Question

Briefly: Question is related to bitwise operations on hex – language C ; O.S: linux

I would simply like to do some bitwise operations on a “long” hex string.
I tried the following:

First try:

I cannot use the following because of overflow:

long  t1 = 0xabefffcccaadddddffff;
and t2 = 0xdeeefffffccccaaadacd;

Second try: Does not work because abcdef are interpreted as string instead of hex

char* t1 = "abefffcccaadddddffff";
char* t2 = "deeefffffccccaaadacd";

int len = strlen(t1);

for (int i = 0; i < len; i++ )
    {
        char exor = *(t1 + i) ^ *(t2 + i);
    printf("%x", exor);
}

Could someone please let me know how to do this? thx

Answer 1

Bitwise operations are usually very easily extended to larger numbers.

The best way to do this is to split them up into 4 or 8 byte sequences, and store them as an array of uints. In this case you need at least 80 bits for those particular strings.

For AND it is pretty simple, something like:

unsigned int A[3] = { 0xabef, 0xffcccaad, 0xddddffff };
unsigned int B[3] = { 0xdeee, 0xfffffccc, 0xcaaadacd };
unsigned int R[3] = { 0 };

for (int b = 0; b < 3; b++) {
    R[b] = A[b] & B[b];
}

A more full example including scanning hex strings and printing them:

#include<stdio.h>
#include<string.h>
#include<stdlib.h>

typedef unsigned int uint;

void long_Print(int size, const uint a[]) {
    printf("0x");
    for (int i = 0; i < size; i++) {
        printf("%x", a[i]);
    }
}

void long_AND(int size, const uint a[], const uint b[], uint r[]) {
    for (int i = 0; i < size; i++) {
        r[i] = a[i] & b[i];
    }
}

// Reads a long hex string and fills an array. Returns the number of elements filled.
int long_Scan(int size, const char* str, uint r[]) {
    int len = strlen(str);
    int ri = size;

    for (const char* here = &str[len]; here != str; here -= 8) {
        if (here < str) {
            char* tmp = (char*)malloc(4);

            tmp[0] = '%';
            tmp[1] = (char)(str - here + '0');
            tmp[2] = 'x';
            tmp[3] = '\0';

            sscanf(str, tmp, &r[ri--]);

            free(tmp);

            break;
        }
        else {
            sscanf(here, "%8x", &r[ri--]);
        }
    }

    for (; ri >= 0; ri--) {
        r[ri] == 0;
    }

    return size - ri;
}

int main(int argc, char* argv[])
{
    uint A[3] = { 0 };
    uint B[3] = { 0 };
    uint R[3] = { 0 };

    long_Scan(3, "abefffcccaadddddffff", A);
    long_Scan(3, "deeefffffccccaaadacd", B);

    long_Print(3, A);
    puts("\nAND");
    long_Print(3, B);
    puts("\n=");

    long_AND(3, A, B, R);
    long_Print(3, R);

    getchar();

    return 0;
}