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.
dirfd — extract the file descriptor used by a DIR stream
int dirfd(DIR *dirp);
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.
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
The dirfd() function may fail if:
- The dirp argument does not refer to a valid directory stream.
The following sections are informative.
The dirfd() function is intended to be a mechanism by which an
application may obtain a file descriptor to use for the fchdir()
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
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
closedir(), fchdir(), fdopendir(),
fileno(), open(), readdir()
The Base Definitions volume of POSIX.1‐2017,
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 .
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