Possible to allocate a small function at run-time?

[Baker]

[Spike]

I really don't get what you're asking.

If you're talking about function pointers, void (*foo)(void); foo = realfunction; foo();
If you're not then you can't just copy blocks of instructions around. For one thing you've no idea where the function ends. Most instructions have hard-coded addresses in them, any given function will have both relative and absolute addresses. Even in PIC code, the data address is typically calculated relative to the instruction pointer. Change the instruction pointer and you change the data address too, and that's not something you want.

If you want to generate native instructions yourself then by all means do so, but this of course requires that you know how to write out the correct addresses in each instruction that you use. Either way, you need full control over the instructions and the addressing used. A C compiler doesn't give you that sort of control, you'll only get that from writing the instructions yourself (either via asm, or via C code).

[andrewj]

Also program code is stored in write-protected memory areas on modern OSes -- trying to memcpy Think2 over Think1 is going to cause a general protection fault or segmentation violation.

Unless you absolutely need to create executable code yourself -- don't do it. You will need OS-specific code to allocate or mark the memory area as executable, and the instructions will only work on a single architecture of CPU.

Some existing code to look at: Quake3 source, code/vm_x86.c

[frag.machine]

^^^ what Spike and andrewj said. You cannot directly manipulate executable code in any modern OS like you do with data; it lives in protected, usually read-only (at least at OS user level) memory segments. In a OS with a really good security model, you can't even figure out *where* in the memory the code is allocated, let alone change something on it.

What exactly are the uses you have in mind for this ?

[Baker]