Question

I want to compile a bunch of C++ files into raw machine code and the run it with a platform-dependent starter written in C. Something like

fread(buffer, 1, len, file);
a=((*int(*)(int))buffer)(b);

How can I tell g++ to output raw code?

Will function calls work? How can I make it work?

I think the calling conventions of Linux and Windows differ. Is this a problem? How can I solve it?

EDIT: I know that PE and ELF prevent the DIRECT starting of the executable. But that’s what I have the starter for.

Answer 1

There is one (relatively) simple way of achieving some of this, and that’s called "position independent code". See your compiler documentation for this.

Meaning you can compile some sources into a binary which will execute no matter where in the address space you place it. If you have such a piece of x86 binary code in a file and mmap() it (or the Windows equivalent) it is possible to invoke it from both Linux and Windows.

Limitations already mentioned are of course still present – namely, the binary code must restrict itself to using a calling convention that’s identical on both platforms / can be represented on both platforms (for 32bit x86, that’d be passing args on the stack and returning values in EAX), and of course the code must be fully self-contained – no DLL function calls as resolving these is system dependent, no system calls either.

I.e.:

1. You need position-independent code
2. You must create self-contained code without any external dependencies
3. You must extract the machine code from the object file.

Then mmap() that file, initialize a function pointer, and (*myblob)(someArgs) may do.

If you’re using gcc, the -ffreestanding -nostdinc -fPIC options should give you most of what you want regarding the first two, then use objdump to extract the binary blob from the ELF object file afterwards.