C-on-ObjC experiments...

hello.c

// which is then used like this
// (helper functions like oc_alloc() and oc_alloc_init() will move into a header later)

#include <stdio.h>
#include <assert.h>
#include "hello_macos.h"

Class app_delegate_class;
id app_delegate;

id oc_alloc(Class cls) {
    return ((id(*)(id,SEL))objc_msgSend)((id)cls, sel_getUid("alloc"));
}

id oc_alloc_init(Class cls) {
    return ((id(*)(id,SEL))objc_msgSend)(oc_alloc(cls), sel_getUid("init"));
}

void didFinishLaunching(id obj, SEL sel, NSNotification* notification) {
    NSWindow* win = (NSWindow*) oc_alloc(objc_lookUpClass("NSWindow"));
    uint64_t style_mask = 
        NSWindowStyleMaskTitled |
        NSWindowStyleMaskClosable |
        NSWindowStyleMaskMiniaturizable |
        NSWindowStyleMaskResizable;
    NSWindow_initWithContentRect_styleMask_backing_defer(
        win,
        (NSRect){ .origin = { 50, 50 }, .size = { 300, 300 } },
        style_mask,
        NSBackingStoreBuffered,
        false);
    NSWindow_makeKeyAndOrderFront(win, 0);
}

void init_delegate(void) {
    // create and register a new app-delegate class
    app_delegate_class = objc_allocateClassPair(objc_lookUpClass("NSObject"), "HelloAppDelegate", 0);
    assert(app_delegate_class);
    class_addMethod(app_delegate_class, sel_getUid("applicationDidFinishLaunching:"), (IMP)didFinishLaunching, "v@@");
    objc_registerClassPair(app_delegate_class); 
    app_delegate = oc_alloc_init(app_delegate_class);
    printf("app delegate: %p\n", app_delegate);
}

int main() {
    init_delegate();
    NSApplication* app = NSApplication_sharedApplication();
    NSApplication_activateIgnoringOtherApps(app, true);
    NSApplication_setActivationPolicy(app, NSApplicationActivationPolicyRegular);
    NSApplication_setDelegate(app, (NSApplicationDelegate*)app_delegate);
    printf("app: %p\n", app);
    NSApplication_run(app);
    return 0;
}

hello_macos.h

// which yields this generated C header...

#include <stdint.h>
#include <objc/objc-runtime.h>

typedef struct NSNotification { } NSNotification;
typedef struct NSApplication { } NSApplication;
typedef struct NSApplicationDelegate { } NSApplicationDelegate;
typedef struct NSWindow { } NSWindow;

typedef enum NSBackingStoreType: uint64_t {
    NSBackingStoreRetained = 0,
    NSBackingStoreNonretained = 1,
    NSBackingStoreBuffered = 2,
} NSBackingStoreType;

typedef enum NSApplicationActivationPolicy: int64_t {
    NSApplicationActivationPolicyRegular,
    NSApplicationActivationPolicyAccessory,
    NSApplicationActivationPolicyProhibited,
} NSApplicationActivationPolicy;

typedef enum NSWindowStyleMask: uint64_t {
    NSWindowStyleMaskBorderless = 0,
    NSWindowStyleMaskTitled = 1<<0,
    NSWindowStyleMaskClosable = 1<<1,
    NSWindowStyleMaskMiniaturizable = 1<<2,
    NSWindowStyleMaskResizable = 1<<3,
    NSWindowStyleMaskTexturedBackground = 1<<8,
    NSWindowStyleMaskUnifiedTitleAndToolbar = 1<<12,
    NSWindowStyleMaskFullScreen = 1<<14,
    NSWindowStyleMaskFullSizeContentView = 1<<15,
    NSWindowStyleMaskUtilityWindow = 1<<4,
    NSWindowStyleMaskDocModalWindow = 1<<6,
    NSWindowStyleMaskNonactivatingPanel = 1<<7,
    NSWindowStyleMaskHUDWindow = 1<<13,
} NSWindowStyleMask;

typedef struct CGPoint {
    double x;
    double y;
} CGPoint;

typedef struct CGSize {
    double width;
    double height;
} CGSize;

typedef struct NSRect {
    CGPoint origin;
    CGSize size;
} NSRect;

static void NSApplication_activateIgnoringOtherApps(NSApplication * self, bool flag) {
    return ((void(*)(void*,void*,bool))objc_msgSend)((void*)self, (void*)sel_getUid("activateIgnoringOtherApps:"), flag);
}
static void NSApplication_run(NSApplication * self) {
    return ((void(*)(void*,void*))objc_msgSend)((void*)self, (void*)sel_getUid("run"));
}
static bool NSApplication_setActivationPolicy(NSApplication * self, NSApplicationActivationPolicy activationPolicy) {
    return ((bool(*)(void*,void*,NSApplicationActivationPolicy))objc_msgSend)((void*)self, (void*)sel_getUid("setActivationPolicy:"), activationPolicy);
}
static NSApplication * NSApplication_sharedApplication() {
    return ((NSApplication *(*)(void*,void*))objc_msgSend)((void*)objc_getClass("NSApplication"), (void*)sel_getUid("sharedApplication"));
}
static void NSApplication_setDelegate(NSApplication * self, NSApplicationDelegate * delegate) {
    return ((void(*)(void*,void*,NSApplicationDelegate *))objc_msgSend)((void*)self, (void*)sel_getUid("setDelegate:"), delegate);
}
static NSWindow* NSWindow_initWithContentRect_styleMask_backing_defer(NSWindow * self, NSRect contentRect, NSWindowStyleMask style, NSBackingStoreType backingStoreType, bool flag) {
    return ((NSWindow*(*)(void*,void*,NSRect,NSWindowStyleMask,NSBackingStoreType,bool))objc_msgSend)((void*)self, (void*)sel_getUid("initWithContentRect:styleMask:backing:defer:"), contentRect, style, backingStoreType, flag);
}
static void NSWindow_makeKeyAndOrderFront(NSWindow * self, id sender) {
    return ((void(*)(void*,void*,id))objc_msgSend)((void*)self, (void*)sel_getUid("makeKeyAndOrderFront:"), sender);
}

test.json

// this describes the macOS types we want to extract

{
    "c_prefix": "",
    "headers": {
        "imports": [ "Cocoa/Cocoa.h" ],
        "includes": []
    },
    "exports": {
        "CGPoint": [ ],
        "CGSize": [ ],
        "NSRect": [ ],
        "NSApplicationActivationPolicy": [  ],
        "NSNotification": [ ],
        "NSApplicationDelegate": [ ],
        "NSApplication": [
            "sharedApplication",
            "setActivationPolicy:",
            "activateIgnoringOtherApps:",
            "setDelegate:",
            "run"
        ],
        "NSWindowStyleMask": [ ],
        "NSBackingStoreType": [ ],
        "NSWindow": [
            "initWithContentRect:styleMask:backing:defer:",
            "makeKeyAndOrderFront:"
        ]
    }
}