According to the 1999 ISO C standard (C99), size_t is an unsigned integer type of at least 16 bit (see sections 7.17 and 7.18.3).
size_t 能存储理论上可行的任何类型（包括数组）对象的最大大小。
other articles
1. https://www.embedded.com/why-size_t-matters/
2. https://stackoverflow.com/questions/131803/unsigned-int-vs-size-t?noredirect=1&lq=1