Question

I am a java programmer, but i have few things to be done in C. So, i started with a simple example as below. If i have compiled it and generate a executable file (hello), can i run the executable file (hello) in any unix platform without the original file (hello.c)? And also is there a way to read the data from executable file means, decompile the executable file to original file (hello.c)?

[oracle@oracleapps test]$ cat hello.c
#include <stdio.h>
int main(){
int i,data =0;
for(i=1;i<=64;i+=1){
data = i*2;
printf("data=%d\n",data);
}
return 0;
}

To compile

gcc -Wall -W -Werror hello.c -o hello

Answer 1

You can run the resulting executable on platforms that are ABI-compatible with the one which you have compiled the executable for. ABI-compatibility basically means that the same physical processor architecture and OS-interfaces (plus calling convention) is used on two (possibly different) OSes. For example, you can run binaries compiled for Linux on a FreeBSD system (with the same processor type), because FreeBSD includes Linux ABI-compatibility. However, it may not be possible to run a binary on all other types of Unices, unless some hackery is done. For example, you can’t run Mac OS X applications on linux, however this guy has a solution with which it’s possible to use some OS X command line tools (including the GCC compiler itself) on Linux.

Reverse engineering: there are indeed decompilers which aim to generate C code from machine code, but they’re not (yet) very powerful. The reason for this is they’re by nature extremely hard to write. Machine code patterns have to be recognized, and even then you can’t gather all the original info. For example, types of loops, comments and non-static local variable names and most of the types are all gone during the compilation process. For example, if you have a C source file like this:

int main(int argc, char **argv)
{
    int i;
    for (i = 0; i < 10; i++)
    {
        printf("I is: %d\n", i); /* Write the value of I */
    }

    return 0;
}

a C decompiler may be able to reconstruct the following code:

int main(int _var1, void *_var2)
{
    int _var3 = 0;
    while (_var3 < 10)
    {
        printf("I is: %d\n", _var3);
        _var3 = _var3 + 1;
    }

    return 0;
}

But this would be a rather advanced decompiler, such as this one.