Implicit declaration of functions

Question

I am getting this error from the isdigit and isalpha functions

 warning: implicit declaration of function ‘isdigit’ [-Wimplicit-function-declaration]
   numberCheck = isdigit(englishWords[i]);
  warning: implicit declaration of function ‘isalpha’ [-Wimplicit-function-declaration]
   letterCheck = isalpha(englishWords[i]);

my code is:

char * inputEnglish()
{
    char englishWords[MAX_LENGTH];
    char required[MAX_LENGTH] = {"message="};
    char * messageToBeSent;
    int i;
    int numberCheck;
    int letterCheck;
    i = 0;

    printf("Please enter the word you'd like to translate\n");
    fgets(englishWords, MAX_LENGTH, stdin);
    for(i = 0; i < (strlen(englishWords) + 1); i++)
    {
            numberCheck = isdigit(englishWords[i]);
            letterCheck = isalpha(englishWords[i]);
            if((numberCheck != 0)  || (letterCheck != 0))
            {
                    printf("Please enter valid Input");
            }
    } 
    strcat(required, englishWords);
    messageToBeSent = malloc(sizeof(char)*(strlen(required)+1));
    strcpy(messageToBeSent, required);

    return (messageToBeSent);
}

How do i get rid of these warnings?

Answer 1

You're using functions that the compiler has not been informed about, so it makes assumptions, in particular about the return type. As @ShafikYaghmour says in the comments, the proper include file will provide the compiler with the necessary information, even though (in this case) your code will probably work.