LINK(3P) POSIX Programmer's Manual LINK(3P)
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This manual page is part of the POSIX Programmer's Manual. The Linux
implementation of this interface may differ (consult the corresponding
Linux manual page for details of Linux behavior), or the interface may
not be implemented on Linux.
NAME
link, linkat -- link one file to another file relative to two directory
file descriptors
SYNOPSIS
#include <unistd.h>
int link(const char *path1, const char *path2);
int linkat(int fd1, const char *path1, int fd2,
const char *path2, int flag);
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.
If path1 names a symbolic link, it is implementation-defined whether
link() follows the symbolic link, or creates a new link to the symbolic
link itself.
Upon successful completion, link() shall mark for update the last file
status change timestamp of the file. Also, the last data modification
and last file status change timestamps 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.
The linkat() function shall be equivalent to the link() function except
that symbolic links shall be handled as specified by the value of flag
(see below) and except in the case where either path1 or path2 or both
are relative paths. In this case a relative path path1 is interpreted
relative to the directory associated with the file descriptor fd1
instead of the current working directory and similarly for path2 and
the file descriptor fd2. If the file descriptor was opened without
O_SEARCH, the function shall check whether directory searches are per-
mitted using the current permissions of the directory underlying the
file descriptor. If the file descriptor was opened with O_SEARCH, the
function shall not perform the check.
Values for flag are constructed by a bitwise-inclusive OR of flags from
the following list, defined in <fcntl.h>:
AT_SYMLINK_FOLLOW
If path1 names a symbolic link, a new link for the target of the
symbolic link is created.
If linkat() is passed the special value AT_FDCWD in the fd1 or fd2
parameter, the current working directory shall be used for the respec-
tive path argument. If both fd1 and fd2 have value AT_FDCWD, the behav-
ior shall be identical to a call to link(), except that symbolic links
shall be handled as specified by the value of flag.
If the AT_SYMLINK_FOLLOW flag is clear in the flag argument and the
path1 argument names a symbolic link, a new link is created for the
symbolic link path1 and not its target.
RETURN VALUE
Upon successful completion, these functions shall return 0. Otherwise,
these functions shall return -1 and set errno to indicate the error.
ERRORS
These functions 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 directory entry 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 a component of a pathname 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 names an existing file that is
neither a directory nor a symbolic link to a directory, or the
path1 argument contains at least one non-<slash> character and
ends with one or more trailing <slash> characters and the last
pathname component names an existing file that is neither a
directory nor a symbolic link to a directory, or the path1 argu-
ment names an existing non-directory file and the path2 argument
names a nonexistent file, contains at least one non-<slash>
character, and ends with one or more trailing <slash> charac-
ters.
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 linkat() function shall fail if:
EBADF The path1 or path2 argument does not specify an absolute path
and the fd1 or fd2 argument, respectively, is neither AT_FDCWD
nor a valid file descriptor open for reading or searching.
ENOTDIR
The path1 or path2 argument is not an absolute path and fd1 or
fd2, respectively, is a file descriptor associated with a non-
directory file.
These functions may fail if:
ELOOP More than {SYMLOOP_MAX} symbolic links were encountered during
resolution of the path1 or path2 argument.
ENAMETOOLONG
The length of a pathname exceeds {PATH_MAX}, or pathname resolu-
tion of a symbolic link produced an intermediate result with a
length that exceeds {PATH_MAX}.
The linkat() function may fail if:
EINVAL The value of the flag argument is not valid.
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.
If path1 refers to a symbolic link, application developers should use
linkat() with appropriate flags to select whether or not the symbolic
link should be resolved.
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
POSIX.1-2008 continues that philosophy by prohibiting link() and
unlink() from doing this. Other functions could do it if the implemen-
tor 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.
The purpose of the linkat() function is to link files in directories
other than the current working directory without exposure to race con-
ditions. Any part of the path of a file could be changed in parallel to
a call to link(), resulting in unspecified behavior. By opening a file
descriptor for the directory of both the existing file and the target
location and using the linkat() function it can be guaranteed that the
both filenames are in the desired directories.
The AT_SYMLINK_FOLLOW flag allows for implementing both common behav-
iors of the link() function. The POSIX specification requires that if
path1 is a symbolic link, a new link for the target of the symbolic
link is created. Many systems by default or as an alternative provide a
mechanism to avoid the implicit symbolic link lookup and create a new
link for the symbolic link itself.
Earlier versions of this standard specified only the link() function,
and required it to behave like linkat() with the AT_SYMLINK_FOLLOW
flag. However, historical practice from SVR4 and Linux kernels had
link() behaving like linkat() with no flags, and many systems that
attempted to provide a conforming link() function did so in a way that
was rarely used, and when it was used did not conform to the standard
(e.g., by not being atomic, or by dereferencing the symbolic link
incorrectly). Since applications could not rely on link() following
links in practice, the linkat() function was added taking a flag to
specify the desired behavior for the application.
FUTURE DIRECTIONS
None.
SEE ALSO
rename(), symlink(), unlink()
The Base Definitions volume of POSIX.1-2008, <fcntl.h>, <unistd.h>
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, Copyright (C) 2013 by the Institute of Electri-
cal and Electronics Engineers, Inc and The Open Group. (This is
POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) 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.unix.org/online.html .
Any typographical or formatting errors that appear in this page are
most likely to have been introduced during the conversion of the source
files to man page format. To report such errors, see https://www.ker-
nel.org/doc/man-pages/reporting_bugs.html .
IEEE/The Open Group 2013 LINK(3P)
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