OPENAT(2) Linux Programmer's Manual OPENAT(2)
openat - open a file relative to a directory file descriptor
int openat(int dirfd, const char *pathname, int flags);
int openat(int dirfd, const char *pathname, int flags, mode_t mode);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
Since glibc 2.10:
_XOPEN_SOURCE >= 700 || _POSIX_C_SOURCE >= 200809L
Before glibc 2.10:
The openat() system call operates in exactly the same way as open(2), except for the dif-
ferences described in this manual page.
If the pathname given in pathname is relative, then it is interpreted relative to the
directory referred to by the file descriptor dirfd (rather than relative to the current
working directory of the calling process, as is done by open(2) for a relative pathname).
If pathname is relative and dirfd is the special value AT_FDCWD, then pathname is inter-
preted relative to the current working directory of the calling process (like open(2)).
If pathname is absolute, then dirfd is ignored.
On success, openat() returns a new file descriptor. On error, -1 is returned and errno is
set to indicate the error.
The same errors that occur for open(2) can also occur for openat(). The following addi-
tional errors can occur for openat():
EBADF dirfd is not a valid file descriptor.
pathname is relative and dirfd is a file descriptor referring to a file other than
openat() was added to Linux in kernel 2.6.16; library support was added to glibc in ver-
POSIX.1-2008. A similar system call exists on Solaris.
openat() and other similar system calls suffixed "at" are supported for two reasons.
First, openat() allows an application to avoid race conditions that could occur when using
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
open(2) could be changed in parallel with the call to open(2). Such races can be avoided
by opening a file descriptor for the target directory, and then specifying that file
descriptor as the dirfd argument of openat().
Second, 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 /proc/self/fd/dirfd, but less efficiently.)
faccessat(2), fchmodat(2), fchownat(2), fstatat(2), futimesat(2), linkat(2), mkdirat(2),
mknodat(2), open(2), readlinkat(2), renameat(2), symlinkat(2), unlinkat(2), utimensat(2),
This page is part of release 3.53 of the Linux man-pages project. A description of the
project, and information about reporting bugs, can be found at
Linux 2012-05-04 OPENAT(2)