A macOS app entirely in plain C

helloworld.c

// A macOS GUI app written in pure C, using the Objective-C runtime directly.
// No Objective-C syntax anywhere. Just raw objc_msgSend and runtime calls.
// This is cursed. Enjoy.

// Compile with clang -Wall -Wextra -o hello_runtime hello_runtime.c \
//                    -framework Cocoa -lobjc

#include <objc/objc.h>
#include <objc/message.h>
#include <objc/runtime.h>
#include <stdbool.h>

typedef struct { double x, y, width, height; } CGRect;
typedef unsigned long NSUInteger;

// Every objc_msgSend call needs a cast to the correct function pointer type.
// These typedefs save some sanity.
typedef id   (*send_t)(id, SEL);
typedef id   (*send_id_t)(id, SEL, id);
typedef id   (*send_rect_t)(id, SEL, CGRect);
typedef id   (*send_long_t)(id, SEL, long);
typedef id   (*send_double_t)(id, SEL, double);
typedef id   (*send_bool_t)(id, SEL, bool);
typedef id   (*send_cstr_t)(id, SEL, const char *);
typedef id   (*send_init_window_t)(id, SEL, CGRect, NSUInteger, NSUInteger, bool);
typedef void (*send_void_t)(id, SEL);
typedef void (*send_void_id_t)(id, SEL, id);
typedef void (*send_void_bool_t)(id, SEL, bool);
typedef void (*send_void_long_t)(id, SEL, long);

#define cls(name) (id)objc_getClass(name)
#define sel(name) sel_registerName(name)

// Delegate callback: quit when the window closes
static BOOL should_terminate(id self, SEL cmd, id sender) {
    (void)self; (void)cmd; (void)sender;
    return YES;
}

int main(void) {
    // NSApplication *app = [NSApplication sharedApplication];
    id app = ((send_t)objc_msgSend)(cls("NSApplication"), sel("sharedApplication"));

    // [app setActivationPolicy:NSApplicationActivationPolicyRegular];
    ((send_void_long_t)objc_msgSend)(app, sel("setActivationPolicy:"), 0);

    // --- Menu bar (so Cmd+Q works) ---

    // NSMenu *menubar = [[NSMenu alloc] init];
    id menubar = ((send_t)objc_msgSend)(cls("NSMenu"), sel("alloc"));
    menubar = ((send_t)objc_msgSend)(menubar, sel("init"));

    // NSMenuItem *appMenuItem = [[NSMenuItem alloc] init];
    id app_menu_item = ((send_t)objc_msgSend)(cls("NSMenuItem"), sel("alloc"));
    app_menu_item = ((send_t)objc_msgSend)(app_menu_item, sel("init"));
    ((send_void_id_t)objc_msgSend)(menubar, sel("addItem:"), app_menu_item);

    // NSMenu *appMenu = [[NSMenu alloc] init];
    id app_menu = ((send_t)objc_msgSend)(cls("NSMenu"), sel("alloc"));
    app_menu = ((send_t)objc_msgSend)(app_menu, sel("init"));

    // NSString *quitTitle = @"Quit";
    id quit_title = ((send_cstr_t)objc_msgSend)(cls("NSString"), sel("stringWithUTF8String:"), "Quit");
    id quit_key = ((send_cstr_t)objc_msgSend)(cls("NSString"), sel("stringWithUTF8String:"), "q");

    // NSMenuItem *quitItem = [[NSMenuItem alloc] initWithTitle:@"Quit" action:@selector(terminate:) keyEquivalent:@"q"];
    id quit_item = ((send_t)objc_msgSend)(cls("NSMenuItem"), sel("alloc"));
    quit_item = ((id (*)(id, SEL, id, SEL, id))objc_msgSend)(
        quit_item, sel("initWithTitle:action:keyEquivalent:"),
        quit_title, sel("terminate:"), quit_key
    );

    ((send_void_id_t)objc_msgSend)(app_menu, sel("addItem:"), quit_item);
    ((send_void_id_t)objc_msgSend)(app_menu_item, sel("setSubmenu:"), app_menu);
    ((send_void_id_t)objc_msgSend)(app, sel("setMainMenu:"), menubar);

    // --- App delegate (so closing the window quits the app) ---

    // Create a new class at runtime: @interface AppDelegate : NSObject <NSApplicationDelegate>
    Class delegate_class = objc_allocateClassPair(
        (Class)objc_getClass("NSObject"), "AppDelegate", 0
    );
    // Add method: - (BOOL)applicationShouldTerminateAfterLastWindowClosed:(NSApplication *)sender
    class_addMethod(delegate_class,
        sel("applicationShouldTerminateAfterLastWindowClosed:"),
        (IMP)should_terminate, "B@:@"
    );
    objc_registerClassPair(delegate_class);

    id delegate = ((send_t)objc_msgSend)((id)delegate_class, sel("alloc"));
    delegate = ((send_t)objc_msgSend)(delegate, sel("init"));
    ((send_void_id_t)objc_msgSend)(app, sel("setDelegate:"), delegate);

    // --- Window ---

    // NSWindow *window = [[NSWindow alloc] initWithContentRect:... styleMask:... backing:... defer:NO];
    id window = ((send_t)objc_msgSend)(cls("NSWindow"), sel("alloc"));
    window = ((send_init_window_t)objc_msgSend)(
        window, sel("initWithContentRect:styleMask:backing:defer:"),
        (CGRect){200, 200, 500, 300},
        15,     // NSWindowStyleMaskTitled | Closable | Miniaturizable | Resizable
        2,      // NSBackingStoreBuffered
        false
    );

    // [window setTitle:@"Hello from Pure C"];
    id title = ((send_cstr_t)objc_msgSend)(cls("NSString"), sel("stringWithUTF8String:"), "Hello from Pure C");
    ((send_void_id_t)objc_msgSend)(window, sel("setTitle:"), title);

    // --- Text label ---

    // NSTextField *label = [[NSTextField alloc] initWithFrame:...];
    id label = ((send_t)objc_msgSend)(cls("NSTextField"), sel("alloc"));
    label = ((send_rect_t)objc_msgSend)(label, sel("initWithFrame:"), (CGRect){50, 100, 400, 80});

    // [label setStringValue:@"Hello, World!\nWritten in pure C.\nNo Objective-C syntax. Just vibes."];
    id text = ((send_cstr_t)objc_msgSend)(cls("NSString"),
        sel("stringWithUTF8String:"),
        "Hello, World!\nWritten in pure C.\nNo Objective-C syntax. Just vibes."
    );
    ((send_void_id_t)objc_msgSend)(label, sel("setStringValue:"), text);

    // Make it look like a label, not a text field
    ((send_void_bool_t)objc_msgSend)(label, sel("setBezeled:"), false);
    ((send_void_bool_t)objc_msgSend)(label, sel("setDrawsBackground:"), false);
    ((send_void_bool_t)objc_msgSend)(label, sel("setEditable:"), false);
    ((send_void_bool_t)objc_msgSend)(label, sel("setSelectable:"), false);

    // [label setFont:[NSFont systemFontOfSize:24]];
    id font = ((send_double_t)objc_msgSend)(cls("NSFont"), sel("systemFontOfSize:"), 24.0);
    ((send_void_id_t)objc_msgSend)(label, sel("setFont:"), font);

    // [[window contentView] addSubview:label];
    id content_view = ((send_t)objc_msgSend)(window, sel("contentView"));
    ((send_void_id_t)objc_msgSend)(content_view, sel("addSubview:"), label);

    // --- Show and run ---

    // [window makeKeyAndOrderFront:nil];
    ((send_void_id_t)objc_msgSend)(window, sel("makeKeyAndOrderFront:"), nil);

    // [app activateIgnoringOtherApps:YES];
    ((send_void_bool_t)objc_msgSend)(app, sel("activateIgnoringOtherApps:"), true);

    // [app run];
    ((send_void_t)objc_msgSend)(app, sel("run"));

    return 0;
}