Undefined symbols for architecture x86_64 in hello world - objective-c

Question

Today I'm learning Objective-C without Xcode following this Guide
Here's the hello world:
makclass.m
#import "makclass.h"
#import <stdio.h>
#implementation MakClass
-(void) age {
printf("Age is %d", age);
}
-(void) setAge: (int) a {
age = a;
}
+(void) say {
printf("Haha");
}
#end
makclass.h
#import <Foundation/NSObject.h>
#interface MakClass: NSObject {
int age;
}
-(void) age;
-(void) setAge: (int) a;
+(void) say;
#end
helloworld.m
#import <stdio.h>
#import "makclass.h"
int main(void) {
printf("Hello World");
MakClass *makclass = [[MakClass alloc] init];
[makclass setAge: 1];
[makclass age];
[MakClass say];
[makclass release];
return 0;
}
And clang helloworld.m give me the following errors:
Undefined symbols for architecture x86_64:
"_OBJC_CLASS_$_MakClass", referenced from:
objc-class-ref in helloworld-XEijke.o
"_objc_msgSend", referenced from:
_main in helloworld-XEijke.o
"_objc_msgSend_fixup", referenced from:
l_objc_msgSend_fixup_alloc in helloworld-XEijke.o
l_objc_msgSend_fixup_release in helloworld-XEijke.o
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)

Answer 1

You need to make sure you pass in the framework and all the files as well:
clang -framework Foundation -o helloworld helloworld.m makclass.m

Answer 2

Welcome to Objective-C!
When you compile, you need to specify all of the implementation files that helloworld.m requires. Just because the header files are included does not mean that the implementation files will be included as well.
Additionally, since this is Objective-C, make sure you also include the -lObjC flag to include the Objective-C libraries.
So, the command that you should run will look like clang -lObjC helloworld.m makclass.m.
clang will then spit out a program called a.out, which you can run. If you want to change the name from a.out to, say, helloworld, you can specify that with the -o flag. For example:
clang -lObjc -o helloworld helloworld.m makclass.m
Hope this helps!