title: Newbie Objective C Error such as _main
body: <p>I've been tinkering all day, and I can't seem to fix this error. </p>
  
  <p>Here's the code: </p>
  
  <pre><code>//
  //  main.m
  //  Learning ObjC
  //
  //  Created by Nickirv on 8/9/15.
  //  Copyright (c) 2015 Nickirv. All rights reserved.
  //
  
  #import &lt;Foundation/Foundation.h&gt;
  
  @interface Person: NSObject{
      int age;
      int weight;
  }
  -(void) print;
  -(void) setAge: (int) a;
  -(void) SetWeight: (int) w;
  @end
  </code></pre>
  
  <p>And it outputs this issue: </p>
  
  <pre><code> "_main", referenced from:
       implicit entry/start for main executable
  ld: symbol(s) not found for architecture x86_64
  clang: error: linker command failed with exit code 1 (use -v to see invocation)
  </code></pre>
  
  <p>I would appreciate any help! Thank you very much! </p>