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 [XSI] 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 error.
The dirfd() function may fail if:
- The dirp argument does not refer to a valid directory stream.
The dirfd() function is intended to be a mechanism by which an application may obtain a file descriptor to use for the fchdir() function.
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().
closedir, fchdir, fdopendir, fileno, open, readdir
First released in Issue 7.
POSIX.1-2008, Technical Corrigendum 1, XSH/TC1-2008/0067  is applied.
POSIX.1-2008, Technical Corrigendum 2, XSH/TC2-2008/0076  is applied.
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