Without including #include <ctype.h>

Question

I have written the below programs without including #include <ctype.h>. I am able to execute the program. Where are these prototypes declared? I am using gcc.

1.

#include <stdio.h>
int main()
{
    if(isalnum(';'))
        printf("character ; is not alphanumeric");
    if(isalnum('A'))
        printf("character A is alphanumeric ");
    return 0;
}

2.

#include <stdio.h>
int main()
{
    printf("Lower case of A is %c \n", tolower('A'));
    printf("Lower case of 9 is %c \n", tolower('9'));
    printf("Lower case of g is %c \n", tolower('g'));
    printf("ASCII value of B is %d \n", toascii('B'));
    printf("Upper case of g is %c \n", toupper('g'));
    return 0;
}

Answer 1

1. In your code these functions are implicitly declared, so they are not included from any particular header. If you crank up warning level for your compiler, you will see (e.g. GCC):

   $ gcc -Wall -o a.c
   a.c: In function ‘main’:
   a.c:4: warning: implicit declaration of function ‘isalnum’
   

   If the definition of the function is not available, the compiler assumes i's a function taking any number of arguments and returning int. For example, the following compiles:

   main(){fgetc(1,2,3,4,5);}
   

2. As to where they should be declared, it's the <ctype.h> header. Of course, different C implementations may include this header in other headers, so the code may appear to work without including <ctype.h>, but if you want your code to compile without warnings across different C implementations, you should include this header.