[kohlrak@kohlrak-server tmixed]$ dir
asm.S  main.cpp  makefile
[kohlrak@kohlrak-server tmixed]$ cat makefile
all:
        gcc main.cpp asm.S -o main
run:
        make all
        ./main 37352
[kohlrak@kohlrak-server tmixed]$ cat main.cpp
#include <stdio.h>
#include <stdlib.h>

extern int bswap(int x); //extern "C" so i don't have to bother with mangling

int main(int argc, char** argv){
        int le=atoi(argv[1]);
        printf("LE: 0x%08x\nBE: 0x%08x\n", le, bswap(le));
        return 0;
}
[kohlrak@kohlrak-server tmixed]$ cat asm.S
.intel_syntax noprefix
.code32

.global _Z5bswapi #C++ name mangling: name is bswap, i is for integer input. Don't blame me, blame whomever wrote the ABI, but it could be made simpler if you don't want to bother with that XD

.section .text

_Z5bswapi:
        mov eax, [esp+4] #esp+0 is return address, +4 is first param, +8 is second param, +12 is third param, and so on
        bswap eax #big endian -- little endian conversion instruction
        ret