How do I get the integer argument in calllee?
I have the following code (caller.c):
#include <stdio.h>
extern int callee(int);
int main(int argc, char *argv[]){
callee(4);
return 1;
}
and (callee.s):
.globl callee
callee:
pop %eax
add $4, %eax
ret
I am compiling with: gcc -m32 caller.c callee.s
and run:
./a.out
Segmentation fault (core dump)
I am wondering what my error (s) are as I figured that the main one should now push 32 bites the stack number. I didn't use the modified stack so that the caller can now pop this number from the same stack. Maybe I should add (add $ 4,% esp) before the pop (if the callee's address is in "mode" / actually popped out). I've tried this too with no success. the callee should now get the number from the stack and add 4 to it. The eax register should be where the return value from the callee should be stored (calling convention), but here I am ignoring the return value.
Can anyone help me?
related question: calling build function from c
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(Using the x86-32 calling convention) the function arguments are pushed onto the stack first, followed by the return address. So your team pop
pulled the return address and the subsequent one ret
tried to go back to address 0x00000004, which is untagged memory, causing a crash.
Also, in this convention, the callee must not expose its arguments. The call will do it.
The code you had to write is
callee:
movl 4(%esp), %eax
addl $4, %eax
ret
You can confirm this yourself by compiling
unsigned int callee(unsigned int x) { return x + 4; }
with parameters -m32 -O2 -S -fomit-frame-pointer
and verification of the created file .s
; you should get the same assembly code as above.
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