FLOCKFILE(3)               Linux Programmer's Manual              FLOCKFILE(3)

       flockfile, ftrylockfile, funlockfile - lock FILE for stdio

       #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

       The  stdio functions are thread-safe.  This is achieved by assigning to
       each FILE object a lockcount and (if the lockcount is nonzero) an  own-
       ing 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  inter-
       rupted  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 *filehan-
       dle, 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  ob-
       tains ownership and increments the lockcount otherwise.

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


       For an  explanation  of  the  terms  used  in  this  section,  see  at-

       |Interface                     | Attribute     | Value   |
       |flockfile(),  ftrylockfile(), | Thread safety | MT-Safe |
       |funlockfile()                 |               |         |
       POSIX.1-2001, POSIX.1-2008.

       These functions are available when _POSIX_THREAD_SAFE_FUNCTIONS is  de-


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                                  2017-07-13                      FLOCKFILE(3)
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