读写锁使用代码的疑问？？大神好心答疑...哭了

[拥抱@]

#include <errno.h>
#include <pthread.h>
#include <stdio.h>
#include <stdlib.h>
#include <bits/pthreadtypes.h>

static pthread_rwlock_t rwlock;//读写锁对象

#define WORK_SIZE 1024
char work_area[WORK_SIZE];
int time_to_exit;

void *thread_function_read_o(void *arg);//读线程1
void *thread_function_read_t(void *arg);//读线程2
void *thread_function_write_o(void *arg);//写线程1
void *thread_function_write_t(void *arg);//写线程2


int main(int argc,char *argv[])
{
int res;
pthread_t a_thread,b_thread,c_thread,d_thread;
void *thread_result;

res=pthread_rwlock_init(&rwlock,NULL);//初始化读写锁
if (res != 0)
{
perror("rwlock initialization failed");
exit(EXIT_FAILURE);
}
res = pthread_create(&a_thread, NULL, thread_function_read_o, NULL);//create new thread创建线程
if (res != 0)
{
perror("Thread creation failed");
exit(EXIT_FAILURE);
}

res = pthread_create(&b_thread, NULL, thread_function_read_t, NULL);//create new thread
if (res != 0)
{
perror("Thread creation failed");
exit(EXIT_FAILURE);
}
res = pthread_create(&c_thread, NULL, thread_function_write_o, NULL);//create new thread
if (res != 0)
{
perror("Thread creation failed");
exit(EXIT_FAILURE);
}
res = pthread_create(&d_thread, NULL, thread_function_write_t, NULL);//create new thread
if (res != 0)
{
perror("Thread creation failed");
exit(EXIT_FAILURE);
}

res = pthread_join(a_thread, &thread_result);//等待a_thread线程结束
if (res != 0)
{
perror("Thread join failed");
exit(EXIT_FAILURE);
}
res = pthread_join(b_thread, &thread_result);
if (res != 0)
{
perror("Thread join failed");
exit(EXIT_FAILURE);
}
res = pthread_join(c_thread, &thread_result);
if (res != 0)
{
perror("Thread join failed");
exit(EXIT_FAILURE);
}
res = pthread_join(d_thread, &thread_result);
if (res != 0)
{
perror("Thread join failed");
exit(EXIT_FAILURE);
}

pthread_rwlock_destroy(&rwlock);//销毁读写锁
exit(EXIT_SUCCESS);
}

void *thread_function_read_o(void *arg)
{
printf("thread read one try to get lock\n");

pthread_rwlock_rdlock(&rwlock);//获取读取锁
while(strncmp("end", work_area, 3) != 0)
{
printf("this is thread read one.");
printf("the characters is %s",work_area);
pthread_rwlock_unlock(&rwlock);
sleep(2);
pthread_rwlock_rdlock(&rwlock);
while (work_area[0] == '\0' )
{
pthread_rwlock_unlock(&rwlock);
sleep(2);
pthread_rwlock_rdlock(&rwlock);
}
}
pthread_rwlock_unlock(&rwlock);
time_to_exit=1;
pthread_exit(0);
}

void *thread_function_read_t(void *arg)
{
printf("thread read one try to get lock\n");
pthread_rwlock_rdlock(&rwlock);
while(strncmp("end", work_area, 3) != 0)
{
printf("this is thread read two.");
printf("the characters is %s",work_area);
pthread_rwlock_unlock(&rwlock);
sleep(5);
pthread_rwlock_rdlock(&rwlock);
while (work_area[0] == '\0' )
{
pthread_rwlock_unlock(&rwlock);
sleep(5);
pthread_rwlock_rdlock(&rwlock);
}
}
pthread_rwlock_unlock(&rwlock);
time_to_exit=1;
pthread_exit(0);
}

void *thread_function_write_o(void *arg)
{
printf("this is write thread one try to get lock\n");
while(!time_to_exit)
{
pthread_rwlock_wrlock(&rwlock);
printf("this is write thread one.\nInput some text. Enter 'end' to finish\n");
fgets(work_area, WORK_SIZE, stdin);
pthread_rwlock_unlock(&rwlock);
sleep(15);
}
pthread_rwlock_unlock(&rwlock);
pthread_exit(0);
}

void *thread_function_write_t(void *arg)
{
sleep(10);
while(!time_to_exit)
{
pthread_rwlock_wrlock(&rwlock);//获取写入锁
printf("this is write thread two.\nInput some text. Enter 'end' to finish\n");
fgets(work_area, WORK_SIZE, stdin);//写入
pthread_rwlock_unlock(&rwlock);//解锁
sleep(20);
}
pthread_rwlock_unlock(&rwlock);//解锁
pthread_exit(0);
}



//////////////以上是网上看到的示例代码///////////
但是有好几处疑惑？
1. 读者中 while(strncmp("end", work_area, 3) != 0) 判断条件是什么意思？读者每次最后int time_to_exit=1？

如果先获得读锁，然后循环进不去，解锁，int time_to_exit=1；当写者获得了锁，那么按照写者代码岂不是永远写不进去东西了吗？



2.获得读锁后，假设进入到循环，循环体为什么那样写？反复的加锁解锁的意义是什么？为什么还要 while (work_area[0] == '\0' ) ？？