How can i pass parameters in assembler x86 function call

Question

Look at this assembler code. It is designed for 32 bits x86 and will be compiled by nasm

   ...
   my_function:
        pop %eax
        ...
        ret


   main:
       push 0x08
       call my_function

I have learned a long time ago that we can use stack for passing parameters between main program and functions. I would expect that eax contains 0x08, but this is false and i can not explain why. How should i do for fetching my function parameters ?

Answer 1

Firstly, if you are looking to interface with other languages or libraries on your platform, be sure to read the defined interface for that platform. There are a variety of calling mechanisms that might be used.

In your case, the call instruction is pushing the return address onto the stack. You can access your parameter by using some arithmetic and esp. I will assume 32 bit code (and a 32 bit stack width) since you are using eax. I'm using intel syntax since I can write that without looking anything up:

my_function:
    mov eax, [esp+4]    ; Move the contents of ESP+4 into EAX
                        ; ESP should be pointing at the 32 bit RIP.
                        ; ESP+4 should be the pushed parameter.
    ...
    ret


main:
   push 0x08
   call my_function

In your comments you ask, regarding this answer, if this represents a memory leak. The answer is "No." The reason is that the caller is responsible to clean up anything that it adds to the stack. A more complete example based on the other comments that have been written might look like this:

my_function:
    push ebp            ; Store the current stack frame
    mov  ebp, esp       ; Preserve ESP into EBP for argument references
    and  esp, 0xfffffff0; Align the stack to allow library calls
    mov  eax, [ebp+8]   ; Move the contents of EBP+8 into EAX
                        ; [EBP] should be the saved 32 bit EBP.
                        ; [EBP+4] should be the 32 bit EIP (return address).
                        ; [EBP+8] should be the pushed parameter.
    ...                 ; Do lots of cool stuff
    mov  esp, ebp       ; Restore the stack and ebp
    pop  ebp
    ret


main:
   push 0x08
   call my_function
   pop ebx              ; Clean up the stack

Notice that when we align the stack (if you're not sure why this is happening, you will quickly find it when you research the calling standard for your platform) to a 16 byte boundary, we don't even try to figure out how much esp has changed. Since ebp will act as a "bookmark" for us, we can let esp move for alignment or perhaps local variable allocation without another thought.

In the function epilogue we move ebp back into esp, which restores esp to its original value when the function was called, thus cleaning up any local allocations and alignment operations that have happened. Finally, we pop ebp off of the stack, leaving the return address pointer as the final value on the stack within the function. We now return.

After returning we clean up with a pop.

Alternatively, it is possible to clean up the stack with a return specifying the number of bytes to free on the stack (eg ret 4). It all depends on whether your calling standard specifies caller cleanup or callee cleanup.