native_client_sdk/src/examples/demo/nacl_io_demo/queue.c

   1 /* Copyright (c) 2012 The Chromium Authors. All rights reserved.
   2  * Use of this source code is governed by a BSD-style license that can be
   3  * found in the LICENSE file.
   4  */
   5
   6 #include "queue.h"
   7
   8 #include <pthread.h>
   9 #include <stdlib.h>
  10
  11 #include "ppapi/c/pp_var.h"
  12
  13 #define MAX_QUEUE_SIZE 16
  14
  15 /** A mutex that guards |g_queue|. */
  16 static pthread_mutex_t g_queue_mutex;
  17
  18 /** A condition variable that is signalled when |g_queue| is not empty. */
  19 static pthread_cond_t g_queue_not_empty_cond;
  20
  21 /** A circular queue of messages from JavaScript to be handled.
  22  *
  23  * If g_queue_start < g_queue_end:
  24  *   all elements in the range [g_queue_start, g_queue_end) are valid.
  25  * If g_queue_start > g_queue_end:
  26  *   all elements in the ranges [0, g_queue_end) and
  27  *   [g_queue_start, MAX_QUEUE_SIZE) are valid.
  28  * If g_queue_start == g_queue_end, and g_queue_size > 0:
  29  *   all elements in the g_queue are valid.
  30  * If g_queue_start == g_queue_end, and g_queue_size == 0:
  31  *   No elements are valid. */
  32 static struct PP_Var g_queue[MAX_QUEUE_SIZE];
  33
  34 /** The index of the head of the queue. */
  35 static int g_queue_start = 0;
  36
  37 /** The index of the tail of the queue, non-inclusive. */
  38 static int g_queue_end = 0;
  39
  40 /** The size of the queue. */
  41 static int g_queue_size = 0;
  42
  43 /** Return whether the queue is empty.
  44  *
  45  * NOTE: this function assumes g_queue_mutex lock is held.
  46  * @return non-zero if the queue is empty. */
  47 static int IsQueueEmpty() { return g_queue_size == 0; }
  48
  49 /** Return whether the queue is full.
  50  *
  51  * NOTE: this function assumes g_queue_mutex lock is held.
  52  * @return non-zero if the queue is full. */
  53 static int IsQueueFull() { return g_queue_size == MAX_QUEUE_SIZE; }
  54
  55 /** Initialize the message queue. */
  56 void InitializeMessageQueue() {
  57   pthread_mutex_init(&g_queue_mutex, NULL);
  58   pthread_cond_init(&g_queue_not_empty_cond, NULL);
  59 }
  60
  61 /** Enqueue a message (i.e. add to the end)
  62  *
  63  * If the queue is full, the message will be dropped.
  64  *
  65  * NOTE: this function assumes g_queue_mutex is _NOT_ held.
  66  * @param[in] message The message to enqueue.
  67  * @return non-zero if the message was added to the queue. */
  68 int EnqueueMessage(struct PP_Var message) {
  69   pthread_mutex_lock(&g_queue_mutex);
  70
  71   /* We shouldn't block the main thread waiting for the queue to not be full,
  72    * so just drop the message. */
  73   if (IsQueueFull()) {
  74     pthread_mutex_unlock(&g_queue_mutex);
  75     return 0;
  76   }
  77
  78   g_queue[g_queue_end] = message;
  79   g_queue_end = (g_queue_end + 1) % MAX_QUEUE_SIZE;
  80   g_queue_size++;
  81
  82   pthread_cond_signal(&g_queue_not_empty_cond);
  83
  84   pthread_mutex_unlock(&g_queue_mutex);
  85
  86   return 1;
  87 }
  88
  89 /** Dequeue a message and return it.
  90  *
  91  * This function blocks until a message is available. It should not be called
  92  * on the main thread.
  93  *
  94  * NOTE: this function assumes g_queue_mutex is _NOT_ held.
  95  * @return The message at the head of the queue. */
  96 struct PP_Var DequeueMessage() {
  97   struct PP_Var message;
  98
  99   pthread_mutex_lock(&g_queue_mutex);
 100
 101   while (IsQueueEmpty()) {
 102     pthread_cond_wait(&g_queue_not_empty_cond, &g_queue_mutex);
 103   }
 104
 105   message = g_queue[g_queue_start];
 106   g_queue_start = (g_queue_start + 1) % MAX_QUEUE_SIZE;
 107   g_queue_size--;
 108
 109   pthread_mutex_unlock(&g_queue_mutex);
 110
 111   return message;
 112 }