关于POSIX的互斥锁和条件锁 个人理解

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

static pthread_mutex_t mutex=PTHREAD_MUTEX_INITIALIZER;
static pthread_cond_t cond=PTHREAD_COND_INITIALIZER;

int g_flag=0;
pthread_t tid1,tid2;
void * thread1()
{
    printf("this is thread1\n");
    printf("this is thread1,g_flag=%d,thread id is %u\n",g_flag,(unsigned int)pthread_self());
    pthread_mutex_lock(&mutex);
    if(g_flag==2)
    {
        g_flag=1;
        pthread_cond_signal(&cond);
    }
    g_flag=1;
    pthread_mutex_unlock(&mutex);//不要写程unclock
    printf("this is thread1,g_flag=%d,thread id is %u\n",g_flag,(unsigned int)pthread_self());
    pthread_join(tid2,NULL);
    printf("leave thread1\n");
    pthread_exit(0);

}

void * thread2()
{
    printf("this is thread2\n");
    printf("this is thread2,g_flag=%d,thread id is %u\n",g_flag,(unsigned int)pthread_self());
    pthread_mutex_lock(&mutex);
    if(g_flag==1)
    {
        g_flag=2;
        pthread_cond_signal(&cond);
    }
    g_flag=2;
    pthread_mutex_unlock(&mutex);
    printf("this is thread2,g_flag=%d,thread id is %u\n",g_flag,(unsigned int)pthread_self());
    printf("leave thread2\n");
    pthread_exit(0);

}

int main()
{
    int rc1=pthread_create(&tid1,NULL,thread1,NULL);
    if(rc1!=0)
    {
        printf("can't create rc1\n");
        return(-1);
    }

    int rc2=pthread_create(&tid2,NULL,thread2,NULL);
    if(rc2!=0)
    {
        printf("can't create rc2\n");
        return(-1);
    }

    pthread_cond_wait(&cond,&mutex);
    printf("________________________leave main\n");
    return 1;
}