The Stack and Heap

Question 1

What happens to the stack when we do a function call? What happens when we return from this function?

Answer 1

A new stack frame is created for this new function. When we return from this function, the stack frame we created is no longer accessible.

Question 2

What does a stack frame contain?

Answer 2

The arguments of the procedure, a place to hold local variables, information to be able to return from the call.

Question 3

What are the three main registers used to implement the stack?

Answer 3

1) Instruction Pointer, which references the next instruction to execute. 2) The stack pointer, which references the top of the stack. 3) The stack base pointer, which is a reference to the current stack frame.

Question 4

When we have made a function call to foo, we will have a pointer to the previous stack frame base pointer (EBP). What will we find if we look at the stack addresses: ESB -1, ESB -2, ESB -3.

Answer 4

ESB -1 is a pointer to the next instruction which will be executed after the function, ESB -2 is a pointer to where the results of the function will be stored, ESB -1 is a pointer to the arguments of the function.

Question 5

Why might you not see some of the arguments on the stack in X_86?

Answer 5

Because in X_86, the first 6 arguments are placed in dedicated registers rather than the stack.

Question 6

Why shouldn’t you return pointers to local variables in functions?

Answer 6

These are written on the stack, so may be overwritten. The same goes for returning data structures.

Question 7

How would you use malloc to create space for an array of 5 integers?

Answer 7

int *h = (int *) malloc (5 * sizeof(int));

Question 8

What do calloc and realloc do?

Answer 8

calloc works like malloc, but initialises all values to 0. realloc can be used to change the size of a structure pointed to by ptr.