信号量(Semaphore),是一个特殊类型的变量,它可以被增加或者减少,对其的关键访问被保证是原子操作。
在进入一个关键代码段之前,线程必须获取一个信号量;一旦该关键代码段完成了,那么该线程必须释放信号量。
其它想进入该关键代码段的线程必须等待直到第一个线程释放信号量。
代码:thread3.c
#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <string.h>
#include <pthread.h>
#include <semaphore.h>
void *thread_function(void *arg);
// 定义一个信号量
sem_t bin_sem;
#define WORK_SIZE 1024
char work_area[WORK_SIZE];
int main() {
int res;
pthread_t a_thread;
void *thread_result;
// 初始化信号量bin_sem为0
res = sem_init(&bin_sem, 0, 0);
if (res != 0) {
perror("Semaphore initialization failed");
exit(EXIT_FAILURE);
}
// 启动新线程
res = pthread_create(&a_thread, NULL, thread_function, NULL);
if (res != 0) {
perror("Thread creation failed");
exit(EXIT_FAILURE);
}
printf("Input some text. Enter 'end' to finish\n");
while (strncmp("end", work_area, 3) != 0) {
// 从键盘读取文本放到工作区work_area数组中
fgets(work_area, WORK_SIZE, stdin);
// 增加信号量的值
sem_post(&bin_sem);
}
printf("\nWaiting for thread to finish...\n");
res = pthread_join(a_thread, &thread_result);
if (res != 0) {
perror("Thread join failed");
exit(EXIT_FAILURE);
}
printf("Thread joined\n");
sem_destroy(&bin_sem);
exit(EXIT_SUCCESS);
}
void *thread_function(void *arg) {
// 阻塞进程并等待信号量bin_sem变为非零值
sem_wait(&bin_sem);
// 统计来自输入的字符个数
while (strncmp("end", work_area, 3) != 0) {
printf("You input %d characters\n", strlen(work_area) - 1);
sem_wait(&bin_sem);
}
pthread_exit(NULL);
}
编译(编译时需要加选项 -lphread):
gcc -g thread3.c -o thread3 -lpthread
运行:
./thread3
结果:
Input some text. Enter 'end' to finish
sade
You input 5 characters
afsf
You input 8 characters
You input 7 characters
asdas
You input 6 characters
end
Waiting for thread to finish...
Thread joined