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
4 .\" All rights reserved.
6 .\" %%%LICENSE_START(BSD_4_CLAUSE_UCB)
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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 2021-03-22 "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 *restrict " pathname ", char *restrict " buf ,
52 .BI " size_t " bufsiz );
54 .BR "#include <fcntl.h> " "/* Definition of " AT_* " constants */"
55 .B #include <unistd.h>
57 .BI "ssize_t readlinkat(int " dirfd ", const char *restrict " pathname ,
58 .BI " char *restrict " buf ", size_t " bufsiz );
62 Feature Test Macro Requirements for glibc (see
63 .BR feature_test_macros (7)):
68 _XOPEN_SOURCE >= 500 || _POSIX_C_SOURCE >= 200112L
69 .\" || _XOPEN_SOURCE && _XOPEN_SOURCE_EXTENDED
70 || /* Glibc <= 2.19: */ _BSD_SOURCE
76 _POSIX_C_SOURCE >= 200809L
82 places the contents of the symbolic link
89 does not append a terminating null byte to
91 It will (silently) truncate the contents (to a length of
93 characters), in case the buffer is too small to hold all of the contents.
97 system call operates in exactly the same way as
99 except for the differences described here.
101 If the pathname given in
103 is relative, then it is interpreted relative to the directory
104 referred to by the file descriptor
106 (rather than relative to the current working directory of
107 the calling process, as is done by
109 for a relative pathname).
119 is interpreted relative to the current working
120 directory of the calling process (like
130 .\" commit 65cfc6722361570bfe255698d9cd4dccaf47570d
132 can be an empty string,
133 in which case the call operates on the symbolic link referred to by
135 (which should have been obtained using
145 for an explanation of the need for
148 On success, these calls return the number of bytes placed in
150 (If the returned value equals
152 then truncation may have occurred.)
153 On error, \-1 is returned and
155 is set to indicate the error.
159 Search permission is denied for a component of the path prefix.
161 .BR path_resolution (7).)
170 nor a valid file descriptor.
174 extends outside the process's allocated address space.
179 .\" At the glibc level, bufsiz is unsigned, so this error can only occur
180 .\" if bufsiz==0. However, the in the kernel syscall, bufsiz is signed,
181 .\" and this error can also occur if bufsiz < 0.
182 .\" See: http://thread.gmane.org/gmane.linux.man/380
183 .\" Subject: [patch 0/3] [RFC] kernel/glibc mismatch of "readlink" syscall?
186 The named file (i.e., the final filename component of
188 is not a symbolic link.
191 An I/O error occurred while reading from the filesystem.
194 Too many symbolic links were encountered in translating the pathname.
197 A pathname, or a component of a pathname, was too long.
200 The named file does not exist.
203 Insufficient kernel memory was available.
206 A component of the path prefix is not a directory.
213 is a file descriptor referring to a file other than a directory.
216 was added to Linux in kernel 2.6.16;
217 library support was added to glibc in version 2.4.
222 first appeared in 4.2BSD),
223 POSIX.1-2001, POSIX.1-2008.
228 In versions of glibc up to and including glibc 2.4, the return type of
232 Nowadays, the return type is declared as
234 as (newly) required in POSIX.1-2001.
236 Using a statically sized buffer might not provide enough room for the
237 symbolic link contents.
238 The required size for the buffer can be obtained from the
240 value returned by a call to
243 However, the number of bytes written by
247 should be checked to make sure that the size of the
248 symbolic link did not increase between the calls.
249 Dynamically allocating the buffer for
253 also addresses a common portability problem when using
256 as this constant is not guaranteed to be defined per POSIX
257 if the system does not have such limit.
259 On older kernels where
261 is unavailable, the glibc wrapper function falls back to the use of
265 is a relative pathname,
266 glibc constructs a pathname based on the symbolic link in
268 that corresponds to the
272 The following program allocates the buffer needed by
274 dynamically from the information provided by
276 falling back to a buffer of size
280 reports a size of zero.
283 #include <sys/types.h>
284 #include <sys/stat.h>
291 main(int argc, char *argv[])
295 ssize_t nbytes, bufsiz;
298 fprintf(stderr, "Usage: %s <pathname>\en", argv[0]);
302 if (lstat(argv[1], &sb) == \-1) {
307 /* Add one to the link size, so that we can determine whether
308 the buffer returned by readlink() was truncated. */
310 bufsiz = sb.st_size + 1;
312 /* Some magic symlinks under (for example) /proc and /sys
313 report \(aqst_size\(aq as zero. In that case, take PATH_MAX as
314 a "good enough" estimate. */
319 buf = malloc(bufsiz);
325 nbytes = readlink(argv[1], buf, bufsiz);
331 /* Print only \(aqnbytes\(aq of \(aqbuf\(aq, as it doesn't contain a terminating
332 null byte (\(aq\e0\(aq). */
333 printf("\(aq%s\(aq points to \(aq%.*s\(aq\en", argv[1], (int) nbytes, buf);
335 /* If the return value was equal to the buffer size, then the
336 the link target was larger than expected (perhaps because the
337 target was changed between the call to lstat() and the call to
338 readlink()). Warn the user that the returned target may have
341 if (nbytes == bufsiz)
342 printf("(Returned buffer may have been truncated)\en");
354 .BR path_resolution (7),