isalnum

  #include <ctype.h>
  int isalnum( int ch );

The function isalnum() returns non-zero if its argument is a numeric digit or a letter of the alphabet. Otherwise, zero is returned.

   char c;
   scanf( "%c", &c );
   if( isalnum(c) )
     printf( "You entered the alphanumeric character %c\n", c );