Generating functions at runtime in C - c

Question

I would like to generate a function at runtime in C. And by this I mean I would essentially like to allocate some memory, point at it and execute it via function pointer. I realize this is a very complex topic and my question is naïve. I also realize there are some very robust libraries out there that do this (e.g. nanojit).
But I would like to learn the technique, starting with the basics. Could someone knowledgeable give me a very simple example in C?
EDIT: The answer below is great but here is the same example for Windows:
#include <Windows.h>
#define MEMSIZE 100*1024*1024
typedef void (*func_t)(void);
int main() {
HANDLE proc = GetCurrentProcess();
LPVOID p = VirtualAlloc(
NULL,
MEMSIZE,
MEM_RESERVE|MEM_COMMIT,
PAGE_EXECUTE_READWRITE);
func_t func = (func_t)p;
PDWORD code = (PDWORD)p;
code[0] = 0xC3; // ret
if(FlushInstructionCache(
proc,
NULL,
0))
{
func();
}
CloseHandle(proc);
VirtualFree(p, 0, MEM_RELEASE);
return 0;
}

Answer 1

As said previously by other posters, you'll need to know your platform pretty well.
Ignoring the issue of casting a object pointer to a function pointer being, technically, UB, here's an example that works for x86/x64 OS X (and possibly Linux too). All the generated code does is return to the caller.
#include <unistd.h>
#include <sys/mman.h>
typedef void (*func_t)(void);
int main() {
/*
* Get a RWX bit of memory.
* We can't just use malloc because the memory it returns might not
* be executable.
*/
unsigned char *code = mmap(NULL, getpagesize(),
PROT_READ|PROT_EXEC|PROT_WRITE,
MAP_SHARED|MAP_ANON, 0, 0);
/* Technically undefined behaviour */
func_t func = (func_t) code;
code[0] = 0xC3; /* x86 'ret' instruction */
func();
return 0;
}
Obviously, this will be different across different platforms but it outlines the basics needed: get executable section of memory, write instructions, execute instructions.

Answer 2

This requires you to know your platform. For instance, what is the C calling convention on your platform? Where are parameters stored? What register holds the return value? What registers must be saved and restored? Once you know that, you can essentially write some C code that assembles code into a block of memory, then cast that memory into a function pointer (though this is technically forbidden in ANSI C, and will not work depending if your platform marks some pages of memory as non-executable aka NX bit).
The simple way to go about this is simply to write some code, compile it, then disassemble it and look at what bytes correspond to which instructions. You can write some C code that fills allocated memory with that collection of bytes and then casts it to a function pointer of the appropriate type and executes.
It's probably best to start by reading the calling conventions for your architecture and compiler. Then learn to write assembly that can be called from C (i.e., follows the calling convention).

Answer 3

If you have tools, they can help you get some things right easier. For example, instead of trying to design the right function prologue/epilogue, I can just code this in C:
int foo(void* Data)
{
return (Data != 0);
}
Then (MicrosoftC under Windows) feed it to "cl /Fa /c foo.c". Then I can look at "foo.asm":
_Data$ = 8
; Line 2
push ebp
mov ebp, esp
; Line 3
xor eax, eax
cmp DWORD PTR _Data$[ebp], 0
setne al
; Line 4
pop ebp
ret 0
I could also use "dumpbin /all foo.obj" to see that the exact bytes of the function were:
00000000: 55 8B EC 33 C0 83 7D 08 00 0F 95 C0 5D C3
Just saves me some time getting the bytes exactly right...