1.线程资源回收
（1）每个线程创建后，可以由主线程调用pthread_detach()函数来让它变为unjoinable状态或者每个线程开始的时候自己调用pthread_detach(pthread_self())，这样线程在退出的时候，会自动释放自己占用的系统资源，包括线程描述符和栈等资源。
（2）通过主线程调用**pthread_join()**函数，阻塞地等待线程结束，收到线程返回值，然后释放线程资源。
（3）涉及线程函数
pthread_create()
pthread_detach()
pthread_join()
pthread_self()
（4）代码示例：

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

char* thread_func1(void* arg) {
    pid_t pid;
    pthread_t tid;

    pid = getpid();
    tid = pthread_self();
    printf("%s pid: %u, tid: %u (0x%x)\n", (char*)arg, (unsigned int)pid, (unsigned int)tid, 
        (unsigned int)tid);

    char* msg = "thread_fucn1";
    return msg;
}

void* thread_func2(void* arg) {
    pid_t pid;
    pthread_t tid;

    pid = getpid();
    tid = pthread_self();
    printf("%s pid: %u, tid: %u (0x%x)\n", (char*)arg, (unsigned int)pid, (unsigned int)tid, 
        (unsigned int)tid);

    char* msg = "thread_fucn2";
    while (1) {
        printf("%s running\n", msg);
        sleep(5);
    }

    return NULL;
}

int main(void) {
    pthread_t tid1;
    pthread_t tid2;
    char* ret = NULL;

    if (pthread_create(&tid1, NULL, (void*)thread_func1, "thread1") != 0) {
        printf("pthread_create tid1 error\n");
        exit(EXIT_FAILURE);
    }

    if (pthread_create(&tid2, NULL, (void*)thread_func2, "thread2") != 0) {
        printf("pthread_create tid2 error\n");
        exit(EXIT_FAILURE);
    }
    
    pthread_detach(tid2);

    pthread_join(tid1, (void*)&ret);
    printf("%s return\n", ret);

    sleep(5);
    pthread_cancel(tid2);

    printf("main thread end\n");
    return 0;
}