is...       tolower       toupper
#include <stdio.h>
#include <ctype.h>

int main (int argc, char *argv[])
{
    char *str = "A8@\033b\t,";
    int i;

    for ( i = 0; i < strlen(str); i++ )
    {
        if ( isalnum(str[i]) )
        {
            printf("%c is alnum\n",str[i]);
        }
        if ( isalpha(str[i]) )
        {
            printf("%c is alpha\n",str[i]);
        }
        if ( isascii(str[i]) )
        {
            printf("%c is ascii\n",str[i]);
        }
        if ( isblank(str[i]) )
        {
            printf("%c is blank\n",str[i]);
        }
        if ( iscntrl(str[i]) )
        {
            printf("%c is cntrl\n",str[i]);
        }
        if ( isdigit(str[i]) )
        {
            printf("%c is digit\n",str[i]);
        }
        if ( isgraph(str[i]) )
        {
            printf("%c is graph\n",str[i]);
        }
        if ( islower(str[i]) )
        {
            printf("%c is lower\n",str[i]);
        }
        if ( ispunct(str[i]) )
        {
            printf("%c is punct\n",str[i]);
        }
        if ( isprint(str[i]) )
        {
            printf("%c is print\n",str[i]);
        }
        if ( isupper(str[i]) )
        {
            printf("%c is upper\n",str[i]);
        }
        if ( isxdigit(str[i]) )
        {
            printf("%c is xdigit\n",str[i]);
        }
    }

    return 0;
}
OUTPUT :
//   A is alnum
//   A is alpha
//   A is ascii
//   A is graph
//   A is print
//   A is upper
//   A is xdigit
//   8 is alnum
//   8 is ascii
//   8 is digit
//   8 is graph
//   8 is print
//   8 is xdigit
//   @ is ascii
//   @ is graph
//   @ is punct
//   @ is print
//   \033 is ascii
//   \033 is cntrl
//   b is alnum
//   b is alpha
//   b is ascii
//   b is graph
//   b is lower
//   b is print
//   b is xdigit
//        is ascii
//        is blank
//        is cntrl
//   , is ascii
//   , is graph
//   , is punct
//   , is print

tolower
#include <stdio.h>
#include <stdlib.h>
#include <ctype.h>
#include <string.h>

int Isitsame (void);
// ////////////////////////////////////////////////

int main (int argc, char **argv)
{
    int res;
    res = Isitsame();
    printf ("\n\t%d\n", res);

    return 0;
}
OUTPUT :
// ////////////////////////////////////////////////
int Isitsame (void)
{
     char *arr1, *arr2, *p1, *p2;
     
     arr1 = malloc (256);
     arr2 = malloc (256);
     
     printf ("\nEnter first string : ");
     fgets (arr1, 255, stdin);
     fflush (stdin);

     printf ("Enter second string: ");
     fgets (arr2, 255, stdin);
     fflush (stdin);

     p1 = arr1; p2 = arr2;
     
     while (*p1)
     {
         *p1 = tolower(*p1);
         p1++;
     }
     while (*p2)
     {
         *p2 = tolower(*p2);
         p2++;
     }

     printf ("\n%s", arr1);
     printf ("%s", arr2);

     if ( strcmp (arr1, arr2) == 0 )
         return 1;
     else
         return 0;
}
OUTPUT :
//   Enter first string : Hello
//   Enter second string: HELLO
//   
//   hello
//   hello
//   
//       1

toupper
#include <stdio.h>
#include <string.h>
#include <ctype.h>

int no_case_comp(char c1, char c2)
{
    //  return true (1) if equal
    return toupper(c1) == toupper(c2);
}
OUTPUT :
// //////////////////////////////////////////
int main ( int argc, char *argv[] )
{
    char s1[] = "String";
    char s2[] = "STRING";
    int flag = 0;
    int i, res;

    if ( strlen(s1) == strlen(s2) ) 
    {
        for ( i = 0; i < strlen(s1); i++ )
        {
            if ( !no_case_comp(s1[i], s2[i]) )
            {
                flag = 1;
                break;
            }
        }
        if ( flag )
            puts("Strings are not equal");
        else
            puts("Strings are equal");
    }
    else
        puts("Strings are not equal");

    return 0;
}
OUTPUT :
//  Strings are equal