c – 如何使用pthreads读/写共享变量？

我有两个线程,在 linux上使用C pthreads.其中一个写入数据,另一个正在读取数据.我正在使用一个变量来允许读取线程,当允许读取时,写入一个是允许的.因此,互斥锁适用于这个名为“newData”的布尔变量.我的问题是：我是否需要在“if”条件内的访问周围锁定/解锁互斥锁？两种方式都有效,但我认为只是因为在这个变量上重写写/读的可能性很小.我展示了两种选择来更好地解释我的问题：

线程1：

pthread_mutex_lock( &lattice_mutex );
if (!newData) {
    pthread_mutex_unlock( &lattice_mutex );
    uchar *lattice_pos = lattice;
    int i;
    for(i=0; i<size; i++) {
        *lattice_pos = rand()%CHAR_MAX;
        lattice_pos++;
    }
    pthread_mutex_lock( &lattice_mutex );
    newData = TRUE;
    pthread_mutex_unlock( &lattice_mutex );
} else {
    pthread_mutex_unlock( &lattice_mutex );
}

线程2：

pthread_mutex_lock( &lattice_mutex );
if(newData) {
    pthread_mutex_unlock( &lattice_mutex );
    renderUpdate();
    pthread_mutex_lock( &lattice_mutex );
    newData = FALSE;
    pthread_mutex_unlock( &lattice_mutex );
} else {
    pthread_mutex_unlock( &lattice_mutex );
}

第二个版本,但有效,但我不知道它是否正确：

线程1：

if (!newData) {
    uchar *lattice_pos = lattice;
    int i;
    for(i=0; i<size; i++) {
        *lattice_pos = rand()%CHAR_MAX;
        lattice_pos++;
    }
    pthread_mutex_lock( &lattice_mutex );
    newData = TRUE;
    pthread_mutex_unlock( &lattice_mutex );
}

线程2：

if(newData) {
    renderUpdate();
    pthread_mutex_lock( &lattice_mutex );
    newData = FALSE;
    pthread_mutex_unlock( &lattice_mutex );
}

pthread_mutex_lock(&lattice_mutex);
if (!newData) {
    pthread_mutex_unlock(&lattice_mutex);  // Omit?
    uchar *lattice_pos = lattice;
    int i;
    for (i = 0; i < size; i++)
        *lattice_pos++ = rand() % CHAR_MAX;
    pthread_mutex_lock(&lattice_mutex);   // Omit?
    newData = TRUE;
}
pthread_mutex_unlock(&lattice_mutex);

pthread_mutex_lock(&lattice_mutex);
if (newData) {
    pthread_mutex_unlock(&lattice_mutex);   // Omit?
    renderUpdate();
    pthread_mutex_lock(&lattice_mutex);     // Omit?
    newData = FALSE;
}
pthread_mutex_unlock(&lattice_mutex);