在C中为’atoi’功能发出警告

我正在收到一条警告，内容如下：
int countdownStart = atoi(numInput);

我得到的警告说：
函数’atoi’的隐式声明在C99中无效

#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!nn",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("%dn",i);
        if (i % 5 == 0){
            printf("Found one!n");
        }
    }
}

return 0;

}