2021-05-02

c/c++多线程和线程同步

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c/c++线程的使用

编译-l指定动态链接库pthread

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gcc xxx.c -lpthread -o xxx

1.线程创建

pthread_t是long类型的线程id
pthread_create时callback调用子线程执行的函数，第四个参数是callback传入的void* arg
sleep让子线程执行完再挂断主线程

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#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <pthread.h>

void* callback(void* arg) {
	for (int i = 0; i < 5; i++) {
		printf("子线程i=: %d\n", i);
	}
	printf("子线程id: %ld\n", pthread_self());
	return NULL;
}

int main() {
	pthread_t tid;
	pthread_create(&tid, NULL, callback, NULL);
	for (int i = 0; i < 5; i++) {
		printf("主线程i=: %d\n", i);
	}
	printf("主线程id: %ld\n", pthread_self());
	sleep(1);
	return 0;
}

2.线程退出

主线程退出，子线程变主，回收空间

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#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <pthread.h>

void* callback(void* arg) {
	for (int i = 0; i < 5; i++) {
		printf("子线程i=: %d\n", i);
	}
	printf("子线程id: %ld\n", pthread_self());
	return NULL;
}

int main() {
	pthread_t tid;
	pthread_create(&tid, NULL, callback, NULL);
	//for (int i = 0; i < 5; i++) {
	//	printf("主线程i=: %d\n", i);
	//}
	printf("主线程id: %ld\n", pthread_self());
	//sleep(1);
	pthread_exit(NULL);
	return 0;
}

3.线程回收

join会让主线程阻塞
接受个Test的struct作为资源
直接在子线程开struct Test t会开在栈空间，回收后释放了，主线程肯定拿不到这个资源
要么：struct Test t开在全局变量，join后拿到Test，然后强转下struct Test* pt = (struct Test*)ptr;
要么：在主线程里开struct Test t，然后pthread_join(tid, &ptr)给子线程，子线程在arg拿到，也要强转下struct Test* t = (struct Test*)arg

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#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <pthread.h>

struct Test {
	int num;
	int age;
};

//struct Test t; //开在这里

void* callback(void* arg) {
	for (int i = 0; i < 5; i++) {
		printf("子线程i=: %d\n", i);
	}
	printf("子线程id: %ld\n", pthread_self());

	struct Test* t = (struct Test*)arg;
	t->num = 100;
	t->age = 22;

	pthread_exit(&t); //Test t如果是在子线程中开栈空间后会被释放

	return NULL;
}

int main() {
	struct Test t; //或者开在这里
	pthread_t tid;
	pthread_create(&tid, NULL, callback, &t);
	//for (int i = 0; i < 5; i++) {
	//	printf("主线程i=: %d\n", i);
	//}
	printf("主线程id: %ld\n", pthread_self());
	//sleep(1);
	//pthread_exit(NULL);
	void* ptr;
	pthread_join(tid, &ptr);
	//struct Test* pt = (struct Test*)ptr;
	printf("%主线程: pt.num=%d, pt.age=%d\n", t.num, t.age);
	return 0;
}

4.线程分离

让主线程退出后不会释放子线程的资源
这里子线程创建后和主线程分离，子线程死后由内核调度回收

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#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <pthread.h>

struct Test {
	int num;
	int age;
};

void* callback(void* arg) {
	for (int i = 0; i < 5; i++) {
		printf("子线程i=: %d\n", i);
	}
	printf("子线程id: %ld\n", pthread_self());

	struct Test* t = (struct Test*)arg;
	t->num = 100;
	t->age = 22;

	pthread_exit(&t);

	return NULL;
}

int main() {
	struct Test t; //或者开在这里
	pthread_t tid;
	pthread_create(&tid, NULL, callback, &t);
	printf("主线程id: %ld\n", pthread_self());

	pthread_detach(tid);
	pthread_exit(NULL);

	return 0;
}

5.其他线程函数

pthread_cancel退出子线程，但是会在系统调用之后（printf就是）

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#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <pthread.h>

struct Test {
	int num;
	int age;
};

void* callback(void* arg) {
	for (int i = 0; i < 5; i++) {
		printf("子线程i=: %d\n", i);
	}
	printf("子线程id: %ld\n", pthread_self()); //printf系统调用了

	struct Test* t = (struct Test*)arg;
	t->num = 100;
	t->age = 22;

	pthread_exit(&t);

	return NULL;
}

int main() {
	struct Test t; //或者开在这里
	pthread_t tid;
	pthread_create(&tid, NULL, callback, &t);
	printf("主线程id: %ld\n", pthread_self());

	pthread_detach(tid);
	pthread_cancel(tid); //杀死子进程

	pthread_exit(NULL); //主线程退出
	

	return 0;
}

c/c++线程的同步和互斥锁

不同步

很明显这没有同步，最后不是100

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#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <pthread.h>

int number = 0; //全局资源

void* funcA(void* arg) {
	for (int i = 0; i < 50; i++) {
		int cur = number;
		cur++;
		usleep(10);
		number = cur;
		printf("Thread A, id = %lu, number = %d\n", pthread_self(), number);
	}
	return NULL;
}

void* funcB(void* arg) {
	for (int i = 0; i < 50; i++) {
		int cur = number;
		cur++;
		number = cur;
		usleep(5);
		printf("Thread B, id = %lu, number = %d\n", pthread_self(), number);
	}
	return NULL;
}

int main() {
	pthread_t p1, p2;

	pthread_create(&p1, NULL, funcA, NULL);
	pthread_create(&p2, NULL, funcB, NULL);

	// 阻塞 & 回收
	pthread_join(p1, NULL);
	pthread_join(p2, NULL);

	return 0;
}

互斥锁

锁：状态（锁/没锁）；加锁信息（线程id）

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#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <pthread.h>

int number = 0; //全局资源
pthread_mutex_t mutex;

void* funcA(void* arg) {
	for (int i = 0; i < 50; i++) {
		pthread_mutex_lock(&mutex);
		int cur = number;
		cur++;
		usleep(10);
		number = cur;
		pthread_mutex_unlock(&mutex);
		printf("Thread A, id = %lu, number = %d\n", pthread_self(), number);
	}
	return NULL;
}

void* funcB(void* arg) {
	for (int i = 0; i < 50; i++) {
		pthread_mutex_lock(&mutex);
		int cur = number;
		cur++;
		number = cur;
		pthread_mutex_unlock(&mutex);
		printf("Thread B, id = %lu, number = %d\n", pthread_self(), number);
		usleep(5);
	}
	return NULL;
}

int main() {
	pthread_t p1, p2;

	pthread_mutex_init(&mutex, NULL);

	pthread_create(&p1, NULL, funcA, NULL);
	pthread_create(&p2, NULL, funcB, NULL);

	// 阻塞 & 回收
	pthread_join(p1, NULL);
	pthread_join(p2, NULL);

	pthread_mutex_destroy(&mutex);

	return 0;
}
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