FILENO(3) Linux Programmer's Manual FILENO(3)

fileno - obtain file descriptor of a stdio stream

#include <stdio.h>
int fileno(FILE *stream);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

fileno():

_POSIX_C_SOURCE

The function fileno() examines the argument stream and returns the integer 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).

On success, fileno() returns the file descriptor associated with stream. On failure, -1 is returned and errno is set to indicate the error.

EBADF
stream is not associated with a file.

For an explanation of the terms used in this section, see attributes(7).
Interface Attribute Value
fileno () Thread safety MT-Safe

The function fileno() conforms to POSIX.1-2001 and POSIX.1-2008.

open(2), fdopen(3), stdio(3), unlocked_stdio(3)

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2021-03-22