为什么字符串的副本不生效？

Question

我正在做一些多线程编程。 它是一个多读者，单一作家模型。 我想我以正确的方式使用pthread_rwlock。当我分配给字符串并进入睡眠状态时，对字符串的赋值总是为空。 有人能告诉我我的代码有什么问题吗？ THX。

请参阅代码中的FIXME注释。

#include "util/tc_thread.h"
#include "util/tc_thread_rwlock.h"

using namespace taf;

static string g_buffer;
static pthread_rwlock_t g_lock;

class TestThread: public TC_Thread
{
    public:
        TestThread(int type, const string &_msg): m_type(type), m_msg(_msg)
    {
        if (type == 2)
        {
            printf("writer get m_msg: %s\n", m_msg.c_str());
        }
    }
        ~TestThread()
        {
            //printf("Destroy thread of type %d\n", m_type);
        }
    protected:
        void run()
        {
            if (m_type == 1)
            {
                //Reader
                pthread_rwlock_rdlock(&g_lock);
                printf("%s\n", g_buffer.c_str());
                usleep(2000000); //FIXME::If I drop this line or move it below pthread_rwlock_unlock, the two ways of assign to g_buffer both take effects.
                pthread_rwlock_unlock(&g_lock);

            }
            else if (m_type == 2)
            {
                //Writer
                pthread_rwlock_wrlock(&g_lock);
                printf("==succ: %s\n", g_buffer.c_str());
                printf("==succ: %s\n", m_msg.c_str());
                g_buffer = m_msg; //FIXME::Here, the g_buffer will be empty
                //g_buffer = m_msg.data(); //FIXME:: If I assign g_buffer like this, the result is right.
                printf("==succ: %s\n", g_buffer.c_str());
                printf("==succ: %s\n", m_msg.c_str());
                pthread_rwlock_unlock(&g_lock);
            }
            else if (m_type == 3)
            {
                while (1)
                {
                    printf("type 3 thread is working\n");
                    sleep(1);
                }
            }
            pthread_exit((void *)(0));
        }
    private:
        int m_type;
        string m_msg;
};

int main(int argc, char **argv)
{
    pthread_rwlockattr_t attr;
    pthread_rwlockattr_setkind_np(&attr, PTHREAD_RWLOCK_PREFER_WRITER_NONRECURSIVE_NP);
    pthread_rwlockattr_setpshared(&attr, PTHREAD_PROCESS_SHARED);
    pthread_rwlock_init(&g_lock, &attr);

    TestThread trd(3, "Hello1");
    trd.start();

    TestThread fst(2, "Hello1");
    fst.start();
    for (int i = 0; i < 10; ++i)
    {
        TestThread t(1, "Hello 9");
        t.start();
    }
    TestThread snd(2, "Hello2");
    snd.start();
    for (int i = 0; i < 10; ++i)
    {
        TestThread t(1, "Hello 10");
        t.start();
    }
    pthread_exit((void *)(0));
}

=============================================== ==

当作者线程snd尝试将g_buffer设置为＆＃34; Hello2＆＃34;时，g_buffer变为空。 TC_Thread是一个私人图书馆，我没有代码。但它经过了彻底的测试，所以不可能成为这个问题的原因。