printf函数size_t的替换字符串zu

参考：
https://stackoverflow.com/questions/2524611/how-can-one-print-a-size-t-variable-portably-using-the-printf-family

size_t x = 111;
ssize_t y = -222;
printf("%zu\n", x);  // prints as unsigned decimal
printf("%zx\n", x);  // prints as hex
printf("%zd\n", y);  // prints as signed decimal