1FILENO(3P)                 POSIX Programmer's Manual                FILENO(3P)
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PROLOG

6       This  manual  page is part of the POSIX Programmer's Manual.  The Linux
7       implementation of this interface may differ (consult the  corresponding
8       Linux  manual page for details of Linux behavior), or the interface may
9       not be implemented on Linux.
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NAME

12       fileno — map a stream pointer to a file descriptor
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SYNOPSIS

15       #include <stdio.h>
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17       int fileno(FILE *stream);
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DESCRIPTION

20       The fileno() function shall return the integer file descriptor  associ‐
21       ated with the stream pointed to by stream.
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RETURN VALUE

24       Upon  successful completion, fileno() shall return the integer value of
25       the file descriptor associated with stream.  Otherwise,  the  value  -1
26       shall be returned and errno set to indicate the error.
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ERRORS

29       The fileno() function shall fail if:
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31       EBADF  The stream is not associated with a file.
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33       The fileno() function may fail if:
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35       EBADF  The  file  descriptor  underlying  stream  is  not  a valid file
36              descriptor.
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38       The following sections are informative.
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EXAMPLES

41       None.
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APPLICATION USAGE

44       None.
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RATIONALE

47       Without some specification of which  file  descriptors  are  associated
48       with  these  streams, it is impossible for an application to set up the
49       streams for another application it starts with  fork()  and  exec.   In
50       particular, it would not be possible to write a portable version of the
51       sh command interpreter (although there may be  other  constraints  that
52       would prevent that portability).
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FUTURE DIRECTIONS

55       None.
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SEE ALSO

58       Section  2.5.1,  Interaction  of  File  Descriptors  and  Standard  I/O
59       Streams, dirfd(), fdopen(), fopen(), stdin
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61       The Base Definitions volume of POSIX.1‐2017, <stdio.h>
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64       Portions of this text are reprinted and reproduced in  electronic  form
65       from  IEEE Std 1003.1-2017, Standard for Information Technology -- Por‐
66       table Operating System Interface (POSIX), The Open Group Base  Specifi‐
67       cations  Issue  7, 2018 Edition, Copyright (C) 2018 by the Institute of
68       Electrical and Electronics Engineers, Inc and The Open Group.   In  the
69       event of any discrepancy between this version and the original IEEE and
70       The Open Group Standard, the original IEEE and The Open Group  Standard
71       is  the  referee document. The original Standard can be obtained online
72       at http://www.opengroup.org/unix/online.html .
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74       Any typographical or formatting errors that appear  in  this  page  are
75       most likely to have been introduced during the conversion of the source
76       files to man page format. To report such errors,  see  https://www.ker‐
77       nel.org/doc/man-pages/reporting_bugs.html .
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81IEEE/The Open Group                  2017                           FILENO(3P)
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