invoke Obj-C from pure-C

invokeObjC.c

#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <ctype.h>
#include <objc/message.h>


SEL sel_propSetter(Class clazz, const char* pname)
{
    char* attr = property_copyAttributeValue(class_getProperty(clazz, pname), "S");
    if (!attr) {
        attr = malloc(strlen(pname) + sizeof("set") + 1/*:*/);
        sprintf(attr, "set%c%s:", toupper(*pname), pname + 1);
    }
    SEL uid = sel_getUid(attr ? attr : pname);
    free(attr);
    return uid;
}

SEL sel_propGetter(Class clazz, const char* pname)
{
    char* attr = property_copyAttributeValue(class_getProperty(clazz, pname), "G");
    SEL uid = sel_getUid(attr ? attr : pname);
    free(attr);
    return uid;
}

void invokeObjC(void)
{
    Class clsTestClass = objc_getClass("TestClass");
    id obj = objc_msgSend(objc_msgSend((id)clsTestClass, sel_registerName("alloc")), sel_registerName("init"));

    printf("intValue=%d\n", (int)objc_msgSend(obj, sel_propGetter(clsTestClass, "intValue")));

    printf("boolValue=%d\n", (BOOL)objc_msgSend(obj, sel_propGetter(clsTestClass, "boolValue")));

    objc_msgSend(obj, sel_propSetter(clsTestClass, "intValue"), 100);

    objc_msgSend(obj, sel_propSetter(clsTestClass, "boolValue"), 0);

    objc_msgSend(obj, sel_registerName("method"));
    
    objc_msgSend(obj, sel_registerName("dealloc"));
}

main.m

#import <Foundation/Foundation.h>

@interface TestClass : NSObject

- (void)method;

@property(nonatomic) int intValue;
@property(nonatomic) BOOL boolValue;

@end

@implementation TestClass
- (instancetype)init {
    NSLog(@"TestClass init");
    if (self = [super init]) {
        _intValue = 42;
        _boolValue = YES;
    }
    return self;
}

- (void)dealloc {
    NSLog(@"TestClass dealloc");
}

- (void)method {
    NSLog(@"TestClass method i=%d b=%d", _intValue, _boolValue);
}
@end

#pragma mark -
void runObjC() {
    TestClass *obj = [[TestClass alloc] init];
    NSLog(@"intValue=%d", obj.intValue);
    NSLog(@"boolValue=%d", obj.boolValue);
    obj.intValue = 100;
    obj.boolValue = 0;
    [obj method];
}

extern void invokeObjC(void);

int main(int argc, const char * argv[]) {
    @autoreleasepool { runObjC(); }
    invokeObjC();
    return 0;
}