title: error: expected '#pragma omp' clause before 'num_thread'
body: <p>new to openmp. My environment as follows:</p>
  
  <pre><code>ubuntu 16.04TLS
  gcc 7.3.0
  </code></pre>
  
  <p>My code as follows:</p>
  
  <pre><code>#include &lt;omp.h&gt;
  #include &lt;iostream&gt;
  
  int main()
  {
      #pragma omp parallel num_thread(2)
      {
          std::cout &lt;&lt; "Hello World!\n";
      }
  
      return 0;
  }
  </code></pre>
  
  <p>and the command:</p>
  
  <pre><code>gcc -fopenmp main.cpp -o main
  </code></pre>
  
  <p>the error shows:</p>
  
  <blockquote>
    <p>error: expected ‘#pragma omp’ clause before ‘num_thread’ </p>
    
    <p>#pragma omp parallel num_thread(2)</p>
  </blockquote>
  
  <p>what should i do to solve this problem? Thx!</p>