最好不要同时使用多线程多进程,两者择其一。比如在多线程程序中调用fork容易出现死锁。下面取一个例子说明这种情况,先看死锁代码。
#include <stdio.h>
#include <time.h>
#include <pthread.h>
#include <unistd.h>
pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;
void *doit(void *arg)
{
printf("pid = %d begin doit ...\n", static_cast<int>(getpid()));
pthread_mutex_lock(&mutex);
struct timespec ts = {2, 0};
nanosleep(&ts, NULL);
pthread_mutex_unlock(&mutex);
printf("pid = %d end doit ...\n", static_cast<int>(getpid()));
return NULL;
}
int main(void)
{
printf("pid = %d Entering main ...\n", static_cast<int>(getpid()));
pthread_t tid;
pthread_create(&tid, NULL, doit, NULL);
struct timespec ts = {1, 0};
nanosleep(&ts, NULL);
if (fork() == 0)
{
doit(NULL);
}
pthread_join(tid, NULL);
printf("pid = %d Exiting main ...\n", static_cast<int>(getpid()));
return 0;
}
上面函数主线程先调用pthread_create()创建一个子线程执行doit(),doit()里面先加锁,睡眠2s;主线程睡眠1s后调用fork(),子进程会复制父进程的内存映像,此时全局变量mutex处于加锁的状态,所以子进程自己的mutex也是加锁的
,此时子进程是独立运行的,也去执行doit(),在里面试图加锁,因为本来mutex已经加锁
,而且根本没有人会来解锁,所以子进程就会死锁。
pthread_atfork()函数
#include <pthread.h>
int pthread_atfork(void (*prepare)(void), void (*parent)(void), void (*child)(void));
pthread_atfork()在fork()之前
调用。当调用fork时,内部创建子进程前
在父进程中会调用prepare,内部创建子进程成功后
,父进程会调用parent ,子进程会调用child。
#include <stdio.h>
#include <time.h>
#include <pthread.h>
#include <unistd.h>
void prepare(void)
{
printf("pid = %d prepare ...\n", static_cast<int>(getpid()));
}
void parent(void)
{
printf("pid = %d parent ...\n", static_cast<int>(getpid()));
}
void child(void)
{
printf("pid = %d child ...\n", static_cast<int>(getpid()));
}
int main(void)
{
printf("pid = %d Entering main ...\n", static_cast<int>(getpid()));
pthread_atfork(prepare, parent, child);
fork();
printf("pid = %d Exiting main ...\n", static_cast<int>(getpid()));
return 0;
}
用pthread_atfork()来解决死锁问题
#include <stdio.h>
#include <time.h>
#include <pthread.h>
#include <unistd.h>
pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;
void *doit(void *arg)
{
printf("pid = %d begin doit ...\n", static_cast<int>(getpid()));
pthread_mutex_lock(&mutex);
struct timespec ts = {2, 0};
nanosleep(&ts, NULL);
pthread_mutex_unlock(&mutex);
printf("pid = %d end doit ...\n", static_cast<int>(getpid()));
return NULL;
}
void prepare(void)
{
pthread_mutex_unlock(&mutex);
}
void parent(void)
{
pthread_mutex_lock(&mutex);
}
int main(void)
{
pthread_atfork(prepare, parent, NULL);
printf("pid = %d Entering main ...\n", static_cast<int>(getpid()));
pthread_t tid;
pthread_create(&tid, NULL, doit, NULL);
struct timespec ts = {1, 0};
nanosleep(&ts, NULL);
if (fork() == 0) // 子进程执行doit
{
doit(NULL);
}
pthread_join(tid, NULL);
printf("pid = %d Exiting main ...\n", static_cast<int>(getpid()));
return 0;
在执行fork()创建子进程之前,先执行prepare(),将子线程加锁的mutex解锁下,然后为了与doit()配对,在创建子进程成功后,父进程调用parent()再次加锁,这时父进程的doit()就可以接着解锁执行下去
。而对于子进程来说,由于在fork()创建子进程之前,mutex已经被解锁,故复制的状态也是解锁的
,所以执行doit()就不会死锁了
有关锁的小实验
没有加锁的情况下,解锁会不会有问题
加两次锁,会死锁,解两次呢