workbench/libs/thread/signalcondition.c

   1 /*
   2  * thread.library - threading and synchronisation primitives
   3  *
   4  * Copyright © 2007 Robert Norris
   5  *
   6  * This program is free software; you can redistribute it and/or modify it
   7  * under the same terms as AROS itself.
   8  */
   9
  10 #include "thread_intern.h"
  11
  12 #include <exec/tasks.h>
  13 #include <exec/lists.h>
  14 #include <proto/exec.h>
  15 #include <assert.h>
  16
  17 /*****************************************************************************
  18
  19     NAME */
  20         AROS_LH1(void, SignalCondition,
  21
  22 /*  SYNOPSIS */
  23         AROS_LHA(void *, cond, A0),
  24
  25 /*  LOCATION */
  26         struct ThreadBase *, ThreadBase, 18, Thread)
  27
  28 /*  FUNCTION
  29         Signals a thread waiting on condition variable.
  30
  31     INPUTS
  32         cond - the condition to signal.
  33
  34     RESULT
  35         This function always succeeds.
  36
  37     NOTES
  38         Before calling this function you should lock the mutex that protects
  39         the condition. WaitCondition() atomically unlocks the mutex and waits
  40         on the condition, so by locking the mutex first before sending the
  41         signal, you ensure that the signal cannot be missed. See
  42         WaitCondition() for more details.
  43
  44         If no threads are waiting on the condition, nothing happens. If more
  45         than one thread is waiting, only one will be signalled. Which one is
  46         undefined.
  47
  48     EXAMPLE
  49         LockMutex(mutex);
  50         SignalCondition(cond);
  51         UnlockMutex(mutex);
  52
  53     BUGS
  54
  55     SEE ALSO
  56         CreateCondition(), DestroyCondition(), WaitCondition(),
  57         BroadcastCondition()
  58
  59     INTERNALS
  60         SIGF_SIGNAL is used to signal the selected waiting thread.
  61
  62 *****************************************************************************/
  63 {
  64     AROS_LIBFUNC_INIT
  65
  66     struct _Condition *c = (struct _Condition *) cond;
  67     struct _CondWaiter *waiter;
  68
  69     assert(c != NULL);
  70
  71     /* safely remove a waiter from the list */
  72     ObtainSemaphore(&c->lock);
  73     waiter = (struct _CondWaiter *) REMHEAD(&c->waiters);
  74     if (waiter != NULL)
  75         c->count--;
  76     ReleaseSemaphore(&c->lock);
  77
  78     /* noone waiting */
  79     if (waiter == NULL)
  80         return;
  81
  82     /* signal the task */
  83     Signal(waiter->task, SIGF_SINGLE);
  84
  85     /* all done */
  86     FreeMem(waiter, sizeof(struct _CondWaiter));
  87
  88     AROS_LIBFUNC_EXIT
  89 } /* SignalCondition */