+/*
+ * double-buffering - an example of double-buffering with pthreads
+ *
+ * Copyright (C) 2013 Antonio Ospite <ospite@studenti.unina.it>
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program. If not, see <http://www.gnu.org/licenses/>.
+ */
+
+/*
+ * NOTE: the synchronization scheme used in this program assume that we want
+ * to consume ALL the buffer produced, exactly ONE time.
+ *
+ * In other cases when buffer skipping or buffer duplication in wanted the
+ * synchronization scheme can change.
+ */
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <sys/types.h>
+#include <unistd.h>
+#include <pthread.h>
+#include <time.h>
+
+#define BUFFER_SIZE 10
+#define MAXLEN 1024
+
+/*
+ * The time spent by the producer and the consumer thread is quite predictable
+ * in this program, this may not be the case when some communication with
+ * actual hardware influences the production or consumption phase.
+ */
+#define PRODUCER_DELAY 50000
+#define CONSUMER_DELAY 20000
+
+#define NUM_ITERATIONS 23
+
+struct buffer {
+ int *data;
+ unsigned int size;
+};
+
+struct double_buffer_context {
+ struct buffer buffer[2];
+ int back_buffer_index;
+ int font_buffer_is_consumable;
+ pthread_cond_t front_buffer_cond;
+ pthread_mutex_t mutex;
+};
+
+#define BACK_BUFFER(ctx) ((ctx)->buffer[(ctx)->back_buffer_index])
+#define FRONT_BUFFER(ctx) ((ctx)->buffer[(ctx)->back_buffer_index ^ 1])
+#define SWAP_BUFFERS(ctx) ((ctx)->back_buffer_index ^= 1)
+
+static void do_log(char *message)
+{
+ struct timespec now;
+ double timestamp;
+
+ clock_gettime(CLOCK_MONOTONIC, &now);
+ timestamp = now.tv_sec + now.tv_nsec / 1000000000.0;
+ fprintf(stderr, "[%f] %s\n", timestamp, message);
+}
+
+/* the consumer thread will only read from the front buffer */
+static void *consumer_cb(void *arg)
+{
+ struct double_buffer_context *ctx = (struct double_buffer_context *) arg;
+ unsigned int i;
+
+ char message[MAXLEN];
+ int len;
+
+ while(1) {
+ pthread_mutex_lock(&ctx->mutex);
+ while (!ctx->font_buffer_is_consumable) {
+ pthread_cond_wait(&ctx->front_buffer_cond, &ctx->mutex);
+ }
+ pthread_mutex_unlock(&ctx->mutex);
+
+ /* consume front buffer */
+ len = snprintf(message, MAXLEN, "Consuming value: ");
+ for (i = 0; i < FRONT_BUFFER(ctx).size; i++) {
+ len += snprintf(message + len, MAXLEN - len, "%d ", FRONT_BUFFER(ctx).data[i]);
+ }
+ usleep(CONSUMER_DELAY);
+ do_log(message);
+
+ pthread_mutex_lock(&ctx->mutex);
+ ctx->font_buffer_is_consumable = 0;
+ pthread_cond_signal(&ctx->front_buffer_cond);
+ pthread_mutex_unlock(&ctx->mutex);
+ }
+
+ pthread_exit(0);
+ return (void *) 0;
+}
+
+int main(void)
+{
+ int ret;
+ pthread_t consumer_tid;
+ struct double_buffer_context *ctx;
+
+ unsigned int iterations;
+ unsigned int i;
+
+ char message[MAXLEN];
+ int len;
+
+ ctx = malloc(sizeof(*ctx));
+ if (ctx == NULL) {
+ perror("malloc");
+ exit(EXIT_FAILURE);
+ }
+ memset(ctx, 0, sizeof(*ctx));
+
+ ctx->buffer[0].data = malloc(BUFFER_SIZE * sizeof(*ctx->buffer[0].data));
+ ctx->buffer[0].size = BUFFER_SIZE;
+ ctx->buffer[1].data = malloc(BUFFER_SIZE * sizeof(*ctx->buffer[1].data));
+ ctx->buffer[1].size = BUFFER_SIZE;
+
+ ctx->back_buffer_index = 0;
+ ctx->font_buffer_is_consumable = 0;
+ pthread_mutex_init(&ctx->mutex, NULL);
+ pthread_cond_init(&ctx->front_buffer_cond, NULL);
+
+ ret = pthread_create(&consumer_tid, NULL, consumer_cb, (void *)ctx);
+ if (ret != 0)
+ {
+ fprintf(stderr, "can't create thread: %s\n", strerror(ret));
+ exit(1);
+ }
+
+ /* fixed seed, always the same sequence */
+ srand(0xa02);
+ iterations = NUM_ITERATIONS;
+ while (iterations--) {
+ /* produce: in the back buffer */
+ len = snprintf(message, MAXLEN, "Producing value: ");
+ for (i = 0; i < BACK_BUFFER(ctx).size; i++) {
+ BACK_BUFFER(ctx).data[i] = rand();
+ len += snprintf(message + len, MAXLEN - len, "%d ", BACK_BUFFER(ctx).data[i]);
+ }
+ usleep(PRODUCER_DELAY);
+ do_log(message);
+
+ pthread_mutex_lock(&ctx->mutex);
+ while (ctx->font_buffer_is_consumable) {
+ pthread_cond_wait(&ctx->front_buffer_cond, &ctx->mutex);
+ }
+
+ SWAP_BUFFERS(ctx);
+
+ /* signal that the front buffer is ready to be consumed */
+ ctx->font_buffer_is_consumable = 1;
+ pthread_cond_signal(&ctx->front_buffer_cond);
+
+ pthread_mutex_unlock(&ctx->mutex);
+ }
+
+ /* wait for the last consumer run before terminating the thread */
+ pthread_mutex_lock(&ctx->mutex);
+ while (ctx->font_buffer_is_consumable) {
+ pthread_cond_wait(&ctx->front_buffer_cond, &ctx->mutex);
+ }
+ pthread_mutex_unlock(&ctx->mutex);
+
+ pthread_cancel(consumer_tid);
+ pthread_join(consumer_tid, NULL);
+
+ free(ctx->buffer[1].data);
+ free(ctx->buffer[0].data);
+ free(ctx);
+
+ exit(0);
+}
projects / experiments / double-buffering.git / commitdiff