FILENO(3)                  POSIX Programmer's Manual                 FILENO(3)



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

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
       associated 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 shall fail if:

       EBADF  The stream is not associated with a file.

       The fileno() function may fail if:

       EBADF  The file descriptor underlying stream is not a valid file
              descriptor.

       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.1-2017, <stdio.h>

COPYRIGHT
       Portions of this text are reprinted and reproduced in electronic form
       from IEEE Std 1003.1-2017, Standard for Information Technology --
       Portable Operating System Interface (POSIX), The Open Group Base
       Specifications Issue 7, 2018 Edition, Copyright (C) 2018 by the
       Institute of Electrical and Electronics Engineers, Inc and The Open
       Group.  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.opengroup.org/unix/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.kernel.org/doc/man-pages/reporting_bugs.html .



IEEE/The Open Group                  2017                            FILENO(3)
