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FILENO(3P)                 POSIX Programmer's Manual                FILENO(3P)
PROLOG
       This  manual  page is part of the POSIX Programmer's Manual.  The Linux
       implementation of this interface may differ (consult the  corresponding
       Linux  manual page for details of Linux behavior), or the interface may
       not be implemented on Linux.
NAME
       fileno -- map a stream pointer to a file descriptor
SYNOPSIS
       #include <stdio.h>
       int fileno(FILE *stream);
DESCRIPTION
       The fileno() function shall return the integer file descriptor  associ-
       ated with the stream pointed to by stream.
RETURN VALUE
       Upon  successful completion, fileno() shall return the integer value of
       the file descriptor associated with stream.  Otherwise,  the  value  -1
       shall be returned and errno set to indicate the error.
ERRORS
       The fileno() function may fail if:
       EBADF  The  stream argument is not a valid stream, or the stream is not
              associated with a file.
       The following sections are informative.
EXAMPLES
       None.
APPLICATION USAGE
       None.
RATIONALE
       Without some specification of which  file  descriptors  are  associated
       with  these  streams, it is impossible for an application to set up the
       streams for another application it starts with  fork()  and  exec.   In
       particular, it would not be possible to write a portable version of the
       sh command interpreter (although there may be  other  constraints  that
       would prevent that portability).
FUTURE DIRECTIONS
       None.
SEE ALSO
       Section  2.5.1,  Interaction  of  File  Descriptors  and  Standard  I/O
       Streams, dirfd(), fdopen(), fopen(), stdin
       The Base Definitions volume of POSIX.1-2008, <stdio.h>
COPYRIGHT
       Portions of this text are reprinted and reproduced in  electronic  form
       from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology
       -- Portable Operating System Interface (POSIX),  The  Open  Group  Base
       Specifications Issue 7, Copyright (C) 2013 by the Institute of Electri-
       cal and Electronics Engineers,  Inc  and  The  Open  Group.   (This  is
       POSIX.1-2008  with  the  2013  Technical Corrigendum 1 applied.) In the
       event of any discrepancy between this version and the original IEEE and
       The  Open Group Standard, the original IEEE and The Open Group Standard
       is the referee document. The original Standard can be  obtained  online
       at http://www.unix.org/online.html .
       Any  typographical  or  formatting  errors that appear in this page are
       most likely to have been introduced during the conversion of the source
       files  to  man page format. To report such errors, see https://www.ker-
       nel.org/doc/man-pages/reporting_bugs.html .
IEEE/The Open Group                  2013                           FILENO(3P)