1 .\" Copyright (c) 1983, 1991 The Regents of the University of California.
2 .\" And Copyright (C) 2011 Guillem Jover <guillem@hadrons.org>
3 .\" And Copyright (C) 2006, 2014 Michael Kerrisk
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36 .\" @(#)readlink.2 6.8 (Berkeley) 3/10/91
38 .\" Modified Sat Jul 24 00:10:21 1993 by Rik Faith (faith@cs.unc.edu)
39 .\" Modified Tue Jul 9 23:55:17 1996 by aeb
40 .\" Modified Fri Jan 24 00:26:00 1997 by aeb
41 .\" 2011-09-20, Guillem Jover <guillem@hadrons.org>:
42 .\" Added text on dynamically allocating buffer + example program
44 .TH READLINK 2 2016-10-08 "Linux" "Linux Programmer's Manual"
46 readlink, readlinkat \- read value of a symbolic link
49 .B #include <unistd.h>
51 .BI "ssize_t readlink(const char *" pathname ", char *" buf \
54 .BR "#include <fcntl.h> " "/* Definition of AT_* constants */"
55 .B #include <unistd.h>
57 .BI "ssize_t readlinkat(int " dirfd ", const char *" pathname ,
58 .BI " char *" buf ", size_t " bufsiz );
62 Feature Test Macro Requirements for glibc (see
63 .BR feature_test_macros (7)):
69 _XOPEN_SOURCE\ >=\ 500 || _POSIX_C_SOURCE\ >=\ 200112L
70 .\" || _XOPEN_SOURCE\ &&\ _XOPEN_SOURCE_EXTENDED
71 || /* Glibc versions <= 2.19: */ _BSD_SOURCE
80 _POSIX_C_SOURCE\ >=\ 200809L
89 places the contents of the symbolic link
96 does not append a null byte to
98 It will (silently) truncate the contents (to a length of
100 characters), in case the buffer is too small to hold all of the contents.
104 system call operates in exactly the same way as
106 except for the differences described here.
108 If the pathname given in
110 is relative, then it is interpreted relative to the directory
111 referred to by the file descriptor
113 (rather than relative to the current working directory of
114 the calling process, as is done by
116 for a relative pathname).
126 is interpreted relative to the current working
127 directory of the calling process (like
137 .\" commit 65cfc6722361570bfe255698d9cd4dccaf47570d
139 can be an empty string,
140 in which case the call operates on the symbolic link referred to by
142 (which should have been obtained using
152 for an explanation of the need for
155 On success, these calls return the number of bytes placed in
157 (If the returned value equals
159 then truncation may have occurred.)
160 On error, \-1 is returned and
162 is set to indicate the error.
166 Search permission is denied for a component of the path prefix.
168 .BR path_resolution (7).)
172 extends outside the process's allocated address space.
177 .\" At the glibc level, bufsiz is unsigned, so this error can only occur
178 .\" if bufsiz==0. However, the in the kernel syscall, bufsiz is signed,
179 .\" and this error can also occur if bufsiz < 0.
180 .\" See: http://thread.gmane.org/gmane.linux.man/380
181 .\" Subject: [patch 0/3] [RFC] kernel/glibc mismatch of "readlink" syscall?
184 The named file (i.e., the final filename component of
186 is not a symbolic link.
189 An I/O error occurred while reading from the filesystem.
192 Too many symbolic links were encountered in translating the pathname.
195 A pathname, or a component of a pathname, was too long.
198 The named file does not exist.
201 Insufficient kernel memory was available.
204 A component of the path prefix is not a directory.
206 The following additional errors can occur for
211 is not a valid file descriptor.
217 is a file descriptor referring to a file other than a directory.
220 was added to Linux in kernel 2.6.16;
221 library support was added to glibc in version 2.4.
226 first appeared in 4.2BSD),
227 POSIX.1-2001, POSIX.1-2008.
232 In versions of glibc up to and including glibc 2.4, the return type of
236 Nowadays, the return type is declared as
238 as (newly) required in POSIX.1-2001.
240 Using a statically sized buffer might not provide enough room for the
241 symbolic link contents.
242 The required size for the buffer can be obtained from the
244 value returned by a call to
247 However, the number of bytes written by
251 should be checked to make sure that the size of the
252 symbolic link did not increase between the calls.
253 Dynamically allocating the buffer for
257 also addresses a common portability problem when using
260 as this constant is not guaranteed to be defined per POSIX
261 if the system does not have such limit.
263 On older kernels where
265 is unavailable, the glibc wrapper function falls back to the use of
269 is a relative pathname,
270 glibc constructs a pathname based on the symbolic link in
272 that corresponds to the
276 The following program allocates the buffer needed by
278 dynamically from the information provided by
280 falling back to a buffer of size
284 reports a size of zero.
287 #include <sys/types.h>
288 #include <sys/stat.h>
295 main(int argc, char *argv[])
302 fprintf(stderr, "Usage: %s <pathname>\\n", argv[0]);
306 if (lstat(argv[1], &sb) == \-1) {
311 bufsiz = sb.st_size + 1;
313 /* Some magic symlinks under (for example) /proc and /sys
314 report \(aqst_size\(aq as zero. In that case, take PATH_MAX as
315 a "good enough" estimate */
320 printf("%zd\\n", bufsiz);
322 linkname = malloc(bufsiz);
323 if (linkname == NULL) {
328 r = readlink(argv[1], linkname, bufsiz);
334 linkname[r] = \(aq\\0\(aq;
336 printf("\(aq%s\(aq points to \(aq%s\(aq\\n", argv[1], linkname);
339 printf("(Returned buffer may have been truncated)\\n");
351 .BR path_resolution (7),