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       openat - open a file relative to a directory file descriptor


       #define _ATFILE_SOURCE
       #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  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 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 relative and dirfd is a file descriptor referring to
              a file other than a directory.


       openat() was added to Linux in kernel 2.6.16.


       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() 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), mkfifoat(3), path_resolution(7)


       This  page  is  part of release 2.77 of the Linux man-pages project.  A
       description of the project, and information about reporting  bugs,  can
       be found at