compat/win32/pthread.c

   1 /*
   2  * Copyright (C) 2009 Andrzej K. Haczewski <ahaczewski@gmail.com>
   3  *
   4  * DISCLAIMER: The implementation is Git-specific, it is subset of original
   5  * Pthreads API, without lots of other features that Git doesn't use.
   6  * Git also makes sure that the passed arguments are valid, so there's
   7  * no need for double-checking.
   8  */
   9
  10 #include "../../git-compat-util.h"
  11 #include "pthread.h"
  12
  13 #include <errno.h>
  14 #include <limits.h>
  15
  16 static unsigned __stdcall win32_start_routine(void *arg)
  17 {
  18         pthread_t *thread = arg;
  19         thread->arg = thread->start_routine(thread->arg);
  20         return 0;
  21 }
  22
  23 int pthread_create(pthread_t *thread, const void *unused,
  24                    void *(*start_routine)(void*), void *arg)
  25 {
  26         thread->arg = arg;
  27         thread->start_routine = start_routine;
  28         thread->handle = (HANDLE)
  29                 _beginthreadex(NULL, 0, win32_start_routine, thread, 0, NULL);
  30
  31         if (!thread->handle)
  32                 return errno;
  33         else
  34                 return 0;
  35 }
  36
  37 int win32_pthread_join(pthread_t *thread, void **value_ptr)
  38 {
  39         DWORD result = WaitForSingleObject(thread->handle, INFINITE);
  40         switch (result) {
  41                 case WAIT_OBJECT_0:
  42                         if (value_ptr)
  43                                 *value_ptr = thread->arg;
  44                         return 0;
  45                 case WAIT_ABANDONED:
  46                         return EINVAL;
  47                 default:
  48                         return err_win_to_posix(GetLastError());
  49         }
  50 }
  51
  52 int pthread_cond_init(pthread_cond_t *cond, const void *unused)
  53 {
  54         cond->waiters = 0;
  55         cond->was_broadcast = 0;
  56         InitializeCriticalSection(&cond->waiters_lock);
  57
  58         cond->sema = CreateSemaphore(NULL, 0, LONG_MAX, NULL);
  59         if (!cond->sema)
  60                 die("CreateSemaphore() failed");
  61
  62         cond->continue_broadcast = CreateEvent(NULL,    /* security */
  63                                 FALSE,                  /* auto-reset */
  64                                 FALSE,                  /* not signaled */
  65                                 NULL);                  /* name */
  66         if (!cond->continue_broadcast)
  67                 die("CreateEvent() failed");
  68
  69         return 0;
  70 }
  71
  72 int pthread_cond_destroy(pthread_cond_t *cond)
  73 {
  74         CloseHandle(cond->sema);
  75         CloseHandle(cond->continue_broadcast);
  76         DeleteCriticalSection(&cond->waiters_lock);
  77         return 0;
  78 }
  79
  80 int pthread_cond_wait(pthread_cond_t *cond, CRITICAL_SECTION *mutex)
  81 {
  82         int last_waiter;
  83
  84         EnterCriticalSection(&cond->waiters_lock);
  85         cond->waiters++;
  86         LeaveCriticalSection(&cond->waiters_lock);
  87
  88         /*
  89          * Unlock external mutex and wait for signal.
  90          * NOTE: we've held mutex locked long enough to increment
  91          * waiters count above, so there's no problem with
  92          * leaving mutex unlocked before we wait on semaphore.
  93          */
  94         LeaveCriticalSection(mutex);
  95
  96         /* let's wait - ignore return value */
  97         WaitForSingleObject(cond->sema, INFINITE);
  98
  99         /*
 100          * Decrease waiters count. If we are the last waiter, then we must
 101          * notify the broadcasting thread that it can continue.
 102          * But if we continued due to cond_signal, we do not have to do that
 103          * because the signaling thread knows that only one waiter continued.
 104          */
 105         EnterCriticalSection(&cond->waiters_lock);
 106         cond->waiters--;
 107         last_waiter = cond->was_broadcast && cond->waiters == 0;
 108         LeaveCriticalSection(&cond->waiters_lock);
 109
 110         if (last_waiter) {
 111                 /*
 112                  * cond_broadcast was issued while mutex was held. This means
 113                  * that all other waiters have continued, but are contending
 114                  * for the mutex at the end of this function because the
 115                  * broadcasting thread did not leave cond_broadcast, yet.
 116                  * (This is so that it can be sure that each waiter has
 117                  * consumed exactly one slice of the semaphor.)
 118                  * The last waiter must tell the broadcasting thread that it
 119                  * can go on.
 120                  */
 121                 SetEvent(cond->continue_broadcast);
 122                 /*
 123                  * Now we go on to contend with all other waiters for
 124                  * the mutex. Auf in den Kampf!
 125                  */
 126         }
 127         /* lock external mutex again */
 128         EnterCriticalSection(mutex);
 129
 130         return 0;
 131 }
 132
 133 /*
 134  * IMPORTANT: This implementation requires that pthread_cond_signal
 135  * is called while the mutex is held that is used in the corresponding
 136  * pthread_cond_wait calls!
 137  */
 138 int pthread_cond_signal(pthread_cond_t *cond)
 139 {
 140         int have_waiters;
 141
 142         EnterCriticalSection(&cond->waiters_lock);
 143         have_waiters = cond->waiters > 0;
 144         LeaveCriticalSection(&cond->waiters_lock);
 145
 146         /*
 147          * Signal only when there are waiters
 148          */
 149         if (have_waiters)
 150                 return ReleaseSemaphore(cond->sema, 1, NULL) ?
 151                         0 : err_win_to_posix(GetLastError());
 152         else
 153                 return 0;
 154 }
 155
 156 /*
 157  * DOUBLY IMPORTANT: This implementation requires that pthread_cond_broadcast
 158  * is called while the mutex is held that is used in the corresponding
 159  * pthread_cond_wait calls!
 160  */
 161 int pthread_cond_broadcast(pthread_cond_t *cond)
 162 {
 163         EnterCriticalSection(&cond->waiters_lock);
 164
 165         if ((cond->was_broadcast = cond->waiters > 0)) {
 166                 /* wake up all waiters */
 167                 ReleaseSemaphore(cond->sema, cond->waiters, NULL);
 168                 LeaveCriticalSection(&cond->waiters_lock);
 169                 /*
 170                  * At this point all waiters continue. Each one takes its
 171                  * slice of the semaphor. Now it's our turn to wait: Since
 172                  * the external mutex is held, no thread can leave cond_wait,
 173                  * yet. For this reason, we can be sure that no thread gets
 174                  * a chance to eat *more* than one slice. OTOH, it means
 175                  * that the last waiter must send us a wake-up.
 176                  */
 177                 WaitForSingleObject(cond->continue_broadcast, INFINITE);
 178                 /*
 179                  * Since the external mutex is held, no thread can enter
 180                  * cond_wait, and, hence, it is safe to reset this flag
 181                  * without cond->waiters_lock held.
 182                  */
 183                 cond->was_broadcast = 0;
 184         } else {
 185                 LeaveCriticalSection(&cond->waiters_lock);
 186         }
 187         return 0;
 188 }