在linux中c++ size_t定义在哪里

Now I'm talking about new type definition by a programmer using typedef keyword. As long as my pupils are used to the type size_t (for example by using funciton length ()), for which I had to ask them a little effort to just "believe" it is an integer type, I think it woud be great to show noew them where this type is defined.Now I'm talking about new type definition by a