1READLINK(2) Linux Programmer's Manual READLINK(2)
2
6 readlink, readlinkat - read value of a symbolic link
7
9 #include <unistd.h>
10 ssize_t readlink(const char *restrict pathname, char *restrict buf,
12 size_t bufsiz);
13 #include <fcntl.h> /* Definition of AT_* constants */
15 #include <unistd.h>
16 ssize_t readlinkat(int dirfd, const char *restrict pathname,
18 char *restrict buf, size_t bufsiz);
19 Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
21 readlink():
23 _XOPEN_SOURCE >= 500 || _POSIX_C_SOURCE >= 200112L
24 || /* Glibc <= 2.19: */ _BSD_SOURCE
25 readlinkat():
27 Since glibc 2.10:
28 _POSIX_C_SOURCE >= 200809L
29 Before glibc 2.10:
30 _ATFILE_SOURCE
31
33 readlink() places the contents of the symbolic link pathname in the
34 buffer buf, which has size bufsiz. readlink() does not append a termi‐
35 nating null byte to buf. It will (silently) truncate the contents (to
36 a length of bufsiz characters), in case the buffer is too small to hold
37 all of the contents.
38 readlinkat()
40 The readlinkat() system call operates in exactly the same way as read‐
41 link(), except for the differences described here.
42 If the pathname given in pathname is relative, then it is interpreted
44 relative to the directory referred to by the file descriptor dirfd
45 (rather than relative to the current working directory of the calling
46 process, as is done by readlink() for a relative pathname).
47 If pathname is relative and dirfd is the special value AT_FDCWD, then
49 pathname is interpreted relative to the current working directory of
50 the calling process (like readlink()).
51 If pathname is absolute, then dirfd is ignored.
53 Since Linux 2.6.39, pathname can be an empty string, in which case the
55 call operates on the symbolic link referred to by dirfd (which should
56 have been obtained using open(2) with the O_PATH and O_NOFOLLOW flags).
57 See openat(2) for an explanation of the need for readlinkat().
59
61 On success, these calls return the number of bytes placed in buf. (If
62 the returned value equals bufsiz, then truncation may have occurred.)
63 On error, -1 is returned and errno is set to indicate the error.
64
66 EACCES Search permission is denied for a component of the path prefix.
67 (See also path_resolution(7).)
68 EFAULT buf extends outside the process's allocated address space.
70 EINVAL bufsiz is not positive.
72 EINVAL The named file (i.e., the final filename component of pathname)
74 is not a symbolic link.
75 EIO An I/O error occurred while reading from the filesystem.
77 ELOOP Too many symbolic links were encountered in translating the
79 pathname.
80 ENAMETOOLONG
82 A pathname, or a component of a pathname, was too long.
83 ENOENT The named file does not exist.
85 ENOMEM Insufficient kernel memory was available.
87 ENOTDIR
89 A component of the path prefix is not a directory.
90 The following additional errors can occur for readlinkat():
92 EBADF dirfd is not a valid file descriptor.
94 ENOTDIR
96 pathname is relative and dirfd is a file descriptor referring to
97 a file other than a directory.
98
100 readlinkat() was added to Linux in kernel 2.6.16; library support was
101 added to glibc in version 2.4.
102
104 readlink(): 4.4BSD (readlink() first appeared in 4.2BSD), POSIX.1-2001,
105 POSIX.1-2008.
106 readlinkat(): POSIX.1-2008.
108
110 In versions of glibc up to and including glibc 2.4, the return type of
111 readlink() was declared as int. Nowadays, the return type is declared
112 as ssize_t, as (newly) required in POSIX.1-2001.
113 Using a statically sized buffer might not provide enough room for the
115 symbolic link contents. The required size for the buffer can be ob‐
116 tained from the stat.st_size value returned by a call to lstat(2) on
117 the link. However, the number of bytes written by readlink() and read‐
118 linkat() should be checked to make sure that the size of the symbolic
119 link did not increase between the calls. Dynamically allocating the
120 buffer for readlink() and readlinkat() also addresses a common porta‐
121 bility problem when using PATH_MAX for the buffer size, as this con‐
122 stant is not guaranteed to be defined per POSIX if the system does not
123 have such limit.
124 Glibc notes
126 On older kernels where readlinkat() is unavailable, the glibc wrapper
127 function falls back to the use of readlink(). When pathname is a rela‐
128 tive pathname, glibc constructs a pathname based on the symbolic link
129 in /proc/self/fd that corresponds to the dirfd argument.
130
132 The following program allocates the buffer needed by readlink() dynami‐
133 cally from the information provided by lstat(2), falling back to a buf‐
134 fer of size PATH_MAX in cases where lstat(2) reports a size of zero.
135 #include <sys/types.h>
137 #include <sys/stat.h>
138 #include <limits.h>
139 #include <stdio.h>
140 #include <stdlib.h>
141 #include <unistd.h>
142 int
144 main(int argc, char *argv[])
145 {
146 struct stat sb;
147 char *buf;
148 ssize_t nbytes, bufsiz;
149 if (argc != 2) {
151 fprintf(stderr, "Usage: %s <pathname>\n", argv[0]);
152 exit(EXIT_FAILURE);
153 }
154 if (lstat(argv[1], &sb) == -1) {
156 perror("lstat");
157 exit(EXIT_FAILURE);
158 }
159 /* Add one to the link size, so that we can determine whether
161 the buffer returned by readlink() was truncated. */
162 bufsiz = sb.st_size + 1;
164 /* Some magic symlinks under (for example) /proc and /sys
166 report 'st_size' as zero. In that case, take PATH_MAX as
167 a "good enough" estimate. */
168 if (sb.st_size == 0)
170 bufsiz = PATH_MAX;
171 buf = malloc(bufsiz);
173 if (buf == NULL) {
174 perror("malloc");
175 exit(EXIT_FAILURE);
176 }
177 nbytes = readlink(argv[1], buf, bufsiz);
179 if (nbytes == -1) {
180 perror("readlink");
181 exit(EXIT_FAILURE);
182 }
183 /* Print only 'nbytes' of 'buf', as it doesn't contain a terminating
185 null byte ('\0'). */
186 printf("'%s' points to '%.*s'\n", argv[1], (int) nbytes, buf);
187 /* If the return value was equal to the buffer size, then the
189 the link target was larger than expected (perhaps because the
190 target was changed between the call to lstat() and the call to
191 readlink()). Warn the user that the returned target may have
192 been truncated. */
193 if (nbytes == bufsiz)
195 printf("(Returned buffer may have been truncated)\n");
196 free(buf);
198 exit(EXIT_SUCCESS);
199 }
200
202 readlink(1), lstat(2), stat(2), symlink(2), realpath(3), path_resolu‐
203 tion(7), symlink(7)
204
206 This page is part of release 5.12 of the Linux man-pages project. A
207 description of the project, information about reporting bugs, and the
208 latest version of this page, can be found at
209 https://www.kernel.org/doc/man-pages/.
210Linux 2021-03-22 READLINK(2)