man3/fileno.3

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