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openat() - Unix, Linux System Call

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openat - open a file relative to a directory file descriptor


#include <fcntl.h> 

int openat(int dirfd, const char *pathname, int flags); int openat(int dirfd, const char *pathname, int flags ", mode_t " mode );


The openat() system call operates in exactly the same way as open(2), except for the differences 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 the pathname given in pathname is relative and dirfd is the special value AT_FDCWD, then pathname is interpreted relative to the current working directory of the calling process (like open(2)).

If the pathname given in 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 additional errors can occur for openat():
EBADF dirfd is not a valid file descriptor.
  pathname is a relative path and dirfd is a file descriptor referring to a file other than a directory.


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() could be changed in parallel with the call to open(). 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.)


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.


openat() was added to Linux in kernel 2.6.16.


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