Convert a hexadecimal string to an integer efficiently in C?

Question

In C, what is the most efficient way to convert a string of hex digits into a binary unsigned int or unsigned long?

For example, if I have

Answer 1

Hexadecimal to decimal. Don't run it on online compilers, because it won't work.

#include
void main()
{
    unsigned int i;
    scanf("%x",&i);
    printf("%d",i);
}