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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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).)
165 extends outside the process's allocated address space.
170 .\" At the glibc level, bufsiz is unsigned, so this error can only occur
171 .\" if bufsiz==0. However, the in the kernel syscall, bufsiz is signed,
172 .\" and this error can also occur if bufsiz < 0.
173 .\" See: http://thread.gmane.org/gmane.linux.man/380
174 .\" Subject: [patch 0/3] [RFC] kernel/glibc mismatch of "readlink" syscall?
177 The named file (i.e., the final filename component of
179 is not a symbolic link.
182 An I/O error occurred while reading from the filesystem.
185 Too many symbolic links were encountered in translating the pathname.
188 A pathname, or a component of a pathname, was too long.
191 The named file does not exist.
194 Insufficient kernel memory was available.
197 A component of the path prefix is not a directory.
199 The following additional errors can occur for
204 is not a valid file descriptor.
210 is a file descriptor referring to a file other than a directory.
213 was added to Linux in kernel 2.6.16;
214 library support was added to glibc in version 2.4.
219 first appeared in 4.2BSD),
220 POSIX.1-2001, POSIX.1-2008.
225 In versions of glibc up to and including glibc 2.4, the return type of
229 Nowadays, the return type is declared as
231 as (newly) required in POSIX.1-2001.
233 Using a statically sized buffer might not provide enough room for the
234 symbolic link contents.
235 The required size for the buffer can be obtained from the
237 value returned by a call to
240 However, the number of bytes written by
244 should be checked to make sure that the size of the
245 symbolic link did not increase between the calls.
246 Dynamically allocating the buffer for
250 also addresses a common portability problem when using
253 as this constant is not guaranteed to be defined per POSIX
254 if the system does not have such limit.
256 On older kernels where
258 is unavailable, the glibc wrapper function falls back to the use of
262 is a relative pathname,
263 glibc constructs a pathname based on the symbolic link in
265 that corresponds to the
269 The following program allocates the buffer needed by
271 dynamically from the information provided by
273 falling back to a buffer of size
277 reports a size of zero.
280 #include <sys/types.h>
281 #include <sys/stat.h>
288 main(int argc, char *argv[])
292 ssize_t nbytes, bufsiz;
295 fprintf(stderr, "Usage: %s <pathname>\en", argv[0]);
299 if (lstat(argv[1], &sb) == \-1) {
304 /* Add one to the link size, so that we can determine whether
305 the buffer returned by readlink() was truncated. */
307 bufsiz = sb.st_size + 1;
309 /* Some magic symlinks under (for example) /proc and /sys
310 report \(aqst_size\(aq as zero. In that case, take PATH_MAX as
311 a "good enough" estimate. */
316 buf = malloc(bufsiz);
322 nbytes = readlink(argv[1], buf, bufsiz);
328 /* Print only \(aqnbytes\(aq of \(aqbuf\(aq, as it doesn't contain a terminating
329 null byte (\(aq\e0\(aq). */
330 printf("\(aq%s\(aq points to \(aq%.*s\(aq\en", argv[1], (int) nbytes, buf);
332 /* If the return value was equal to the buffer size, then the
333 the link target was larger than expected (perhaps because the
334 target was changed between the call to lstat() and the call to
335 readlink()). Warn the user that the returned target may have
338 if (nbytes == bufsiz)
339 printf("(Returned buffer may have been truncated)\en");
351 .BR path_resolution (7),