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.TH OPENAT 2 2012-05-04 "Linux" "Linux Programmer's Manual"
.SH NAME
openat \- open a file relative to a directory file descriptor
.SH SYNOPSIS
.nf
.B #include <fcntl.h>
.sp
.BI "int openat(int " dirfd ", const char *" pathname ", int " flags );
.BI "int openat(int " dirfd ", const char *" pathname ", int " flags \
", mode_t " mode );
.fi
.sp
.in -4n
Feature Test Macro Requirements for glibc (see
.BR feature_test_macros (7)):
.in
.sp
.BR openat ():
.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 openat ()
system call operates in exactly the same way as
.BR open (2),
except for the differences described in this manual page.

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

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

If
.I pathname
is absolute, then
.I dirfd
is ignored.
.SH RETURN VALUE
On success,
.BR openat ()
returns a new file descriptor.
On error, \-1 is returned and
.I errno
is set to indicate the error.
.SH ERRORS
The same errors that occur for
.BR open (2)
can also occur for
.BR openat ().
The following additional errors can occur for
.BR openat ():
.TP
.B EBADF
.I dirfd
is not a valid file descriptor.
.TP
.B ENOTDIR
.I pathname
is relative and
.I dirfd
is a file descriptor referring to a file other than a directory.
.SH VERSIONS
.BR openat ()
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.
A similar system call exists on Solaris.
.\" The 'at' suffix in Solaris is actually double sensed.  It
.\" primarily referred to "extended *at*tributes", which are
.\" handled by Solaris' O_XATTR flag, but was also intended
.\" to refer to the notion of "at a relative location".
.\"
.\" See the following for a discussion of the inconsistent
.\" naming of the *at() functions:
.\" http://www.opengroup.org/austin/mailarchives/ag/msg09103.html
.\" Subject: 	RE: The naming of at()s is a difficult matter
.\" From: 	Don Cragun
.\" Date: 	Tue, 14 Feb 2006 14:56:50 -0800 (PST)
.\"
.SH NOTES
.BR openat ()
and other similar system calls suffixed "at" are supported
for two reasons.

First,
.BR openat ()
allows an application to avoid race conditions that could
occur when using
.BR open (2)
to open files in directories other than the current working directory.
These race conditions result from the fact that some component
of the directory prefix given to
.BR open (2)
could be changed in parallel with the call to
.BR open (2).
Such races can be avoided by
opening a file descriptor for the target directory,
and then specifying that file descriptor as the
.I dirfd
argument of
.BR openat ().

Second,
.BR openat ()
allows the implementation of a per-thread "current working
directory", via file descriptor(s) maintained by the application.
(This functionality can also be obtained by tricks based
on the use of
.IR /proc/self/fd/ dirfd,
but less efficiently.)
.SH SEE ALSO
.BR faccessat (2),
.BR fchmodat (2),
.BR fchownat (2),
.BR fstatat (2),
.BR futimesat (2),
.BR linkat (2),
.BR mkdirat (2),
.BR mknodat (2),
.BR open (2),
.BR readlinkat (2),
.BR renameat (2),
.BR symlinkat (2),
.BR unlinkat (2),
.BR utimensat (2),
.BR mkfifoat (3),
.BR path_resolution (7)
.SH COLOPHON
This page is part of release 3.53 of the Linux
.I man-pages
project.
A description of the project,
and information about reporting bugs,
can be found at
\%http://www.kernel.org/doc/man\-pages/.
