rom/exec/procure.c

   1 /*
   2     Copyright © 1995-2007, The AROS Development Team. All rights reserved.
   3     $Id$
   4
   5     Desc: Try to lock a semaphore.
   6     Lang: english
   7 */
   8 #include "exec_intern.h"
   9 #include "semaphores.h"
  10 #include <exec/semaphores.h>
  11 #include <proto/exec.h>
  12
  13 /*****************************************************************************
  14
  15     NAME */
  16
  17         AROS_LH2(ULONG, Procure,
  18
  19 /*  SYNOPSIS */
  20         AROS_LHA(struct SignalSemaphore  *, sigSem, A0),
  21         AROS_LHA(struct SemaphoreMessage *, bidMsg, A1),
  22
  23 /*  LOCATION */
  24         struct ExecBase *, SysBase, 90, Exec)
  25
  26 /*  FUNCTION
  27         Tries to get a lock on a semaphore in an asynchronous manner.
  28         If the semaphore is not free this function will not wait but
  29         just post a request to the semaphore. As soon as the semaphore is
  30         available the bidMsg will return and make you owner of the semaphore.
  31
  32     INPUTS
  33         sigSem - pointer to semaphore structure
  34         bidMsg - pointer to a struct SemaphoreMessage. This should lie in
  35                  public or at least shared memory.
  36
  37     RESULT
  38         Principly none. Don't know. Just ignore it.
  39
  40     NOTES
  41         Locks obtained with Procure() must be released with Vacate().
  42
  43     EXAMPLE
  44
  45     BUGS
  46
  47     SEE ALSO
  48         Vacate()
  49
  50     INTERNALS
  51
  52 *****************************************************************************/
  53 {
  54     AROS_LIBFUNC_INIT
  55
  56     /* Prepare semaphore message to be a sent message */
  57     bidMsg->ssm_Message.mn_Length=sizeof(struct SemaphoreMessage);
  58
  59     /*
  60      *  If ln_Name field of message == 1, then this is a shared message, so
  61      *  we set the field to NULL. Otherwise, set it to the task requesting
  62      *  the semaphore. By the end, the ssm_Semaphore field contains the new
  63      *  owner of the semaphore.
  64      */
  65     if( (IPTR)(bidMsg->ssm_Message.mn_Node.ln_Name) == SM_SHARED )
  66         bidMsg->ssm_Semaphore = NULL;
  67     else
  68         bidMsg->ssm_Semaphore = (struct SignalSemaphore *)FindTask(NULL);
  69
  70     /* Arbitrate for the semaphore structure - following like ObtainSema() */
  71     Forbid();
  72
  73     sigSem->ss_QueueCount++;
  74     /*
  75         Check if we own it
  76     */
  77     if( sigSem->ss_QueueCount == 0 )
  78     {
  79         /* No, and neither does anybody else - claim it as ours. */
  80         sigSem->ss_Owner = (struct Task *)bidMsg->ssm_Semaphore;
  81         sigSem->ss_NestCount++;
  82         bidMsg->ssm_Semaphore = sigSem;
  83         ReplyMsg(&bidMsg->ssm_Message);
  84     }
  85     /*
  86         Otherwise check if we already own it
  87     */
  88     else if( sigSem->ss_Owner == (struct Task *)bidMsg->ssm_Semaphore )
  89     {
  90         /* Yes we do... */
  91         sigSem->ss_NestCount++;
  92         bidMsg->ssm_Semaphore = sigSem;
  93         ReplyMsg(&bidMsg->ssm_Message);
  94     }
  95
  96     /*
  97         It is owned by somebody else, set up the waiter
  98     */
  99     else
 100     {
 101         struct SemaphoreRequest *sr;
 102         /*
 103          *  Unholy Hack v1.
 104          *
 105          *  Whoever came up with this obviously has a twisted mind. We
 106          *  pretend that the SemaphoreMessage is really a SemaphoreRequest.
 107          *  Now, the code in ReleaseSemaphore that deals with this checks
 108          *  (after clearing bit 0) whether the sr_Waiter field is NULL. If
 109          *  so then it is really a SemaphoreMessage and should be returned.
 110          *
 111          *  Now the bad part about this is what we are doing. There are two
 112          *  cases, in the Amiga case (bincompat), we are overwriting the
 113          *  ln_Type, ln_Pri and (half the) ln_Name fields. In the other
 114          *  case we are overwriting the ln_Name field completely. Now, in
 115          *  either case this doesn't matter as they do not contain any
 116          *  useful information that we haven't already claimed.
 117          *
 118          *  Thank goodness C is so type unsafe.
 119          */
 120         sr = (struct SemaphoreRequest *)bidMsg;
 121         if (bidMsg->ssm_Semaphore != NULL)
 122             sr->sr_Waiter = NULL;
 123         else
 124             sr->sr_Waiter = (APTR)SM_SHARED;
 125
 126         AddTail((struct List *)&sigSem->ss_WaitQueue, (struct Node *)bidMsg);
 127     }
 128     /* All done. */
 129     Permit();
 130
 131     /* Huh? */
 132     return 0;
 133     AROS_LIBFUNC_EXIT
 134 } /* Procure */