low-level objc runtime apis

.gitignore

*
!*.m
!Makefile

array.m

// To compile: gcc -o array array.m -lobjc -framework Foundation

#include <objc/runtime.h>
#include <dlfcn.h>
#include <stdio.h>

int main() {

  void *sdl_library = dlopen("/System/Library/Frameworks/Foundation.framework/Versions/Current/Foundation", RTLD_LAZY);
  if (sdl_library == NULL) {
    printf("Got dlopen error!\n");
  } else {
    printf("dlopen successful!\n");

    // Set up an NSAutoreleasePool
    Class nsautoreleasepool = objc_getClass("NSAutoreleasePool");
    id pool = class_createInstance(nsautoreleasepool, 0);
    SEL initSel = sel_registerName("init");
    id poolAfterInit = objc_msgSend(pool, initSel);

    // Create an NSMutableArray
    Class nsmutablearray = objc_getClass("NSMutableArray");

    // Regular way to create an NSMutableArray,
    // equivalent to [[NSMutableArray alloc] initWithCapacity: 10]
    id array = objc_msgSend(nsmutablearray, sel_registerName("alloc"));
    id arrayAfterInit = objc_msgSend(array, sel_registerName("initWithCapacity:"), 10);

    // Alternative, shorthand way of creating an NSMutableArray,
    // equivalent to [NSMutableArray arrayWithCapacity: 10]
    //id arrayAfterInit = objc_msgSend(nsmutablearray, sel_registerName("arrayWithCapacity:"), 10);

    // Add an NSString and the nsma class reference to the array
    objc_msgSend(arrayAfterInit, sel_registerName("addObject:"), @"test");
    objc_msgSend(arrayAfterInit, sel_registerName("addObject:"), nsmutablearray);

    // Print out the length and debug printout
    NSLog(@"%d", objc_msgSend(arrayAfterInit, sel_registerName("count")));
    NSLog(@"%@", arrayAfterInit);
  }

  return 0;
}

hello.m

// To compile: gcc -o hello hello.m -lobjc

#include <objc/runtime.h>
#include <dlfcn.h>
#include <stdio.h>

int main() {

  void *sdl_library = dlopen("/System/Library/Frameworks/Foundation.framework/Versions/Current/Foundation", RTLD_LAZY);
  if (sdl_library == NULL) {
    printf("Got dlopen error!\n");
  } else {
    printf("dlopen successful!\n");

    // Set up an NSAutoreleasePool
    Class nsautoreleasepool = objc_getClass("NSAutoreleasePool");
    id pool = class_createInstance(nsautoreleasepool, 0);
    SEL initSel = sel_registerName("init");
    id poolAfterInit = objc_msgSend(pool, initSel);

    // Set up a NSString with the contents "Hello World" from a C string
    Class nsstring = objc_getClass("NSString");
    SEL stringUTF8sel = sel_registerName("stringWithUTF8String:");
    id hello = objc_msgSend(nsstring, stringUTF8sel, "Hello World\n");

    // Print it back out as a C string
    printf("%s", (char *)objc_msgSend(hello, sel_registerName("UTF8String")));
  }

  return 0;
}

Makefile

CC = gcc
SRC_FILES = $(wildcard *.m)
OUT_FILES = $(SRC_FILES:.m=)

.PHONY: all
all: $(OUT_FILES)

%: %.m
	$(CC) -g -lobjc -framework Foundation -o $@ $<

.PHONY: clean
clean:
	rm -f $(OUT_FILES)

scriptingbridge.m

// To compile: gcc -o scriptingbridge scriptingbridge.m -lobjc -framework Foundation

#import <Foundation/Foundation.h>
#include <objc/runtime.h>
#include <dlfcn.h>
#include <stdio.h>

int main(int argc, char **argv) {

  dlopen("/System/Library/Frameworks/Foundation.framework/Versions/Current/Foundation", RTLD_LAZY);
  dlopen("/System/Library/Frameworks/ScriptingBridge.framework/Versions/Current/ScriptingBridge", RTLD_LAZY);

  // Set up an NSAutoreleasePool
  Class nsautoreleasepool = objc_getClass("NSAutoreleasePool");
  id pool = class_createInstance(nsautoreleasepool, 0);
  SEL initSel = sel_registerName("init");
  id poolAfterInit = objc_msgSend(pool, initSel);

  // Create an instance of the local iTunes installation
  Class sbapplication = objc_getClass("SBApplication");
  id itunes = objc_msgSend(sbapplication, sel_registerName("applicationWithBundleIdentifier:"), @"com.apple.iTunes");

  // Is iTunes running?
  id running = objc_msgSend(itunes, sel_registerName("isRunning"));
  NSLog(@"iTunes Running: %d", running);

  // Print out the name of the iTunes application
  id name = objc_msgSend(itunes, sel_registerName("name"));
  NSLog(@"Name: %@", name);

  // Print out the current volume
  NSInteger curVol = (NSInteger) objc_msgSend(itunes, sel_registerName("soundVolume"));
  NSLog(@"Current Volume: %d", curVol);

  // If an argument was passed, assume it's an integer and set iTunes' volume
  if (argc > 1) {
    NSInteger volToSet = atoi(argv[1]);
    objc_msgSend(itunes, sel_registerName("setSoundVolume:"), volToSet);
    NSLog(@"Set Volume to: %d", volToSet);
  }

  return 0;
}

Realise this was created a long time ago but is a really useful resource.

objc_msgSend appears to have a odd definition where the params are just inserted wherever. Do you know if it's possible to send parameters gradually to an object by repeatedly sending objc_msgSend() (or some other function) for parameters instead?

id itunesPre = objc_msgSend(sbapplication, sel_registerName("applicationWithBundleIdentifier:"));
id itunes = objc_msgSend(itunesPre, @"com.apple.iTunes")

Additionally, do you know how to send a named parameter?