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4.15 fileno, fileno_unlocked—return file descriptor associated with stream

Synopsis 

     #include <stdio.h>
     int fileno(FILE *fp);
     
     #define _BSD_SOURCE
     #include <stdio.h>
     int fileno_unlocked(FILE *fp);

Description
You can use fileno to return the file descriptor identified by fp.

fileno_unlocked is a non-thread-safe version of fileno. fileno_unlocked may only safely be used within a scope protected by flockfile() (or ftrylockfile()) and funlockfile(). This function may safely be used in a multi-threaded program if and only if they are called while the invoking thread owns the (FILE *) object, as is the case after a successful call to the flockfile() or ftrylockfile() functions. If threads are disabled, then fileno_unlocked is equivalent to fileno.
Returns
fileno returns a non-negative integer when successful. If fp is not an open stream, fileno returns -1.
Portability
fileno is not part of ANSI C. POSIX requires fileno.

fileno_unlocked is a BSD extension also provided by GNU libc.

Supporting OS subroutines required: none.