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int dirfd(DIR *dirp);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
_BSD_SOURCE || _SVID_SOURCE
|| /* Since glibc 2.10: */
(_POSIX_C_SOURCE >= 200809L || _XOPEN_SOURCE >= 700)
The function dirfd() returns the file descriptor associated with the
directory stream dirp.
This descriptor is the one used internally by the directory stream. As
a result, it is useful only for functions which do not depend on or
alter the file position, such as fstat(2) and fchdir(2). It will be
automatically closed when closedir(3) is called.
On success, a nonnegative file descriptor is returned. On error, -1 is
returned, and errno is set to indicate the cause of the error.
POSIX.1-2008 specifies two errors, neither of which is returned by the
EINVAL dirp does not refer to a valid directory stream.
The implementation does not support the association of a file
descriptor with a directory.
Multithreading (see pthreads(7))
The dirfd() function is thread-safe.
POSIX.1-2008. This function was a BSD extension, present in 4.3BSD-
Reno, not in 4.2BSD.
The prototype for dirfd() is available only if _BSD_SOURCE or
_SVID_SOURCE is defined.
open(2), closedir(3), opendir(3), readdir(3), rewinddir(3), scandir(3),