Calling in matlab is an application program that passes a const pointer and an integer. The cal function passes a const pointer and an integer application. In cal, first assign an integer value of 0, and then start the loop. When *p is zero, the loop ends. Here, the timing of the loop is determined by the memory space allocated for str in the system. Because you didn't assign a value to str, the program will execute according to the unknown number originally stored in memory, and stop the loop when it encounters 0.