c - Stack variables vs. Heap variables

Question

Am I correct in thinking that:

char *buff[500];

... creates a stack variable, and:

char *buff = (char *)malloc(500);

... creates a heap variable?

If that's correct, when and why would you use heap variables over stack variables and vice versa. I understand the stack is faster is there anything else.

One last question, is the main function a stack frame on the stack?

Answer 1

Yes, first one creates an array of char pointers in the stack, about 500*4 bytes and second one allocates 500 chars in the heap and points a stack char ptr to them.

Allocating in the stack is easy and fast, but stack is limited, heap is slower but much bigger. Apart from that, stack allocated values are "deleted" once you leave the scope, so it is very good for small local values like primitive variables.

If you allocate too much in the stack you might run out of stack and die, main as all the functions you execute has a stack frame in the stack and all the local variables to the function are stored there, so going too deep into function calling might get you into a stackoverflow as well.

In general is a good rule of thumb to allocate anything that you use often and is bigger than a hundred bytes in the heap, and small variables and pointers in the stack.