fileno(3) — Linux manual page

fileno(3)                Library Functions Manual               fileno(3)

NAME         top

       fileno - obtain file descriptor of a stdio stream

LIBRARY         top

       Standard C library (libc, -lc)

SYNOPSIS         top

       #include <stdio.h>

       int fileno(FILE *stream);

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

       fileno():
           _POSIX_C_SOURCE

DESCRIPTION         top

       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).

RETURN VALUE         top

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

ERRORS         top

       EBADF  stream is not associated with a file.

ATTRIBUTES         top

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

STANDARDS         top

       POSIX.1-2008.

HISTORY         top

       POSIX.1-2001.

SEE ALSO         top

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

COLOPHON         top

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Linux man-pages 6.15            2025-05-17                      fileno(3)

Pages that refer to this page: fsync(2),  fclose(3),  ferror(3),  fflush(3),  fmemopen(3),  fopen(3),  open_memstream(3),  pcap_file(3pcap),  stdio(3)

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