fileno(3) Library Functions Manual fileno(3)
NAME
fileno - obtain file descriptor of a stdio stream
LIBRARY
Standard C library (libc, -lc)
SYNOPSIS
#include <stdio.h>
int fileno(FILE *stream);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
fileno():
_POSIX_C_SOURCE
DESCRIPTION
The function fileno() examines the argument stream and returns the inte-
ger file descriptor used to implement this stream. The file descriptor
is still owned by stream and will be closed when fclose(3) is called.
Duplicate the file descriptor with dup(2) before passing it to code that
might close it.
For the nonlocking counterpart, see unlocked_stdio(3).
RETURN VALUE
On success, fileno() returns the file descriptor associated with stream.
On failure, -1 is returned and errno is set to indicate the error.
ERRORS
EBADF stream is not associated with a file.
ATTRIBUTES
For an explanation of the terms used in this section, see attributes(7).
┌────────────────────────────────────────────┬───────────────┬─────────┐
│ Interface │ Attribute │ Value │
├────────────────────────────────────────────┼───────────────┼─────────┤
│ fileno() │ Thread safety │ MT-Safe │
└────────────────────────────────────────────┴───────────────┴─────────┘
STANDARDS
POSIX.1-2008.
HISTORY
POSIX.1-2001.
SEE ALSO
open(2), fdopen(3), stdio(3), unlocked_stdio(3)
Linux man-pages 6.9.1 2024-05-02 fileno(3)
Generated by dwww version 1.16 on Tue Dec 16 04:44:37 CET 2025.