线程信号处理的问题

Hovernoon

之前用信号都是在进程中用的，最近的作业要用到线程的信号处理，自己写了个测试程序，结果很诡异，希望懂行的给指点一二
测试程序的流程是在主线程中安装信号处理句柄，然后创建几个线程，然后主线程向子线程发信号，子线程退出，代码是这样的：
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <errno.h>
#include <sys/types.h>
#include <pthread.h>
#include <signal.h>

#define THREADS 4
#define erraise(string) do{\
    perror(string);\
    exit(-1);\
}while(0)


void *thread_func(void *argv)
{
    while(1);
    return (void *)0;
}

void handler(int sig, siginfo_t *si, void *unused)
{
    printf("Signal recieved!\n");
    pthread_exit((void *)0);
}
int main()
{
    pthread_t thread[THREADS];
    struct sigaction sa;
    sa.sa_flags = SA_SIGINFO;
    sigemptyset(&sa.sa_mask);
    sa.sa_sigaction = handler;
    sigaction(SIGUSR1, &sa, NULL);
    int i;
    for (i = 0; i < THREADS; i++)
    {
        pthread_create(&thread[i], NULL, thread_func, NULL);
        printf("thread %lu created!\n", thread[i]);
    }
    printf("Father thread: %lu\n", (unsigned long)pthread_self());
    for (i = 0; i < THREADS; i++)
        pthread_kill(thread[i], SIGUSR1);
    for (i = 0; i < THREADS; i++)
    {
        pthread_join(thread[i], NULL);

        printf("Thread %lu recieved signal and exited!\n", thread[i]);
    }
    printf("Father thread exit!\n");
    return 0;
}

运行多次，大部分情况下，程序运行会出现断错误：
thread 3078339440 created!
thread 3069946736 created!
thread 3061554032 created!
thread 3053161328 created!
Father thread: 3078342336
Signal recieved!
Signal recieved!
Signal recieved!
Signal recieved!
Segmentation fault

不过少数情况下结果是正确的（四个子线程都正常退出），也就是说我的程序结果是不确定的，有人知道是什么情况吗?