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 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 .