Question

I have read in the K&R II C Programming ANSI C book that the “>>” and “<<” operators control bits, and of course with me being a noob, I don’t understand when to use them. I got interested in figuring out how to build packets manually and I came across the following snippet:

unsigned short csum(unsigned short *buf, int nwords)
{       
        unsigned long sum;
        for(sum=0; nwords>0; nwords--)
                sum += *buf++;
        sum = (sum >> 16) + (sum &0xffff);
        sum += (sum >> 16);
        return (unsigned short)(~sum);
}

I know that this calculates the checksum, but I don’t understand what is going on here. XD

Obviously this is out of my skill range, but I figured I can use this snippet as a scapegoat to figure out some unanswered questions. When do you know when to use the bitwise operators to achieve a certain value, why not just add (+) or subtract (-)? Also, why is there a hexadecimal &0xffff there next to sum, if there are no operators with the two?

P.S. What does ~sum mean?

Answer 1

That’s not a question, that’s a whole bunch. 🙂

1. You use bitwise operators when you want to view a number as a collection of bits, rather than an integer. It’s much easier to say “I want this bit-pattern shifted two bits to the left” than to create the mathematically equivalent operation. They’re conceptually different; if you think of the number as bits, using a bit-operator makes more sense.
2. The & 0xffff makes sure the value is 16 bits, by masking off all higher bits. This assumes the system’s unsigned long is at least 16 bits wide, which is a pretty safe assumption. The & (bitwise AND) is often used for this purpose. Look at the truth table for logical conjunction and think “false is 0, true is 1” to see how this works.
3. The & before the hexadecimal constant is C’s bitwise AND operator, which is used to do the masking I describe above. Basically, for single-bit variables a & b, the result is 1 if and only if both a and b are 1. The operator applies this logic to each pair of bits in its input terms.
4. The ~ operator is C’s bitwise inversion, it “flips” the bits of its argument. It is commonly used to create masks.