Objective C

There isn't too much to know about compiling Objective C sources; you must ensure that you link the program with the libobjc.a using -lobjc. Here's a few sources continuing the 'Hello, world!' theme - the first source file is main.m:

     /* main.m */
     #include "HelloWorld.h"
     
     int main()
     {
       id helloWorld;
       helloWorld = [HelloWorld new];
       [helloWorld world];
       return 0;
     }
     

And the header file containing the interface for the HelloWorld class:

     /* HelloWorld.h */
     #ifndef _HI_WORLD_INCLUDED
     #define _HI_WORLD_INCLUDED
     #include <objc/Object.h>
     #define world_len 13
     
     @interface HelloWorld : Object
     {
       char helloWorld[world_len];
     }
     - world;
     
     @end
     
     #endif
     

And finally, the implementation of HelloWorld:

     /* HelloWorld.m */
     #include "HelloWorld.h"
     
     @implementation HelloWorld
     
     - world
     {
       strcpy(helloWorld, "Hello, world!");
       printf("%s\n", helloWorld);
     }
     
     @end
     

Compiling this using the command

gcc -lobjc -lpthread -o helloWorld main.m HelloWorld.m

creates the executable helloWorld which, as you can predict, prints out the message "Hello, world!". You can use all the standard compilation options already described to compile Objective C programs.

NOTE

You'll notice that I added -lpthread too. I did this, because if I didn't, the following errors popped up:

               /usr/lib/gcc-lib/i486-suse-linux/2.95.2/libobjc.a
               (thr-posix.o):
               In function `__objc_init_thread_system':
               thr-posix.o(.text+0x41): undefined reference to
               `pthread_key_create'
               /usr/lib/gcc-lib/i486-suse-linux/2.95.2/libobjc.a
               (thr-posix.o):
               ...
               In function `__objc_mutex_trylock':
               thr-posix.o(.text+0x1f1): undefined reference to
               `pthread_mutex_trylock'
               collect2: ld returned 1 exit status
               

This is because without the -lpthread flag, a number of references from the archive libobjc.a could not be resolved, so we had to explicitly call in the pthread objects.