How can I undo compilation of a file overwriting another file? - c++

Question

I was trying to compile my c++ program using the command g++ program.cpp -o program in order to create an executable with the same name as the actual program but used g++ program.cpp -o program.cpp and now my code looks like gibberish. Is there anyway to reverse it?

Answer 1

Reversing this process is semi-possible, but I do not recommend it. It is easier to just rewrite the program, as the compilation loses a ton of info. If you had compiled it with -g I believe it would be possible to recover the source (I have no idea though how one would do that).
If you have to reverse it you need to use a decompiler such as Cutter/Ghidra, but the result will need to be cleaned up. Example:
#inlcude <iostream>
#include <string>
int main(){
std::string a = "Hey";
a += '\n';
std::cout << a;
return 0;
}
Compiling with g++ test.cc -o test, and then decompiling with Ghidra, gets us:
undefined8 main(void){
long in_FS_OFFSET;
allocator<char> local_49;
basic_string<char,std::char_traits<char>,std::allocator<char>> local_48 [40];
long local_20;
local_20 = *(long *)(in_FS_OFFSET + 0x28);
std::allocator<char>::allocator();
/* try { // try from 001012a3 to 001012a7 has its CatchHandler # 001012fc */
std::__cxx11::basic_string<char,std::char_traits<char>,std::allocator<char>>::basic_string
((char *)local_48,(allocator *)&DAT_00102005);
std::allocator<char>::~allocator(&local_49);
/* try { // try from 001012c2 to 001012d9 has its CatchHandler # 0010131a */
std::__cxx11::basic_string<char,std::char_traits<char>,std::allocator<char>>::operator+=
(local_48,"\n");
std::operator<<((basic_ostream *)std::cout,(basic_string *)local_48);
std::__cxx11::basic_string<char,std::char_traits<char>,std::allocator<char>>::~basic_string
(local_48);
if (local_20 != *(long *)(in_FS_OFFSET + 0x28)) {
/* WARNING: Subroutine does not return */
__stack_chk_fail();
}
return 0;
}
Pretty far gone, huh. The original program was only a few lines, but this seems a monstrosity. By knowing what to look for you can ignore things the compiler placed there to be safe like __stack_chk_fail(), and the anything that has in_FS_OFFSET or local_20 in it. But it is extremely laboursome in general. With some code it might be easier, but parts of it might be optimised away etc. so it is generally not recommended.
Note: I did not paste it here but at DAT_00102005, the string "Hey" is contained.