man3/fileno.3

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   2 .\" and Copyright (C) 2021 Michael Kerrisk <mtk.manpages@gmail.com>
   3 .\" All rights reserved.
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   5 .\" This code is derived from software contributed to Berkeley by
   6 .\" Chris Torek and the American National Standards Committee X3,
   7 .\" on Information Processing Systems.
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   9 .\" SPDX-License-Identifier: BSD-4-Clause-UC
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  11 .\" Converted for Linux, Mon Nov 29 14:24:40 1993, faith@cs.unc.edu
  12 .\" Added remark on EBADF for fileno, aeb, 2001-03-22
  13 .\"
  14 .TH FILENO 3 2021-03-22 "Linux man-pages (unreleased)"
  15 .SH NAME
  16 fileno \- obtain file descriptor of a stdio stream
  17 .SH LIBRARY
  18 Standard C library
  19 .RI ( libc ", " \-lc )
  20 .SH SYNOPSIS
  21 .nf
  22 .B #include <stdio.h>
  23 .PP
  24 .BI "int fileno(FILE *" stream );
  25 .fi
  26 .PP
  27 .RS -4
  28 Feature Test Macro Requirements for glibc (see
  29 .BR feature_test_macros (7)):
  30 .RE
  31 .PP
  32 .BR fileno ():
  33 .nf
  34     _POSIX_C_SOURCE
  35 .fi
  36 .SH DESCRIPTION
  37 The function
  38 .BR fileno ()
  39 examines the argument
  40 .I stream
  41 and returns the integer file descriptor used to implement this stream.
  42 The file descriptor is still owned by
  43 .I stream
  44 and will be closed when
  45 .BR fclose (3)
  46 is called.
  47 Duplicate the file descriptor with
  48 .BR dup (2)
  49 before passing it to code that might close it.
  50 .PP
  51 For the nonlocking counterpart, see
  52 .BR unlocked_stdio (3).
  53 .SH RETURN VALUE
  54 On success,
  55 .BR fileno ()
  56 returns the file descriptor associated with
  57 .IR stream .
  58 On failure, \-1 is returned and
  59 .I errno
  60 is set to indicate the error.
  61 .SH ERRORS
  62 .TP
  63 .B EBADF
  64 .I stream
  65 is not associated with a file.
  66 .SH ATTRIBUTES
  67 For an explanation of the terms used in this section, see
  68 .BR attributes (7).
  69 .ad l
  70 .nh
  71 .TS
  72 allbox;
  73 lbx lb lb
  74 l l l.
  75 Interface       Attribute       Value
  76 T{
  77 .BR fileno ()
  78 T}      Thread safety   MT-Safe
  79 .TE
  80 .hy
  81 .ad
  82 .sp 1
  83 .SH STANDARDS
  84 The function
  85 .BR fileno ()
  86 conforms to POSIX.1-2001 and POSIX.1-2008.
  87 .SH SEE ALSO
  88 .BR open (2),
  89 .BR fdopen (3),
  90 .BR stdio (3),
  91 .BR unlocked_stdio (3)