A simple implementation of a NSApplicationDelegate and two methods (openFile:, openFiles:) in pure C.

app_delegate.c

/*
app_delegate.c

1. Compile and link with:
  $ clang -g -o AppInC app_delegate.c -lobjc -framework Foundation -framework AppKit -isysroot /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk

2. Create a Info.plist with:
[[
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
        <key>CFBundleDevelopmentRegion</key>
        <string>en</string>
        <key>CFBundleExecutable</key>
        <string>AppInC</string>
        <key>CFBundleIconFile</key>
        <string></string>
        <key>CFBundleIdentifier</key>
        <string>com.foo.AppInC</string>
        <key>CFBundleInfoDictionaryVersion</key>
        <string>6.0</string>
        <key>CFBundleName</key>
        <string>AppInC</string>
        <key>CFBundlePackageType</key>
        <string>APPL</string>
        <key>CFBundleShortVersionString</key>
        <string>1.0</string>
        <key>CFBundleSignature</key>
        <string>????</string>
        <key>CFBundleVersion</key>
        <string>1</string>
        <key>LSApplicationCategoryType</key>
        <string>public.app-category.games</string>
        <key>LSMinimumSystemVersion</key>
        <string></string>
        <key>NSPrincipalClass</key>
        <string>NSApplication</string>
        <key>CFBundleDocumentTypes</key>
        <array>
            <dict>
                <key>CFBundleTypeRole</key>
                <string>Editor</string>
                <key>CFBundleTypeName</key>
                <string>Lua</string>
                <key>CFBundleTypeExtensions</key>
                <array>
                        <string>lua</string>
                </array>
                <key>CFBundleTypeIconFile</key>
                <string>LUA</string>
            </dict>
        </array>
</dict>
</plist>
]]

3. Make a "real" MacOS app of it:
  $ mkdir -p AppInC.app/Contents/MacOS
  $ cp AppInC AppInC.app/Contents/MacOS/
  $ cp Info.plist AppInC.app/Contents/

4. Try to open a Lua file with the AppInC.app, it should display an alertbox with the path of the Lua file.


References:
- https://stackoverflow.com/questions/11319170/c-as-principal-class-or-a-cocoa-app-without-objc

*/

#include <stdio.h>
#include <stdlib.h>

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

#define ALLOC_INIT_OBJC_CLASS(class_name) objc_msgSend(objc_msgSend((id)objc_getClass(class_name), sel_getUid("alloc")), sel_getUid("init"));

extern id NSApp;

struct AppDel
{
    Class isa;
};

// This is a strong reference to the class of the AppDelegate
// (same as [AppDelegate class])
Class AppDelClass;

BOOL AppDel_openFile(struct AppDel *self, SEL _cmd, id sender, id filename)
{
    id t_alertbox = ALLOC_INIT_OBJC_CLASS("NSAlert");
    objc_msgSend(t_alertbox, sel_getUid("setMessageText:"), filename);
    objc_msgSend(t_alertbox, sel_getUid("runModal"));

    return YES;
}

BOOL AppDel_openFiles(struct AppDel *self, SEL _cmd, id sender, id filenames)
{
    // Pick first filename in the array
    id first_nsstr = objc_msgSend(filenames, sel_getUid("firstObject"));

    // Do a little dance here to get the C string, just to see that it's possible.
    const char* c_str = (const char*)objc_msgSend(first_nsstr, sel_getUid("UTF8String"));
    id msg_text = objc_msgSend((id)objc_getClass("NSString"), sel_getUid("alloc"));
    msg_text = objc_msgSend(msg_text, sel_getUid("initWithUTF8String:"), c_str);

    id t_alertbox = ALLOC_INIT_OBJC_CLASS("NSAlert");
    objc_msgSend(t_alertbox, sel_getUid("setMessageText:"), msg_text);
    objc_msgSend(t_alertbox, sel_getUid("runModal"));

    return YES;
}

// Set up the delegate class that has the two openFile and openFiles methods.
static void init_delegate_class()
{
  AppDelClass = objc_allocateClassPair((Class) objc_getClass("NSObject"), "AppDelegate", 0);
  class_addMethod(AppDelClass, sel_getUid("application:openFile:"), (IMP) AppDel_openFile, "i@:@:@");
  class_addMethod(AppDelClass, sel_getUid("application:openFiles:"), (IMP) AppDel_openFiles, "i@:@:@");

  objc_registerClassPair(AppDelClass);
}

void init_app(void)
{
  objc_msgSend((id)objc_getClass("NSApplication"), sel_getUid("sharedApplication"));

  if (NSApp == NULL)
  {
    fprintf(stderr,"Failed to initialized NSApplication...  terminating...\n");
    return;
  }

  id appDelObj = ALLOC_INIT_OBJC_CLASS("AppDelegate");
  objc_msgSend(NSApp, sel_getUid("setDelegate:"), appDelObj);
  objc_msgSend(NSApp, sel_getUid("run"));
}


int main(int argc, char** argv)
{
  init_delegate_class();
  init_app();
  return EXIT_SUCCESS;
}