Passing a function and its size to WriteFile

Question

As a learning exercise, I'm writing a program that outputs a DLL at run-time.

I've written the PE header and have successfully written the DOS header, NT header, optional section header and the .text section header to a file using WriteFile, e.g.:

WriteFile(hFile, &nt_header, sizeof(nt_header), &written, NULL);

I'm now like to add some code to the .text section, but I don't know how to pass a function and its size to WriteFile, e.g.:

static int test(void)
{
    return 10;
}

WriteFile's second parameter has type LPCVOID. I tried passing in test, but that only wrote 1 byte. Passing a pointer to test wrote 4 bytes, as expected.

This is probably obvious, but I'm not understanding where I'm going wrong.

Answer 1

It's obvious, but the answer isn't what you want, probably. It can't be done, at least not portably.

Functions in C don't have sizes. You also can't assume that the concept "address of a function" means "address of the first machine instruction in the compiled code for a function".

In short, what you're doing isn't possible at that level in C, you can't do I/O on functions directly.