man3/fileno.3

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.\" Converted for Linux, Mon Nov 29 14:24:40 1993, faith@cs.unc.edu
.\" Added remark on EBADF for fileno, aeb, 2001-03-22
.\"
.TH FILENO 3  2021-03-22 "" "Linux Programmer's Manual"
.SH NAME
fileno \- obtain file descriptor of a stdio stream
.SH SYNOPSIS
.nf
.B #include <stdio.h>
.PP
.BI "int fileno(FILE *" stream );
.fi
.PP
.RS -4
Feature Test Macro Requirements for glibc (see
.BR feature_test_macros (7)):
.RE
.PP
.BR fileno ():
.nf
    _POSIX_C_SOURCE
.fi
.SH DESCRIPTION
.PP
The function
.BR fileno ()
examines the argument
.I stream
and returns the integer file descriptor used to implement this stream.
The file descriptor is still owned by
.I stream
and will be closed when
.BR fclose (3)
is called.
Duplicate the file descriptor with
.BR dup (2)
before passing it to code that might close it.
.PP
For the nonlocking counterpart, see
.BR unlocked_stdio (3).
.SH RETURN VALUE
On success,
.BR fileno ()
returns the file descriptor associated with
.IR stream .
On failure, \-1 is returned and
.I errno
is set to indicate the error.
.SH ERRORS
.TP
.B EBADF
.I stream
is not associated with a file.
.SH ATTRIBUTES
For an explanation of the terms used in this section, see
.BR attributes (7).
.ad l
.nh
.TS
allbox;
lbx lb lb
l l l.
Interface	Attribute	Value
T{
.BR fileno ()
T}	Thread safety	MT-Safe
.TE
.hy
.ad
.sp 1
.SH CONFORMING TO
The function
.BR fileno ()
conforms to POSIX.1-2001 and POSIX.1-2008.
.SH SEE ALSO
.BR open (2),
.BR fdopen (3),
.BR stdio (3),
.BR unlocked_stdio (3)