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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);

   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
       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

       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; library support was added to glibc in
       version 2.4.


       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),
       mkfifoat(3), path_resolution(7)


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