Getting warning in C for 'atoi' function

Question

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.

I'm getting a warning on the line that reads:

int countdownStart = atoi(numInput); 

The warning I'm getting says:

Implicit declaration of function 'atoi' is invalid in C99

#import <readline/readline.h> #import <stdio.h>  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 >= 0; i--){         if (i % 3 == 0){             printf("%d\n", i);             if (i % 5 == 0){                 printf("Found one!\n");             }         }     }      return 0; } 

Answer 1

You have to include stdlib.h

#include <stdlib.h> 

Next time you encounter similar warnings just run man atoi and the manual pages should state that which header file should be included.