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.TH LINKAT 2 2013-07-21 "Linux" "Linux Programmer's Manual"
.SH NAME
linkat \- create a file link relative to directory file descriptors
.SH SYNOPSIS
.nf
.B #include <fcntl.h>           /* Definition of AT_* constants */
.B #include <unistd.h>
.sp
.BI "int linkat(int " olddirfd ", const char *" oldpath ,
.BI "           int " newdirfd ", const char *" newpath ", int " flags );
.fi
.sp
.in -4n
Feature Test Macro Requirements for glibc (see
.BR feature_test_macros (7)):
.in
.sp
.BR linkat ():
.PD 0
.ad l
.RS 4
.TP 4
Since glibc 2.10:
_XOPEN_SOURCE\ >=\ 700 || _POSIX_C_SOURCE\ >=\ 200809L
.TP
Before glibc 2.10:
_ATFILE_SOURCE
.RE
.ad
.PD
.SH DESCRIPTION
The
.BR linkat ()
system call operates in exactly the same way as
.BR link (2),
except for the differences described in this manual page.

If the pathname given in
.I oldpath
is relative, then it is interpreted relative to the directory
referred to by the file descriptor
.I olddirfd
(rather than relative to the current working directory of
the calling process, as is done by
.BR link (2)
for a relative pathname).

If
.I oldpath
is relative and
.I olddirfd
is the special value
.BR AT_FDCWD ,
then
.I oldpath
is interpreted relative to the current working
directory of the calling process (like
.BR link (2)).

If
.I oldpath
is absolute, then
.I olddirfd
is ignored.

The interpretation of
.I newpath
is as for
.IR oldpath ,
except that a relative pathname is interpreted relative
to the directory referred to by the file descriptor
.IR newdirfd .

The following values can be bitwise ORed in
.IR flags :
.TP
.BR AT_EMPTY_PATH " (since Linux 2.6.39)"
.\" commit 11a7b371b64ef39fc5fb1b6f2218eef7c4d035e3
If
.I oldpath
is an empty string, create a link to the file referenced by
.IR olddirfd
(which may have been obtained using the
.BR open (2)
.B O_PATH
flag).
In this case,
.I olddirfd
can refer to any type of file, not just a directory.
The caller must have the
.BR CAP_DAC_READ_SEARCH
capability in order to use this flag;
this prevents arbitrary users from creating hard links
using file descriptors received via a UNIX domain socket
(see the discussion of
.BR SCM_RIGHTS
in
.BR unix (7)).
.TP
.BR AT_SYMLINK_FOLLOW " (since Linux 2.6.18)"
By default,
.BR linkat (),
does not dereference
.I oldpath
if it is a symbolic link (like
.BR link (2)).
The flag
.B AT_SYMLINK_FOLLOW
can be specified in
.I flags
to cause
.I oldpath
to be dereferenced if it is a symbolic link.
.PP
Before kernel 2.6.18, the
.I flags
argument was unused, and had to be specified as 0.
.SH RETURN VALUE
On success,
.BR linkat ()
returns 0.
On error, \-1 is returned and
.I errno
is set to indicate the error.
.SH ERRORS
The same errors that occur for
.BR link (2)
can also occur for
.BR linkat ().
The following additional errors can occur for
.BR linkat ():
.TP
.B EBADF
.I olddirfd
or
.I newdirfd
is not a valid file descriptor.
.TP
.B ENOENT
.B AT_EMPTY_PATH
was specified in
.IR flags ,
but the caller did not have the
.B CAP_DAC_READ_SEARCH
capability.
.TP
.B ENOTDIR
.TP
.B ENOTDIR
.I oldpath
is relative and
.I olddirfd
is a file descriptor referring to a file other than a directory;
or similar for
.I newpath
and
.I newdirfd
.SH VERSIONS
.BR linkat ()
was added to Linux in kernel 2.6.16;
library support was added to glibc in version 2.4.
.SH CONFORMING TO
POSIX.1-2008.
.SH NOTES
See
.BR openat (2)
for an explanation of the need for
.BR linkat ().
.SH SEE ALSO
.BR link (2),
.BR openat (2),
.BR path_resolution (7),
.BR symlink (7)