This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface

       None.

       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(3POSIX)

       The  fileno()  function shall return the integer file descriptor associated with the stream pointed to by
       stream.

       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.

       Thefollowingsectionsareinformative.

       None.

       None.

       fileno — map a stream pointer to a file descriptor

       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.

       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).

       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.

Section2.5.1, InteractionofFileDescriptorsandStandardI/OStreams,  dirfd(),  fdopen(),  fopen(),
       stdin

       The Base Definitions volume of POSIX.1‐2017, <stdio.h>

       #include <stdio.h>

       int fileno(FILE *stream);