rom/exec/obtainsemaphore.c

   1 /*
   2     Copyright © 1995-2011, The AROS Development Team. All rights reserved.
   3     $Id$
   4
   5     Desc: Lock a semaphore.
   6     Lang: english
   7 */
   8 #include "exec_intern.h"
   9 #include "semaphores.h"
  10 #include <exec/semaphores.h>
  11 #include <aros/atomic.h>
  12
  13 #include <proto/exec.h>
  14
  15 #define CHECK_INITSEM 1
  16
  17 /*****************************************************************************/
  18 #undef  Exec
  19 #ifdef UseExecstubs
  20 #    define Exec _Exec
  21 #endif
  22
  23 /*    NAME */
  24         #include <proto/exec.h>
  25
  26         AROS_LH1(void, ObtainSemaphore,
  27
  28 /*    SYNOPSIS */
  29         AROS_LHA(struct SignalSemaphore *, sigSem, A0),
  30
  31 /*    LOCATION */
  32         struct ExecBase *, SysBase, 94, Exec)
  33
  34 /*    FUNCTION
  35         Obtain an exclusive lock on a semaphore. If the semaphore is already
  36         in use by another task this function will wait until the semaphore
  37         becomes free.
  38
  39     INPUTS
  40         sigSem - Pointer to semaphore structure
  41
  42     RESULT
  43
  44     NOTES
  45         This function preserves all registers.
  46
  47     EXAMPLE
  48
  49     BUGS
  50
  51     SEE ALSO
  52         ReleaseSemaphore()
  53
  54     INTERNALS
  55
  56 *****************************************************************************/
  57 {
  58 #undef  Exec
  59
  60     AROS_LIBFUNC_INIT
  61
  62     struct Task *me;
  63
  64     /* Get pointer to current task */
  65     me=SysBase->ThisTask;
  66
  67     /*
  68      * If there's no ThisTask, the function is called from within memory
  69      * allocator in exec's pre-init code. We are already single-threaded,
  70      * just return. :)
  71      */
  72     if (!me)
  73         return;
  74
  75 #if CHECK_INITSEM
  76     if (sigSem->ss_Link.ln_Type != NT_SIGNALSEM)
  77     {
  78         kprintf("\n\nObtainSemaphore called on a not intialized semaphore!!! "
  79                 "sem = %x  task = %x (%s)\n\n", sigSem, me, me->tc_Node.ln_Name);
  80         Alert(AN_SemCorrupt);
  81     }
  82 #endif
  83
  84     /* Arbitrate for the semaphore structure */
  85     Forbid();
  86
  87     /*
  88         ss_QueueCount == -1 indicates that the semaphore is
  89         free, so we increment this straight away. If it then
  90         equals 0, then we are the first to allocate this semaphore.
  91
  92         Note: This will need protection for SMP machines.
  93     */
  94     sigSem->ss_QueueCount++;
  95     if( sigSem->ss_QueueCount == 0 )
  96     {
  97         /* We now own the semaphore. This is quick. */
  98         sigSem->ss_Owner = me;
  99         sigSem->ss_NestCount++;
 100     }
 101
 102     /* The semaphore was in use, but was it by us? */
 103     else if( sigSem->ss_Owner == me )
 104     {
 105         /* Yes, just increase the nesting count */
 106         sigSem->ss_NestCount++;
 107     }
 108
 109     /*
 110         Else, some other task must own it. We have
 111         to set a waiting request here.
 112     */
 113     else
 114     {
 115         /*
 116             We need a node to mark our semaphore request. Lets use some
 117             stack memory.
 118         */
 119         struct SemaphoreRequest sr;
 120         sr.sr_Waiter = me;
 121
 122         /*
 123             Have to clear the signal to make sure that we don't
 124             return immediately. We then add the SemReq to the
 125             waiters list of the semaphore. We were the last to
 126             request, so we must be the last to get the semaphore.
 127         */
 128
 129         /* This must be atomic! */
 130         AROS_ATOMIC_AND(me->tc_SigRecvd, ~SIGF_SINGLE);
 131
 132         AddTail((struct List *)&sigSem->ss_WaitQueue, (struct Node *)&sr);
 133
 134         /*
 135             Finally, we simply wait, ReleaseSemaphore() will fill in
 136             who owns the semaphore.
 137         */
 138         Wait(SIGF_SINGLE);
 139     }
 140
 141     /* All Done! */
 142     Permit();
 143
 144     AROS_LIBFUNC_EXIT
 145 } /* ObtainSemaphore */