DIRFD(3P)                  POSIX Programmer's Manual                 DIRFD(3P)

PROLOG
       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.

NAME
       dirfd -- extract the file descriptor used by a DIR stream

SYNOPSIS
       #include <dirent.h>

       int dirfd(DIR *dirp);

DESCRIPTION
       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.

RETURN VALUE
       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.

ERRORS
       The dirfd() function may fail if:

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

       The following sections are informative.

EXAMPLES
       None.

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

RATIONALE
       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().

FUTURE DIRECTIONS
       None.

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

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

COPYRIGHT
       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(3P)