NAME

       flockfile, ftrylockfile, funlockfile - lock FILE for stdio

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 versions <= 2.23: */ _POSIX_C_SOURCE
               || /* Glibc versions <= 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(), │ Thread safety │ MT-Safe │
       │funlockfile()                 │               │         │
       └──────────────────────────────┴───────────────┴─────────┘

CONFORMING TO

       POSIX.1-2001, POSIX.1-2008.

AVAILABILITY

       These functions are available when _POSIX_THREAD_SAFE_FUNCTIONS is defined.

SEE ALSO

       unlocked_stdio(3)

COLOPHON

       This  page  is  part of release 5.05 of the Linux man-pages project.  A description of the
       project, information about reporting bugs, and the latest version of  this  page,  can  be
       found at https://www.kernel.org/doc/man-pages/.

                                            2017-07-13                               FLOCKFILE(3)