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.\" Hey Emacs! This file is -*- nroff -*- source.
.\"
.\" This manpage is Copyright (C) 2006, Michael Kerrisk
.\"
.\" Permission is granted to make and distribute verbatim copies of this
.\" manual provided the copyright notice and this permission notice are
.\" preserved on all copies.
.\"
.\" Permission is granted to copy and distribute modified versions of this
.\" manual under the conditions for verbatim copying, provided that the
.\" entire resulting derived work is distributed under the terms of a
.\" permission notice identical to this one.
.\"
.\" Since the Linux kernel and libraries are constantly changing, this
.\" manual page may be incorrect or out-of-date. The author(s) assume no
.\" responsibility for errors or omissions, or for damages resulting from
.\" the use of the information contained herein. The author(s) may not
.\" have taken the same level of care in the production of this manual,
.\" which is licensed free of charge, as they might when working
.\" professionally.
.\"
.\" Formatted or processed versions of this manual, if unaccompanied by
.\" the source, must acknowledge the copyright and authors of this work.
.\"
.\"
.TH OPENAT 2 2006-03-06 "Linux 2.6.16" "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
.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
.IR 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 the pathname given in
.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 the pathname given in
.IR 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 a relative path and
.I dirfd
is a file descriptor referring to a file other than a directory.
.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 ()
could be changed in parallel with the call to
.BR open ().
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 "CONFORMING TO"
This system call is non-standard but is proposed
for inclusion in a future revision of POSIX.1.
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 VERSIONS
.BR openat ()
was added to Linux in kernel 2.6.16.
.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 path_resolution (2),
.BR readlinkat (2),
.BR renameat (2),
.BR symlinkat (2),
.BR unlinkat (2),
.BR mkfifoat (3).
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