一句话总结：父进程创建子进程时，子进程会复制父进程的内存（包括锁状态），需仔细处理。
1、死锁例子
#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;
}
运行结果：
通过查询进程可以发现死锁了，子进程3071无法往下执行。
原因：父进程创建的线程调用doit()后，对mutex加了锁；此时fork了一个子进程，子进程复制父进程的内存，包括此时的mutex锁状态；父进程接着往后执行直到结束，而子进程调用了doit()，此时mutex处于加锁状态，一直等待，造成死锁。2、解决死锁
如何解决上述问题呢，我们需要在fork子进程前将锁释放掉，fork之后再将父进程的锁加上。用
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。
不过还是尽量少将多进程多线程混在一起。
#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)
{
printf("pid = %d prepare ...\n", static_cast<int>(getpid()));
pthread_mutex_unlock(&mutex);
}
void parent(void)
{
printf("pid = %d parent ...\n", static_cast<int>(getpid()));
pthread_mutex_lock(&mutex);
}
void child(void)
{
printf("pid = %d child ...\n", static_cast<int>(getpid()));
}
int main(void)
{
pthread_atfork(prepare, parent, child);
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;
}