1LINK(3P) POSIX Programmer's Manual LINK(3P)
2
6 This manual page is part of the POSIX Programmer's Manual. The Linux
7 implementation of this interface may differ (consult the corresponding
8 Linux manual page for details of Linux behavior), or the interface may
9 not be implemented on Linux.
10
12 link - link to a file
13
15 #include <unistd.h>
16 int link(const char *path1, const char *path2);
18
21 The link() function shall create a new link (directory entry) for the
22 existing file, path1.
23 The path1 argument points to a pathname naming an existing file. The
25 path2 argument points to a pathname naming the new directory entry to
26 be created. The link() function shall atomically create a new link for
27 the existing file and the link count of the file shall be incremented
28 by one.
29 If path1 names a directory, link() shall fail unless the process has
31 appropriate privileges and the implementation supports using link() on
32 directories.
33 Upon successful completion, link() shall mark for update the st_ctime
35 field of the file. Also, the st_ctime and st_mtime fields of the direc‐
36 tory that contains the new entry shall be marked for update.
37 If link() fails, no link shall be created and the link count of the
39 file shall remain unchanged.
40 The implementation may require that the calling process has permission
42 to access the existing file.
43
45 Upon successful completion, 0 shall be returned. Otherwise, -1 shall be
46 returned and errno set to indicate the error.
47
49 The link() function shall fail if:
50 EACCES A component of either path prefix denies search permission, or
52 the requested link requires writing in a directory that denies
53 write permission, or the calling process does not have permis‐
54 sion to access the existing file and this is required by the
55 implementation.
56 EEXIST The path2 argument resolves to an existing file or refers to a
58 symbolic link.
59 ELOOP A loop exists in symbolic links encountered during resolution of
61 the path1 or path2 argument.
62 EMLINK The number of links to the file named by path1 would exceed
64 {LINK_MAX}.
65 ENAMETOOLONG
67 The length of the path1 or path2 argument exceeds {PATH_MAX} or
68 a pathname component is longer than {NAME_MAX}.
69 ENOENT A component of either path prefix does not exist; the file named
71 by path1 does not exist; or path1 or path2 points to an empty
72 string.
73 ENOSPC The directory to contain the link cannot be extended.
75 ENOTDIR
77 A component of either path prefix is not a directory.
78 EPERM The file named by path1 is a directory and either the calling
80 process does not have appropriate privileges or the implementa‐
81 tion prohibits using link() on directories.
82 EROFS The requested link requires writing in a directory on a read-
84 only file system.
85 EXDEV The link named by path2 and the file named by path1 are on dif‐
87 ferent file systems and the implementation does not support
88 links between file systems.
89 EXDEV path1 refers to a named STREAM.
91 The link() function may fail if:
94 ELOOP More than {SYMLOOP_MAX} symbolic links were encountered during
96 resolution of the path1 or path2 argument.
97 ENAMETOOLONG
99 As a result of encountering a symbolic link in resolution of the
100 path1 or path2 argument, the length of the substituted pathname
101 string exceeded {PATH_MAX}.
102 The following sections are informative.
105
107 Creating a Link to a File
108 The following example shows how to create a link to a file named
109 /home/cnd/mod1 by creating a new directory entry named /modules/pass1.
110 #include <unistd.h>
113 char *path1 = "/home/cnd/mod1";
116 char *path2 = "/modules/pass1";
117 int status;
118 ...
119 status = link (path1, path2);
120 Creating a Link to a File Within a Program
122 In the following program example, the link() function links the
123 /etc/passwd file (defined as PASSWDFILE) to a file named /etc/opasswd
124 (defined as SAVEFILE), which is used to save the current password file.
125 Then, after removing the current password file (defined as PASSWDFILE),
126 the new password file is saved as the current password file using the
127 link() function again.
128 #include <unistd.h>
131 #define LOCKFILE "/etc/ptmp"
134 #define PASSWDFILE "/etc/passwd"
135 #define SAVEFILE "/etc/opasswd"
136 ...
137 /* Save current password file */
138 link (PASSWDFILE, SAVEFILE);
139 /* Remove current password file. */
142 unlink (PASSWDFILE);
143 /* Save new password file as current password file. */
146 link (LOCKFILE,PASSWDFILE);
147
149 Some implementations do allow links between file systems.
150
152 Linking to a directory is restricted to the superuser in most histori‐
153 cal implementations because this capability may produce loops in the
154 file hierarchy or otherwise corrupt the file system. This volume of
155 IEEE Std 1003.1-2001 continues that philosophy by prohibiting link()
156 and unlink() from doing this. Other functions could do it if the imple‐
157 mentor designed such an extension.
158 Some historical implementations allow linking of files on different
160 file systems. Wording was added to explicitly allow this optional
161 behavior.
162 The exception for cross-file system links is intended to apply only to
164 links that are programmatically indistinguishable from "hard" links.
165
167 None.
168
170 symlink(), unlink(), the Base Definitions volume of
171 IEEE Std 1003.1-2001, <unistd.h>
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174 Portions of this text are reprinted and reproduced in electronic form
175 from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
176 -- Portable Operating System Interface (POSIX), The Open Group Base
177 Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of
178 Electrical and Electronics Engineers, Inc and The Open Group. In the
179 event of any discrepancy between this version and the original IEEE and
180 The Open Group Standard, the original IEEE and The Open Group Standard
181 is the referee document. The original Standard can be obtained online
182 at http://www.opengroup.org/unix/online.html .
183IEEE/The Open Group 2003 LINK(3P)