dirfd man page

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.

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 may 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.
ENOTSUP
The implementation does not support the association of a file descriptor with a directory.

The following sections are informative.

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.1‐2008 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().

An implementation that does not support file descriptors referring to directories may fail with [ENOTSUP].

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().

See Also

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

The Base Definitions volume of POSIX.1‐2008, <dirent.h>

Info

2013 IEEE/The Open Group POSIX Programmer's Manual