This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface

       The dirfd() function is intended to be a mechanism by which an application may obtain a  file  descriptor
       to use for the fchdir() function.

       Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1-2017, Standard
       for  Information  Technology  --  Portable  Operating  System  Interface  (POSIX),  The  Open  Group Base
       Specifications Issue 7, 2018 Edition, Copyright (C) 2018 by the Institute of Electrical  and  Electronics
       Engineers, Inc and The Open Group.  In the event of any discrepancy between this version and the original
       IEEE  and The Open Group Standard, the original IEEE and The Open Group Standard is the referee document.
       The original Standard can be obtained online at http://www.opengroup.org/unix/online.html .

       Any typographical or formatting errors that appear in this page are most likely to have  been  introduced
       during   the   conversion  of  the  source  files  to  man  page  format.  To  report  such  errors,  see
       https://www.kernel.org/doc/man-pages/reporting_bugs.html .

IEEE/The Open Group                                   2017                                         DIRFD(3POSIX)

       The dirfd() function shall return a file descriptor referring to the same directory as the dirp argument.
       This  file  descriptor shall be closed by a call to closedir().  If any attempt is made to close the file
       descriptor, or to modify the state of the associated description, other  than  by  means  of  closedir(),
       readdir(), readdir_r(), rewinddir(), or seekdir(), the behavior is undefined.

       The dirfd() function may fail if:

       EINVAL The dirp argument does not refer to a valid directory stream.

       Thefollowingsectionsareinformative.

       None.

       None.

       dirfd — extract the file descriptor used by a DIR stream

       This  manual  page  is part of the POSIX Programmer's Manual.  The Linux implementation of this interface
       may differ (consult the corresponding Linux manual page for details of Linux behavior), or the  interface
       may not be implemented on Linux.

       This  interface  was  introduced because the Base Definitions volume of POSIX.1‐2017 does not make public
       the DIR data structure. Applications tend to use the fchdir() function on the file descriptor returned by
       this interface, and this has proven useful for security reasons; in particular, it is a better  technique
       than others where directory names might change.

       The  description uses the term ``a file descriptor'' rather than ``the file descriptor''. The implication
       intended is that an implementation that does not use an fd for opendir() could still open() the directory
       to implement the dirfd() function. Such a descriptor must be closed later during a call to closedir().

       If it is necessary to allocate an fd to be returned by dirfd(), it should be done at the time of  a  call
       to opendir().

       Upon successful completion, the dirfd() function shall return an integer which contains a file descriptor
       for  the  stream  pointed  to by dirp.  Otherwise, it shall return -1 and shall set errno to indicate the
       error.

closedir(), fchdir(), fdopendir(), fileno(), open(), readdir()

       The Base Definitions volume of POSIX.1‐2017, <dirent.h>

       #include <dirent.h>

       int dirfd(DIR *dirp);