flockfile - phpMan

LOCKFILE(3)                Linux Programmer’s Manual               LOCKFILE(3)



NAME
       flockfile, ftrylockfile, funlockfile - lock FILE for stdio

SYNOPSIS
       #include <stdio.h>

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

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 effi-
       ciency.

       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   non-locking  versions  of  the  stdio  functions:  with  getc_unlocked()  and
       putc_unlocked() instead of getc() and putc().

       The flockfile() function waits for *filehandle to be no longer locked by a  differ-
       ent  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 non-blocking version of flockfile(). It does noth-
       ing in case some other thread owns *filehandle, and it obtains ownership and incre-
       ments the lockcount otherwise.

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

ERRORS
       None.

AVAILABILITY
       These  functions  are  available when _POSIX_THREAD_SAFE_FUNCTIONS is defined. They
       are in libc since libc 5.1.1 and in glibc since glibc 2.0.

CONFORMING TO
       POSIX.1

SEE ALSO
       unlocked_stdio(3)




                                  2001-10-18                       LOCKFILE(3)

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