and use in source and binary forms, with or without .\" modification, are permitted provided that the following conditions .\" are met: .\" 1. Redistributions of source code must retain the above copyright .\" notice, this list of conditions and the following disclaimer. .\" 2. Redistributions in binary form must reproduce the above copyright .\" notice, this list of conditions and the following disclaimer in the .\" documentation and/or other materials provided with the distribution. .\" 3. All advertising materials mentioning features or use of this software .\" must display the following acknowledgement: .\" This product includes software developed by the University of .\" California, Berkeley and its contributors. .\" 4. Neither the name of the University nor the names of its contributors .\" may be used to endorse or promote products derived from this software .\" without specific prior written permission. .\" .\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND .\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE .\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE .\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE .\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL .\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS .\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) .\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT .\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF .\" SUCH DAMAGE. .\" %%%LICENSE_END .\" .\" @(#)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 : .\" 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 .PP .BI "ssize_t readlink(const char *" pathname ", char *" buf \ ", size_t " bufsiz ); .BR "#include " "/* Definition of AT_* constants */" .B #include .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 #include #include #include #include #include int main(int argc, char *argv[]) { struct stat sb; char *buf; ssize_t nbytes, bufsiz; if (argc != 2) { fprintf(stderr, "Usage: %s \\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 4.16 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/.