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LINK(3P) POSIX Programmer's Manual LINK(3P)
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.
link, linkat — link one file to another file relative to two
directory file descriptors
#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);
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 permitted 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
respective path argument. If both fd1 and fd2 have value AT_FDCWD,
the behavior 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.
Upon successful completion, these functions shall return 0.
Otherwise, these functions shall return −1 and set errno to indicate
the error.
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
permission 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
argument 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> characters.
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 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
resolution 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.
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);
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.
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 POSIX.1‐2008 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.
The purpose of the linkat() function is to link files in directories
other than the current working directory without exposure to race
conditions. 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
behaviors 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.
None.
rename(3p), symlink(3p), unlink(3p)
The Base Definitions volume of POSIX.1‐2008, fcntl.h(0p),
unistd.h(0p)
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 Electrical 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.kernel.org/doc/man-pages/reporting_bugs.html .
IEEE/The Open Group 2013 LINK(3P)
Pages that refer to this page: unistd.h(0p), link(1p), ln(1p), fstatvfs(3p), open(3p), rename(3p), symlink(3p), unlink(3p)
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