LINK(P) POSIX Programmer's Manual LINK(P)NAMElink - link to a file
SYNOPSIS
#include <unistd.h>
int link(const char *path1, const char *path2);
DESCRIPTION
The link() function shall create a new link (directory entry) for the
existing file, path1.
The path1 argument points to a pathname naming an existing file. The
path2 argument points to a pathname naming the new directory entry to
be created. The link() function shall atomically create a new link for
the existing file and the link count of the file shall be incremented
by one.
If path1 names a directory, link() shall fail unless the process has
appropriate privileges and the implementation supports using link() on
directories.
Upon successful completion, link() shall mark for update the st_ctime
field of the file. Also, the st_ctime and st_mtime fields of the direc‐
tory that contains the new entry shall be marked for update.
If link() fails, no link shall be created and the link count of the
file shall remain unchanged.
The implementation may require that the calling process has permission
to access the existing file.
RETURN VALUE
Upon successful completion, 0 shall be returned. Otherwise, -1 shall be
returned and errno set to indicate the error.
ERRORS
The link() function shall fail if:
EACCES A component of either path prefix denies search permission, or
the requested link requires writing in a directory that denies
write permission, or the calling process does not have permis‐
sion to access the existing file and this is required by the
implementation.
EEXIST The path2 argument resolves to an existing file or refers to a
symbolic link.
ELOOP A loop exists in symbolic links encountered during resolution of
the path1 or path2 argument.
EMLINK The number of links to the file named by path1 would exceed
{LINK_MAX}.
ENAMETOOLONG
The length of the path1 or path2 argument exceeds {PATH_MAX} or
a pathname component is longer than {NAME_MAX}.
ENOENT A component of either path prefix does not exist; the file named
by path1 does not exist; or path1 or path2 points to an empty
string.
ENOSPC The directory to contain the link cannot be extended.
ENOTDIR
A component of either path prefix is not a directory.
EPERM The file named by path1 is a directory and either the calling
process does not have appropriate privileges or the implementa‐
tion prohibits using link() on directories.
EROFS The requested link requires writing in a directory on a read-
only file system.
EXDEV The link named by path2 and the file named by path1 are on dif‐
ferent file systems and the implementation does not support
links between file systems.
EXDEV path1 refers to a named STREAM.
The link() function may fail if:
ELOOP More than {SYMLOOP_MAX} symbolic links were encountered during
resolution of the path1 or path2 argument.
ENAMETOOLONG
As a result of encountering a symbolic link in resolution of the
path1 or path2 argument, the length of the substituted pathname
string exceeded {PATH_MAX}.
The following sections are informative.
EXAMPLES
Creating a Link to a File
The following example shows how to create a link to a file named
/home/cnd/mod1 by creating a new directory entry named /modules/pass1.
#include <unistd.h>
char *path1 = "/home/cnd/mod1";
char *path2 = "/modules/pass1";
int status;
...
status = link (path1, path2);
Creating a Link to a File Within a Program
In the following program example, the link() function links the
/etc/passwd file (defined as PASSWDFILE) to a file named /etc/opasswd
(defined as SAVEFILE), which is used to save the current password file.
Then, after removing the current password file (defined as PASSWDFILE),
the new password file is saved as the current password file using the
link() function again.
#include <unistd.h>
#define LOCKFILE "/etc/ptmp"
#define PASSWDFILE "/etc/passwd"
#define SAVEFILE "/etc/opasswd"
...
/* Save current password file */
link (PASSWDFILE, SAVEFILE);
/* Remove current password file. */
unlink (PASSWDFILE);
/* Save new password file as current password file. */
link (LOCKFILE,PASSWDFILE);
APPLICATION USAGE
Some implementations do allow links between file systems.
RATIONALE
Linking to a directory is restricted to the superuser in most histori‐
cal implementations because this capability may produce loops in the
file hierarchy or otherwise corrupt the file system. This volume of
IEEE Std 1003.1-2001 continues that philosophy by prohibiting link()
and unlink() from doing this. Other functions could do it if the imple‐
mentor designed such an extension.
Some historical implementations allow linking of files on different
file systems. Wording was added to explicitly allow this optional
behavior.
The exception for cross-file system links is intended to apply only to
links that are programmatically indistinguishable from "hard" links.
FUTURE DIRECTIONS
None.
SEE ALSOsymlink() , unlink() , the Base Definitions volume of
IEEE Std 1003.1-2001, <unistd.h>
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of
Electrical and Electronics Engineers, Inc and The Open Group. In the
event of any discrepancy between this version and the original IEEE and
The Open Group Standard, the original IEEE and The Open Group Standard
is the referee document. The original Standard can be obtained online
at http://www.opengroup.org/unix/online.html .
IEEE/The Open Group 2003 LINK(P)
