modulo 2 modulus

I'm having trouble writing a modulo 2 modulus function. I need to divide a 260 byte unsigned integer by a 4 byte unsigned integer without carrying bits (modulo 2) and return the remainder. Currently I'm using loops to perform bitshift and logical operations and I'm thinking it might be cleaner if I wrote separate functions for performing binary operations on arbitrarily large amounts of data. The reason why I know my function isn't currently working is that if I divide a dividend by a divisor, then add the remainder to the dividend and divide again, the second remainder is not zero. I would appreciate some help getting this function working.

My current function is:

Code:

---

char *divide(char *message, char *divisor, char *remainder)
{
        unsigned int *poly=divisor;
        unsigned int *result=remainder;
        int i, j;

        for (i=0; i<260; i++)
        {
                for (j=7; j>=0; j--)
                {
                        int action=(*result>>31)&1;
                        *result<<=1;
                        *result|=(*(message+i)>>j)&1;
                        *result^=action*i;
                }
        }

        for (i=0; i<32; i++)
        {
                int action=(*result>>31)&1;
                *result<<=1;
                *result^=action*i;
        }

        unsigned int temp=(1<<31)-1;

        return (*result & temp);

}

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In case it helps, I uploaded my whole source to show some context.

I changed it to cpp so that I could use booleans and wrote this:

Code:

---

char *divide(char *message, char *divisor, char *remainder)
{
        unsigned int result=*((unsigned int*)message);
        const unsigned int div=*((unsigned int*)divisor);
        bool *p=(bool*)(message+32);
        int i=2048;

        while (i--)
        {
                if (result>div)
                {
                        result^=div;
                }

                result<<1;
                result+=*(p++);
        }

        *((unsigned int*)remainder)=result;

        return remainder;
}

---

but it still isn't comming out with a zero remainder.

Could I have some comments?
Also is char *message the dividend?

If you want I could slap together some pseudo-code to work with numbers as strings and a basic algorithm some hardware uses to divide numbers.

Finally what do you mean by carry? Do you mean you can't save the msb of a string you are shifting left?

Quote:

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I changed it to cpp so that I could use booleans

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You can do that in C (C99), too. Just use _Bool instead of bool or include <stdbool.h> and use bool as before.

In C89, you can use this code:

Code:

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typedef enum {false, true} bool;

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Thanks for the thing about stdbool.h, I hadn't heard of it before. I think my problem is that I'm not understanding how to properly do modulo 2 division. The idea is that since we are concerned about bits, we do not carry when we do arithmatic. So instead of addition or subtraction, we use XOR. I want to be able to check for errors in a message that I send so I want to divide the bits of the message (*message is the dividend), add the remainder and divide again. If the remainder is 0 then I know the message sent without errors. I am having trouble writing a function that can divide the message by a divisor using binary arithmatic (modulo 2). If anyone knows some sort of sudo-code for this type of function, to divide a message by a divisor, it would help a lot.

Currently running the program looks like this:
message: 61736466000000000000000000.....
divisor: 37000000
remainder: ffffff9a506566
message: 61736466000000000000000000.....ffffff9a506566
remainder: ffffffc14b6566

if this were working properly, the second remainder should have been 0.

Code:

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*((unsigned int*)remainder)=result;

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Why not pass remainder as an unsigned int * instead of casting it? My compiler gives a warning about that, something like "use of casts in lvalues is deprecated".