title: Getting warning in C for 'atoi' function
body: <p>I'm currently coding for a challenge question in a book I'm reading. My code executes perfectly with the correct output, but i"m getting a warning in my code and I'm just wondering why.</p>
  
  <p>I'm getting a warning on the line that reads:</p>
  
  <pre><code>int countdownStart = atoi(numInput);
  </code></pre>
  
  <p>The warning I'm getting says:</p>
  
  <blockquote>
    <p>Implicit declaration of function 'atoi' is invalid in C99</p>
  </blockquote>
  
  <pre><code>#import &lt;readline/readline.h&gt;
  #import &lt;stdio.h&gt;
  
  int main(int argc, const char * argv[]){
      printf("Who is cool? ");
      const char *name = readline(NULL);
      printf("%s is cool!\n\n", name);
  
      printf("What should I start counting? ");
      const char *numInput = readline(NULL);
      int countdownStart = atoi(numInput);
      for (int i = countdownStart; i &gt;= 0; i--){
          if (i % 3 == 0){
              printf("%d\n", i);
              if (i % 5 == 0){
                  printf("Found one!\n");
              }
          }
      }
  
      return 0;
  }
  </code></pre>