NAME

       flockfile, ftrylockfile, funlockfile - lock FILE for stdio

LIBRARY

       Standard C library (libc, -lc)

SYNOPSIS

       #include <stdio.h>

       void flockfile(FILE *filehandle);
       int ftrylockfile(FILE *filehandle);
       void funlockfile(FILE *filehandle);

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

       All functions shown above:
           /* Since glibc 2.24: */ _POSIX_C_SOURCE >= 199309L
               || /* glibc <= 2.23: */ _POSIX_C_SOURCE
               || /* glibc <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE

DESCRIPTION

       The  stdio  functions are thread-safe.  This is achieved by assigning to each FILE object a lockcount and
       (if the lockcount is nonzero) an owning thread.  For each library call, these functions  wait  until  the
       FILE object is no longer locked by a different thread, then lock it, do the requested I/O, and unlock the
       object again.

       (Note:  this  locking  has  nothing  to  do  with  the  file  locking done by functions like flock(2) and
       lockf(3).)

       All this is invisible to the C-programmer, but there may  be  two  reasons  to  wish  for  more  detailed
       control.   On  the one hand, maybe a series of I/O actions by one thread belongs together, and should not
       be interrupted by the I/O of some other thread.  On the other hand, maybe the locking overhead should  be
       avoided for greater efficiency.

       To  this  end,  a  thread  can  explicitly  lock the FILE object, then do its series of I/O actions, then
       unlock.  This prevents other threads from coming in between.  If the reason for doing this was to achieve
       greater efficiency, one does  the  I/O  with  the  nonlocking  versions  of  the  stdio  functions:  with
       getc_unlocked(3) and putc_unlocked(3) instead of getc(3) and putc(3).

       The  flockfile()  function waits for *filehandle to be no longer locked by a different thread, then makes
       the current thread owner of *filehandle, and increments the lockcount.

       The funlockfile() function decrements the lock count.

       The ftrylockfile() function is a nonblocking version of flockfile().  It does nothing in case some  other
       thread owns *filehandle, and it obtains ownership and increments the lockcount otherwise.

RETURN VALUE

       The ftrylockfile() function returns zero for success (the lock was obtained), and nonzero for failure.

ERRORS

       None.

ATTRIBUTES

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

STANDARDS

       POSIX.1-2008.

HISTORY

       POSIX.1-2001.

       These functions are available when _POSIX_THREAD_SAFE_FUNCTIONS is defined.

SEE ALSO

       unlocked_stdio(3)

Linux man-pages 6.7                                2023-10-31                                       flockfile(3)