User's Guide

Programming Examples Using the Pthreads Library Module

!******************************************************************
!* Example 5 : Create a thread with Round_Robin scheduling policy.*
!* For simplicity, we do not show any codes for error checking,   *
!* which would be necessary in a real program.                    *
!******************************************************************
        use f_pthread
        integer(4) ret_val
        type(f_pthread_attr_t) attr
        type(f_pthread_t)      thr
 
        ret_val = f_pthread_attr_init(attr)
        ret_val = f_pthread_attr_setschedpolicy(attr, SCHED_RR)
        ret_val = f_pthread_attr_setinheritsched(attr, PTHREAD_EXPLICIT_SCHED)
        ret_val = f_pthread_create(thr, attr, FLAG_DEFAULT, ent, integer_arg)
        ret_val = f_pthread_attr_destroy(attr)
        ......

Before you can manipulate a pthread attribute object, you need to create and initialize it. The appropriate interfaces must be called to manipulate the attribute objects. A call to f_pthread_attr_setschedpolicy sets the scheduling policy attribute to Round_Robin. Note that this does not affect newly created threads that inherit the scheduling property from the creating thread. For these threads, we explicitly call f_pthread_attr_setinheritsched to override the default inheritance attribute. The rest of the code is self-explanatory.

!*****************************************************************
!* Example 6 : Thread safety                                     *
!* In this example, we show that thread safety can be achieved   *
!* by using the push-pop cleanup stack for each thread. We       *
!* assume that the thread is in deferred cancellability-enabled  *
!* state.  This means that any thread-cancel requests will be    *
!* put on hold until a cancellation point is encountered.        *
!* Note that f_pthread_cond_wait provides a                      *
!* cancellation point.                                           *
!*****************************************************************
        use f_pthread
        integer(4) ret_val
        type(f_pthread_mutex_t) mutex
        type(f_pthread_cond_t) cond
        pointer(p, byte)
        ! Initialize mutex and condition variables before using them.
        ! For global variables this should be done in a module, so that they
        ! can be used by all threads. If they are local, other threads
        ! will not see them. Furthermore, they must be managed carefully
        ! (for example, destroy them before returning, to avoid dangling and
        ! undefined objects).
        mutex = PTHREAD_MUTEX_INITIALIZER
        cond  = PTHREAD_COND_INITIALIZER
 
        ......
        ! Doing something
 
        ......
 
        ! This thread needs to allocate some memory area used to
        ! synchronize with other threads. However, when it waits on a
        ! condition variable, this thread may be canceled by another
        ! thread. The allocated memory may be lost if no measures are
        ! taken in advance. This will cause memory leakage.
 
        ret_val = f_pthread_mutex_lock(mutex)
        p = malloc(%val(4096))
 
        ! check condition. If it is not true, going to wait for it.
        ! This should be a loop.
 
        ! Since memory has been allocated, cleanup must be registered
        ! for safety during condition waiting.
 
        ret_val = f_pthread_cleanup_push(mycleanup, FLAG_DEFAULT, p)
        ret_val = f_pthread_cond_wait(cond, mutex)
 
        ! If this thread returns from condition waiting, the cleanup
        ! should be de-registered.
 
        call f_pthread_cleanup_pop(0)      ! not execute
        ret_val = f_pthread_mutex_unlock(mutex)
 
        ! This thread will take care of p for the rest of its life.
        ......
 
 
        ! mycleanup looks like:
 
        subroutine mycleanup(passed_in)
            pointer(passed_in, byte)
            external free
 
            call free(%val(passed_in))
        end subroutine mycleanup