Converting a number from one base to another

Write a program that prompts the user for a nonzero hexadecimal number, uses getchar() to read

the input, and prints out its value in decimal.

The input can be preceded or followed by blanks and will end in carriage return (anything input after the first carriage return will be ignored). An error message should be output in response to inputs that are incorrect, that is, do not start (after possibly some spaces) with 0x or 0X followed by hexadecimal digits, or are too large to fit in an unsigned long, or are followed by anything but spaces before carriage return.

Here are examples of possible inputs and outputs on a machine where an unsigned long occupies 8 bytes.