Procedure Call Sequence in C

 

When you call a function in C, you need to ensure mainly 5 things:

  1. Store the returning address somewhere, so that we can continue executing the current function after completion of called function.
  2. Saving other information about current function. (Later, we'll see that it boils down to saving current frame pointer (EBP) somewhere.)
  3. Providing the callee with the parameters.
  4. Providing called function some space to store its automatic variables.
  5. Providing some mechanism to get return value from the called function. It's implemented using EAX register. Called function stores return calue in EAX.

Stack

In C, first 4 features are implemented using stack.

Salient Features of Stack:

  1. It grows downwards in memory. That is the top of the stack moves down with each push to the stack.
  2. 
                     ________________ <---- Bottom of the stack (fixed)
                    |                |      Higher memory address
                    |                |
                    |                |        
                    |________________|<---- Top of the stack (moving down)
                            |               Lower memory address
                            |
                            V Grows downwards in memory
    
  3. Top of stack (pointer to the last pushed vlaue) is stored in the register ESP.
  4. Only operations allowed are push and pop.

Thus, **ESP alone represents the current status of the stack.

Function Execution:

By convention, procedures will take their parameters from stack. If a procedure returns a value that will fit in a register, it will be returned in AL, AX, EAX, depending on its size. A procedure will in general have local variables on stack as well.

A standard method of accessing both local variables and parameters is to "mark" a place on the stack, and then address both parameters and local variables by their offsets from the "mark". This mark or reference point is called "FRAME POINTER" , because all the information that is pushed into the stack at the time of procedure call is called a frame for that procedure.

Frame Pointer (FP) for the running procedure is always stored in EBP.

Execution in steps

  1. Push the parameters in the reverse order (right to left).
  2. "Call" function now. It implicitly pushes the return address into STACK.
    [[ call func ]]
  3. ------ Now enters the called procedure ------
  4. Push the current EBP ie. Frame Pointer (FP) of calling function into stack. We need to do this to continue with the execution of calling function after return from the called function.
    [[ pushl %ebp ]]
  5. Copy the ESP to EBP. (yes, this location will be new FRAME POINTER)
    [[ movl %esp, %ebp ]]
  6. Allocate space on stack for local variables. It's done by decrementing ESP.
    [[ subl $4, %esp ]]
    ------ Do some processing ------
  7. Put the value to be returned in EAX.
    ----- Start unwinding STACK ------
  8. Copy current EBP to ESP, it will reduce stack's size. Now we have old FP at the top of the stack.
    [[ movl %ebp, %esp]]
  9. Pop a 32 bit value (which is old frame pointer) and stuff it into EBP. (undoing Step 3)
    [[ popl %ebp ]]
  10. The "ret" instruction pops a 32 bit value from the stack and stuffs into the program counter.
    [[ ret ]]
  11. ** steps 7 and 8 are combined in single instruction "leave".

Illustration

Example C Code

static int a = 8; 	/*Static Storage*/
extern c;

static int func1 (int x)
{
	int t = 8;	/* Local Variable */
	return (x+t);	/* Return */
}

int main()
{
	int b = 0;
	b = func1(a);
	c = b;
	return(0);
}

Corresponding Assembly Code (Only relevant part shown):

a:
        .long   8                 &<-- Static variable a

func1:
        pushl   %ebp              <-- Step 3, Push EBP
        movl    %esp, %ebp        <-- Step 4, Copy ESP -> EBP
        subl    $4, %esp          <-- Step 5, Create space on stack for t
        movl    $8, -4(%ebp)      <-- Initialize "t" to 8
        movl    -4(%ebp), %eax    <-- Step 6, Copy "t" to EAX
        addl    8(%ebp), %eax     <-- Step 6, Add "c" to EAX
            leave                 <-- Step 7 and 8:
                                7: Restore ESP (EBP -> ESP)
                                8: Restore EBP (Pop STACK -> EBP)
        ret                       <-- Step 9, (Pop STACK -> Program Counter)

main:
        .........
        pushl   a                 <-- Step 1, push parameters
        call    func1             <-- Step 2, call func1
        addl    $16, %esp
        movl    %eax, -4(%ebp)
        movl    -4(%ebp), %eax
        movl    %eax, c
        .........

During execution of func1, STACK looks like:

                 _______________ <---- Bottom of the stack (fixed)
                |               |
                |     main      |
                |_______________|
func1 frame --> |        a      |<---- Parameters pushed
starts here     |_______________|
                |    RET Addr   |<---- Return address pushed by "call"
                |_______________|
Frame Pointer-->|      EBP      |<---- FP for main pushed by func1
for func1       |_______________|      Copy ESP to EBP
                |        t      |<---- Local variable "t"
                |_______________|
                        |
                        V

From the figure, you can make out that , to refer to parameters, we have to add 8 offset to EBP ie. 8(%ebp). And to refer to local variable, we'll say -4(%ebp).

**Remember, ESP points to the last item pushed, so 0(%ebp) will give us FP of main.


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