fileno − map a stream pointer to a file descriptor
int fileno(FILE *stream);
The fileno() function shall return the integer file descriptor associated with the stream pointed to by stream.
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.
The fileno() function may fail if:
The stream argument is not a valid stream.
The following sections are informative.
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).
Interaction of File Descriptors and Standard I/O Streams , fdopen() , fopen() , stdin() , the Base Definitions volume of IEEE Std 1003.1-2001, <stdio.h>
Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 6, Copyright (C) 2001-2003 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 .