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isalnum()                Test for Alphanumeric Character (Macro)

 #include   <ctype.h>

 int        isalnum(c);
 int        c;                           Integer to be tested

    isalnum() is a macro that tests if 'c' is an alphanumeric character;
    that is, if it is included in the sets:

                        A-Z,  a-z, or  0-9

    Returns:    True (nonzero) if the integer is in one of the sets, and
                0 if not.  The result is undefined if 'c' is other than
                an ASCII character or EOF (i.e., undefined if c > 127).

      Notes:    isalnum() is a macro.

  -------------------------------- Example ---------------------------------

    The following statements continue to get input from stdin until a
    non-alphanumeric character is read.

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

         int x = 0;
         char buffer[80];
         int ch;

         main()
         {
            while (isalnum(ch = getchar()))
                  buffer[x++] = ch;
            printf("data input from console: %s \n",buffer);
         }