WNDPROC trampolines

exebuf.s

	.section .exebuf,"bwx"
	.globl exebuf
exebuf:	.space 1<<21

main.c

// $ cc -mwindows -o main.exe main.c exebuf.s
// This is free and unencumbered software released into the public domain.
#include <assert.h>
#include <stddef.h>
#include <stdio.h>
#include <windows.h>

#define S(s)    (Str){s, sizeof(s)-1}
typedef struct {
    char     *data;
    ptrdiff_t len;
} Str;

typedef struct {
    char *beg;
    char *end;
} Arena;

#define new(a, n, t)    (t *)alloc(a, n, sizeof(t), _Alignof(t))
char *alloc(Arena *a, ptrdiff_t count, ptrdiff_t size, ptrdiff_t align)
{
    ptrdiff_t pad = (ptrdiff_t)-(uintptr_t)a->beg & (align - 1);
    assert(count < (a->end - a->beg - pad)/size);
    char *r = a->beg + pad;
    a->beg += pad + count*size;
    return memset(r, 0, (size_t)(count*size));
}

Arena get_exebuf()
{
    extern char exebuf[1<<21];
    Arena r = {exebuf, exebuf+sizeof(exebuf)};
    return r;
}

Str clone(Arena *a, Str s)
{
    Str r = s;
    r.data = new(a, r.len, char);
    memcpy(r.data, s.data, (size_t)r.len);
    return r;
}

typedef LRESULT Wndproc5(HWND, UINT, WPARAM, LPARAM, void *);

WNDPROC make_wndproc(Arena *a, Wndproc5 proc, void *arg)
{
    Str thunk = S(
        "\x48\x83\xec\x28"      // sub   $40, %rsp
        "\x48\xb8........"      // movq  $arg, %rax
        "\x48\x89\x44\x24\x20"  // mov   %rax, 32(%rsp)
        "\xe8...."              // call  proc
        "\x48\x83\xc4\x28"      // add   $40, %rsp
        "\xc3"                  // ret
    );
    Str r   = clone(a, thunk);
    int rel = (int)((uintptr_t)proc - (uintptr_t)(r.data + 24));
    memcpy(r.data+ 6, &arg, sizeof(arg));
    memcpy(r.data+20, &rel, sizeof(rel));
    return (WNDPROC)r.data;
}

void set_proc_arg(WNDPROC p, void *arg)
{
    memcpy((char *)p+6, &arg, sizeof(arg));
}

LRESULT example(HWND hwnd, UINT msg, WPARAM wp, LPARAM lp, void *arg)
{
    printf(
        "%lld %lld %lld %lld \"%s\"\n",
        (long long)hwnd,
        (long long)msg,
        (long long)wp,
        (long long)lp,
        (char *)arg
    );
    return 0;
}

LRESULT wndproc(HWND hwnd, UINT msg, WPARAM wp, LPARAM lp, void *arg)
{
    switch (msg) {
    case WM_PAINT:
        PAINTSTRUCT ps;
        HDC hdc = BeginPaint(hwnd, &ps);
        RECT rect;
        GetClientRect(hwnd, &rect);
        char *message = arg;
        UINT format = DT_CENTER | DT_VCENTER | DT_SINGLELINE;
        DrawTextA(hdc, message, -1, &rect, format);
        EndPaint(hwnd, &ps);
        return 0;

    case WM_DESTROY:
        PostQuitMessage(0);
        return 0;
    }

    return DefWindowProcA(hwnd, msg, wp, lp);
}

int WINAPI WinMain(HINSTANCE, HINSTANCE, LPSTR, int)
{
    Arena a = get_exebuf();

    // Note: not visible from the windows subsystem
    WNDPROC proc = make_wndproc(&a, example, "hello world");
    proc((void *)1, 2, 3, 4);
    set_proc_arg(proc, "goodbye");
    proc((void *)5, 6, 7, 8);

    RegisterClassA(&(WNDCLASSA){
        .lpfnWndProc   = make_wndproc(&a, wndproc, "Hello World"),
        .lpszClassName = "demo",
    });
    CreateWindowExA(
        0, "demo", "Demo",
        WS_OVERLAPPED | WS_VISIBLE | WS_MINIMIZEBOX | WS_SYSMENU,
        CW_USEDEFAULT, CW_USEDEFAULT,
        400, 200,
        0, 0, 0, 0
    );

    for (MSG msg; GetMessageW(&msg, 0, 0, 0);) {
        if (msg.message == WM_QUIT) {
            break;
        }
        TranslateMessage(&msg);
        DispatchMessageW(&msg);
    }
    return 0;
}