Otdfbt

I'm trying to link x86 assembly and C.

My C program:

extern int plus_10(int);

# include <stdio.h>

int main() 
 int x = plus_10(40);
 printf("%dn", x);
 return 0;

My assembly program:

[bits 32]

section .text

global plus_10
plus_10:
 pop edx
 mov eax, 10
 add eax, edx
 ret

I compile and link the two as follows:

gcc -c prog.c -o prog_c.o -m32
nasm -f elf32 prog.asm -o prog_asm.o
gcc prog_c.o prog_asm.o -m32

However, when I run the resulting file, I get a segmentation fault.

But when I replace

pop edx

with

mov edx, [esp+4]

the program works fine. Can someone please explain why this happens?

This is a possible assembly code of int x = plus_10(40);

 push 40 ; push argument
 call plus_10 ; call function
retadd: add esp, 4 ; clean up stack (dummy pop)
 ; result of the function call is in EAX, per the calling convention

 ; if compiled without optimization, the caller might just store it:
 mov DWORD PTR [ebp-x], eax ; store return value
 ; (in eax) in x

Now when you call plus_10, the address retadd is pushed on the stack by the call instruction. It's effectively a push+jmp, and ret is effectively pop eip.

So your stack looks like this in the plus_10 function:

| ... |
+--------+
| 40 | <- ESP+4 points here (the function argument)
+--------+
| retadd | <- ESP points here
+--------+

ESP points to a memory location that contains the return address.

Now if you use pop edx the return address goes into edx and the stack looks like this:

| ... |
+--------+
| 40 | <- ESP points here
+--------+

Now if you execute ret at this point, the program will actually jump to address 40 and most likely segfault or behave in some other unpredictable way.

The actual assembly code generated by the compiler may be different, but this illustrates the problem.

BTW, a more efficient way to write your function is this: it's what most compilers would do with optimization enabled, for a non-inline version of this tiny function.

global plus_10
plus_10:
 mov eax, [esp+4] ; retval = first arg
 add eax, 10 ; retval += 10
 ret

This is smaller and slightly more efficient than

 mov eax, 10
 add eax, [esp+4] ; decode to a load + add.
 ret