线程互斥。在 main函数中创建一新线程，然后main线程和这个新线程共享同一变量，如 sum。在 main函数中，编写一个循环语句共循环 20 次，在每循环中先把sum的值加 1，然后休眠1毫秒。在新线程中，也 毫秒。在新线程中，也编写一个循环语句，总共循环20 次，在每次循环中先把sum 赋值给一个临时变量t，然后把 t加 1，然后休眠 1毫秒，最后把 t的值赋给sum 。当上述工作全部完成后，在main函数中打印sum的值。(Thread mutual exclusion. Create a new thread in the main function, and then the main thread shares the same variable with the new thread, such as sum. In the main function, a loop statement is written in a total cycle of 20 times, and the value of the sum is first added to 1 in each loop and then dormant for 1 milliseconds. In the new thread, also milliseconds. In the new thread, a circular statement is also written, which has 20 cycles altogether. In each cycle, assign sum to a temporary variable t first, then add t to 1, then sleep 1 milliseconds, and finally assign the value of t to sum. When all of the work is done, the value of the sum is printed in the main function.)