openat(2)

1OPENAT(2)                  Linux Programmer's Manual                 OPENAT(2)
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NAME

6       openat - open a file relative to a directory file descriptor
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SYNOPSIS

9       #define _ATFILE_SOURCE
10       #include <fcntl.h>
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12       int openat(int dirfd, const char *pathname, int flags);
13       int openat(int dirfd, const char *pathname, int flags, mode_t mode);
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DESCRIPTION

16       The  openat()  system call operates in exactly the same way as open(2),
17       except for the differences described in this manual page.
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19       If the pathname given in pathname is relative, then it  is  interpreted
20       relative  to  the  directory  referred  to by the file descriptor dirfd
21       (rather than relative to the current working directory of  the  calling
22       process, as is done by open(2) for a relative pathname).
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24       If  pathname  is relative and dirfd is the special value AT_FDCWD, then
25       pathname is interpreted relative to the current  working  directory  of
26       the calling process (like open(2)).
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28       If pathname is absolute, then dirfd is ignored.
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RETURN VALUE

31       On  success,  openat()  returns a new file descriptor.  On error, -1 is
32       returned and errno is set to indicate the error.
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ERRORS

35       The same errors that occur for open(2) can  also  occur  for  openat().
36       The following additional errors can occur for openat():
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38       EBADF  dirfd is not a valid file descriptor.
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40       ENOTDIR
41              pathname is relative and dirfd is a file descriptor referring to
42              a file other than a directory.
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NOTES

45       openat() and other similar system calls suffixed "at" are supported for
46       two reasons.
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48       First,  openat()  allows  an  application to avoid race conditions that
49       could occur when using open(2) to open files in directories other  than
50       the  current  working directory.  These race conditions result from the
51       fact that some component of the directory prefix given to open()  could
52       be  changed  in  parallel  with  the call to open().  Such races can be
53       avoided by opening a file descriptor for the target directory, and then
54       specifying that file descriptor as the dirfd argument of openat().
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56       Second,  openat()  allows  the  implementation of a per-thread "current
57       working directory", via file descriptor(s) maintained by  the  applica‐
58       tion.   (This functionality can also be obtained by tricks based on the
59       use of /proc/self/fd/dirfd, but less efficiently.)
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CONFORMING TO

62       This system call is non-standard but is proposed  for  inclusion  in  a
63       future revision of POSIX.1.  A similar system call exists on Solaris.
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VERSIONS

66       openat() was added to Linux in kernel 2.6.16.
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