title: isalpha check on argument results in segmentation fault
body: <p>working through some C problems and am hitting a wall with this. I can think of another way or two around this, but I'd like to understand better why my check is failing.</p>
  
  <pre><code>#include &lt;stdio.h&gt;
  #include &lt;ctype.h&gt;
  #include &lt;math.h&gt;
  #include &lt;string.h&gt;
  
  int main(int argc, char *argv[256]){
      //Require one alpha only argument if else exit 1
      if(argc &lt; 2){
          printf("Usage: ./vigenere arg1 [arg2] [arg3]...\n");
          return 1;
      }
      for (int i=1;i&lt;argc;i++){
          if(isalpha(argv[i]) == 1){
              return 1;
          }
          printf("%d\n",i);
      }
  
      //Prompt the user for some plaintext
  
      //Rotate plaintext by argument
  
      //Print Rotated Text
  
      // exit 0
  }
  </code></pre>
  
  <p>The script is working as expected until the <code>isalpha()</code> line. I'd assume argv's that have non-alpha characters in them would !=0 ergo skipping my return(1). However it seems to fail regardless of what is inserted as an argument.</p>
  
  <p>Any thoughts?</p>