isxdigit() is used to check if the argument is a hexadecimal characters in the range A-F,a-f,or 0-9. Those are "0 1 2 3 4 5 6 7 8 9 a b c d e f A B C D E F". If the argument does not have a them, the function returns Zero.

    int isxdigit ( int c );

   #include <stdio.h>
   #include <stdlib.h>
   #include <ctype.h>
   char str[] = { 'B', 'a','0', '@' };
   #define SIZE sizeof( str ) / sizeof( char )
   int main( void )
     {
        int j;
        for( j = 0; j < SIZE; j++ ) 
        {
        if( isxdigit( str[j] ) ) 
          {
            printf( "Char %c is a hexadecimal 
                          digit\n",str[j] );
          } 
        else 
          {
            printf( "Char %c is not a hexadecimal 
                          digit\n",str[j] );
          }
        }
        return 0;
     }

    Char B is a hexadecimal digit
    Char a is a hexadecimal digit
    Char 0 is a hexadecimal digit
    Char @ is not a hexadecimal digit

In the above example, this function is used to check for hexadecimal characters in an array "str". In the array, all the digits are checked and correspondingly they are displayed.