信号量操作。

1. 信号量相关函数

1.1 信号量声明

信号量类型为sem_t，本质上是长整型的数。

//semaphores
sem_t mutex;

1.2 信号量初始化

使用函数sem_init对信号量进行初始化。

#include <semaphore.h>

// initialize an unnamed semaphore
int sem_init(sem_t *sem, int pshared, unsigned int value);

参数：

• pshared，若为0，则信号量在线程间共享；若为非0，则信号量在进程间共享

返回值：

• 0 on success
• -1 on error, errno is set to indicate the error.

1.3 加锁

使用函数sem_wait，

#include <semaphore.h>

// lock a semaphore

// decrements (locks) the semaphore pointed to by sem.
int sem_wait(sem_t *sem);

// same as sem_wait(), except that if the decrement cannot be immediately performed, then call returns an error
int sem_trywait(sem_t *sem);

// same as sem_wait(), except that abs_timeout specifies a limit on the amount of time that the call should block if the decrement cannot be immediately performed.
int sem_timedwait(sem_t *restrict sem, const struct timespec *restrict abs_timeout);

1.4 解锁

使用函数sem_post，

#include <semaphore.h>

// unlock a semaphore
int sem_post(sem_t *sem);

1.5 释放信号量

使用函数sem_destroy，

#include <semaphore.h>

// destroy an unnamed semaphore
int sem_destroy(sem_t *sem);

2. 一个实例

#include <semaphore.h>
#include <stdlib.h>

int main(){
    // declare a semaphore
    sem_t mutex;
    //init
    sem_init(&mutex, 0, 1);

    // lock and unlock
    sem_wait(&mutex);
    // critical section
    sem_post(&mutex);

    // destroy the semaphore
    sem_destroy(&mutex);

    return EXIT_SUCCESS;
}

编译的时候，加上选项-pthread，

gcc -pthread -o sem_example sem_example.c

-pthread Link with the POSIX threads library. On some targets this option also sets flags for the preprocessor, so it should be used consistently for both compilation and linking.