fileno - obtain file descriptor of a stdio stream

       For an explanation of the terms used in this section, see attributes(7).
       ┌─────────────────────────────────────────────────────────────────────────────┬───────────────┬─────────┐
       │ Interface                                                                   │ Attribute     │ Value   │
       ├─────────────────────────────────────────────────────────────────────────────┼───────────────┼─────────┤
       │ fileno()                                                                    │ Thread safety │ MT-Safe │
       └─────────────────────────────────────────────────────────────────────────────┴───────────────┴─────────┘

       The  function  fileno()  examines  the  argument  stream  and returns the integer file descriptor used to
       implement this stream.  The file descriptor is still owned by stream and will be closed when fclose(3) is
       called.  Duplicate the file descriptor with dup(2) before passing it to code that might close it.

       For the nonlocking counterpart, see unlocked_stdio(3).

EBADFstream is not associated with a file.

       POSIX.1-2001.

       Standard C library (libc, -lc)

       fileno - obtain file descriptor of a stdio stream

       On success, fileno() returns the file descriptor associated with stream.  On failure, -1 is returned  and
       errno is set to indicate the error.

open(2), fdopen(3), stdio(3), unlocked_stdio(3)

Linux man-pages 6.9.1                              2024-05-02                                          fileno(3)

       POSIX.1-2008.

#include<stdio.h>intfileno(FILE*stream);

   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

       fileno():
           _POSIX_C_SOURCE