fork 线程与锁

给群里的;

群里的人问, 多线程下调用fork , 但子进程没办法执行下去了,原因是子进程中也访问了同一把锁;

fork的子进程将继承父进程锁的状态 , 如果fork时此锁已经被锁住了;

一个模拟的例子:

#include "util.h"
#include 
#include 
#include 
#include 
#include 
pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;

void * func1(void *arg)
{
    printf("thread start : %ld\n" , pthread_self());
    pthread_mutex_lock(&mutex);
    puts("thread enter lock");
    sleep(5); //睡5秒
    pthread_mutex_unlock(&mutex);
}


int main(int argc, char**argv)
{
    pthread_t t1;
    pthread_create(&t1,0,func1,NULL);
    sleep(1);
    int pid = fork();
    if(0 == pid){
        //子进程
        //解决办法是不管有没有锁住,先解锁再说
        //pthread_mutex_unlock(&mutex); 
    

        puts("child begin");

    //由于此时的mutex 是被锁住的, 因此,子进程将永远等在这里
        pthread_mutex_lock(&mutex);
        puts("??????????");
        pthread_mutex_unlock(&mutex);
        return 0;
    }
    //parent




    pthread_join(t1,0);
    puts("begin wait child");
    wait(NULL);
    return 0;
}

 

代码中注释的是解决办法之一

 

另一种是 pthread_atfork:

#include "util.h"
#include 
#include 
#include 
#include 
#include 
pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;

void * func1(void *arg)
{
    printf("thread start : %ld\n" , pthread_self());
    pthread_mutex_lock(&mutex);
    puts("thread enter lock");
    sleep(5);
    pthread_mutex_unlock(&mutex);
}
void prepare(){
    puts("preapre");
    pthread_mutex_lock(&mutex);
}
void parent_handler(){
    puts("parent handler");
    pthread_mutex_unlock(&mutex);
}
void child_handler(){
    puts(" ---------- child handler");
    pthread_mutex_unlock(&mutex);
}

int main(int argc, char**argv)
{
    pthread_t t1;
    pthread_create(&t1,0,func1,NULL);
    pthread_atfork(0,0,child_handler); //统一处理锁的问题
    sleep(1);
    int pid = fork();
    if(0 == pid){
        puts("child begin");
        pthread_mutex_lock(&mutex);
        puts("??????????");
        pthread_mutex_unlock(&mutex);
        return 0;
    }
    //parent




    pthread_join(t1,0);
    puts("begin wait child");
    wait(NULL);
    return 0;
}

 

 

上面代码中只用了child_handler, 在子进程一开始就调用了;

另2个参数;

    // prepare 在fork调用之前就被调用
    // parent_handler在fork返回父进程前被调用

    pthread_atfork(prepare,parent_handler,child_handler);