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 .