title: How can I just one header file based on some condition?
body: <p>I have two header files:a.h and b.h. I want to use only one of them, based on some condition. How can I do this?</p>
  
  <p>Here some code:</p>
  
  <p>main.c:</p>
  
  <pre><code>#include&lt;stdio.h&gt;
  #include"a.h"
  #include"b.h"
  main() {
      test();
      system("pause");
      return 0;
  }
  </code></pre>
  
  <p>a.h:</p>
  
  <pre><code>#ifndef A_H
  #define A_H
  #include&lt;stdio.h&gt;
  void test();
  #endif
  </code></pre>
  
  <p>a.c:</p>
  
  <pre><code>#include "a.h"
  void test() {
      printf("Got tested from a.\n");
  }
  </code></pre>
  
  <p>b.h:</p>
  
  <pre><code>#ifndef B_H
  #define B_H
  #include&lt;stdio.h&gt;
  void test();
  #endif
  </code></pre>
  
  <p>b.c:</p>
  
  <pre><code>#include "b.h"
  void test() {
      printf("Got tested from b.\n");
  }
  </code></pre>
  
  <p>As I've said before,I only want to use one of them.How can I do this?</p>