1READLINK(2) Linux Programmer's Manual READLINK(2)
2
6 readlink - read value of a symbolic link
7
9 #include <unistd.h>
10 ssize_t readlink(const char *path, char *buf, size_t bufsiz);
12 Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
14 readlink():
16 _BSD_SOURCE || _XOPEN_SOURCE >= 500 ||
17 _XOPEN_SOURCE && _XOPEN_SOURCE_EXTENDED ||
18 _POSIX_C_SOURCE >= 200112L
19
21 readlink() places the contents of the symbolic link path in the buffer
22 buf, which has size bufsiz. readlink() does not append a null byte to
23 buf. It will truncate the contents (to a length of bufsiz characters),
24 in case the buffer is too small to hold all of the contents.
25
27 On success, readlink() returns the number of bytes placed in buf. On
28 error, -1 is returned and errno is set to indicate the error.
29
31 EACCES Search permission is denied for a component of the path prefix.
32 (See also path_resolution(7).)
33 EFAULT buf extends outside the process's allocated address space.
35 EINVAL bufsiz is not positive.
37 EINVAL The named file is not a symbolic link.
39 EIO An I/O error occurred while reading from the file system.
41 ELOOP Too many symbolic links were encountered in translating the
43 pathname.
44 ENAMETOOLONG
46 A pathname, or a component of a pathname, was too long.
47 ENOENT The named file does not exist.
49 ENOMEM Insufficient kernel memory was available.
51 ENOTDIR
53 A component of the path prefix is not a directory.
54
56 4.4BSD (readlink() first appeared in 4.2BSD), POSIX.1-2001.
57
59 In versions of glibc up to and including glibc 2.4, the return type of
60 readlink() was declared as int. Nowadays, the return type is declared
61 as ssize_t, as (newly) required in POSIX.1-2001.
62 Using a statically sized buffer might not provide enough room for the
64 symbolic link contents. The required size for the buffer can be
65 obtained from the stat.st_size value returned by a call to lstat(2) on
66 the link. However, the number of bytes written by readlink() should be
67 checked to make sure that the size of the symbolic link did not
68 increase between the calls. Dynamically allocating the buffer for
69 readlink() also addresses a common portability problem when using
70 PATH_MAX for the buffer size, as this constant is not guaranteed to be
71 defined per POSIX if the system does not have such limit.
72
74 The following program allocates the buffer needed by readlink() dynami‐
75 cally from the information provided by lstat(), making sure there's no
76 race condition between the calls.
77 #include <sys/types.h>
79 #include <sys/stat.h>
80 #include <stdio.h>
81 #include <stdlib.h>
82 #include <unistd.h>
83 int
85 main(int argc, char *argv[])
86 {
87 struct stat sb;
88 char *linkname;
89 ssize_t r;
90 if (argc != 2) {
92 fprintf(stderr, "Usage: %s <pathname>\n", argv[0]);
93 exit(EXIT_FAILURE);
94 }
95 if (lstat(argv[1], &sb) == -1) {
97 perror("lstat");
98 exit(EXIT_FAILURE);
99 }
100 linkname = malloc(sb.st_size + 1);
102 if (linkname == NULL) {
103 fprintf(stderr, "insufficient memory\n");
104 exit(EXIT_FAILURE);
105 }
106 r = readlink(argv[1], linkname, sb.st_size + 1);
108 if (r == -1) {
110 perror("lstat");
111 exit(EXIT_FAILURE);
112 }
113 if (r > sb.st_size) {
115 fprintf(stderr, "symlink increased in size "
116 "between lstat() and readlink()\n");
117 exit(EXIT_FAILURE);
118 }
119 linkname[r] = '\0';
121 printf("'%s' points to '%s'\n", argv[1], linkname);
123 exit(EXIT_SUCCESS);
125 }
126
128 readlink(1), lstat(2), readlinkat(2), stat(2), symlink(2), path_resolu‐
129 tion(7), symlink(7)
130
132 This page is part of release 3.53 of the Linux man-pages project. A
133 description of the project, and information about reporting bugs, can
134 be found at http://www.kernel.org/doc/man-pages/.
135Linux 2013-07-18 READLINK(2)