信号量是编写并发程序的强大而灵活的原语,有程序员会因为简单实用,只用信号量, 不用锁和条件变量
信号量
信号量是一个整数值对象。
int sem_wait(sem_t *s)
{
// 将信号量减一,如果是负数,就将线程挂起,
// 负数的值就是等待唤醒的数量,虽然这个值通常不会暴露给使用者
}
int sem_post(sem_t *s)
{
// 将信号量加一,如果有1个或多个等待,唤醒一个
}
互斥锁
void semaphoreLock()
{
/*
static pthread_mutex_t lock = PTHREAD_MUTEX_INITIALIZER;
pthread_mutex_lock(&lock);
// 临界区
pthread_mutex_unlock(&lock);
*/
static sem_t semaphore;
sem_init(&semaphore, 0, 1);
sem_wait(&semaphore);
// 临界区
sem_post(&semaphore);
}
join实现
遗憾的是Mac OS X废弃了sem_init, 取而代之的是sem_open和sem_unlink
#include <pthread/pthread.h>
#include <semaphore.h>
static sem_t *semaphore;
static void *childThread(void *args)
{
printf("child start\n");
sem_post(semaphore);
printf("child end\n");
return NULL;
}
static int testEntry()
{
sem_unlink("semaphore");
semaphore = sem_open("semaphore", O_CREAT | O_EXCL, S_IRWXU, 0);
printf("parent start\n");
pthread_t p;
pthread_create(&p, NULL, childThread, NULL);
sem_wait(semaphore);
printf("parent end\n");
return 0;
}
生产者消费者
#include <pthread/pthread.h>
#include <semaphore.h>
#define kBufferSize 2
static pthread_mutex_t lock = PTHREAD_MUTEX_INITIALIZER;
static pthread_cond_t empty = PTHREAD_COND_INITIALIZER;
static pthread_cond_t full = PTHREAD_COND_INITIALIZER;
static int buffer[kBufferSize];
static int count = 0; // 大小为1
static int use = 0;
static int fill = 0;
static void put(int value)
{
buffer[fill] = value;
fill = (fill + 1) % kBufferSize;
count++;
}
static int get()
{
int tmp = buffer[use];
use = (use + 1) % kBufferSize;
count--;
return tmp;
}
static void *producer(void *arg)
{
int loops = (int)arg;
for (int i = 0; i < loops; ++i) {
pthread_mutex_lock(&lock);
while (count == kBufferSize) {
pthread_cond_wait(&empty, &lock);
}
put(i);
printf("produce %d \n", i);
pthread_cond_signal(&full);
pthread_mutex_unlock(&lock);
}
return NULL;
}
static void *consumer(void *arg)
{
int loops = (int)arg;
for (int i = 0; i < loops; ++i) {
pthread_mutex_lock(&lock);
while (count == 0) {
pthread_cond_wait(&full, &lock);
}
int tmp = get();
printf("consume %d \n", tmp);
pthread_cond_signal(&empty);
pthread_mutex_unlock(&lock);
}
return NULL;
}
static int testEntry()
{
printf("parent start\n");
pthread_t p1, p2, c1, c2;
pthread_create(&p1, NULL, producer, (void *)10);
pthread_create(&p2, NULL, producer, (void *)10);
pthread_create(&c1, NULL, consumer, (void *)10);
pthread_create(&c2, NULL, consumer, (void *)10);
printf("parent end\n");
return 0;
}
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