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 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  Electrical  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.kernel.org/doc/man-pages/reporting_bugs.html .