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 directory 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 permission
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
implementation 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 different 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 historical
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 implementor 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 ALSO
symlink() , 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 .
