set64bit.c

#include <assert.h>

typedef unsigned __int64 uint64;
typedef unsigned char byte;

void setBit(byte *data, int pos) {
    byte bits[] = {
        0x80, 0x40, 0x20, 0x10, 0x08, 0x04, 0x02, 0x01
    };
    /* note the position starts from 1, not 0 */
    data[pos / 8] |= bits[pos % 8 - 1];
}

#define OFFSET_PTR(i, o) (((byte*)i) + o)

uint64 htoni64(uint64 i) {
    uint64 r = 0;
    *OFFSET_PTR(&r, 0) = *OFFSET_PTR(&i, 7); 
    *OFFSET_PTR(&r, 1) = *OFFSET_PTR(&i, 6); 
    *OFFSET_PTR(&r, 2) = *OFFSET_PTR(&i, 5); 
    *OFFSET_PTR(&r, 3) = *OFFSET_PTR(&i, 4); 
    *OFFSET_PTR(&r, 4) = *OFFSET_PTR(&i, 3); 
    *OFFSET_PTR(&r, 5) = *OFFSET_PTR(&i, 2); 
    *OFFSET_PTR(&r, 6) = *OFFSET_PTR(&i, 1); 
    *OFFSET_PTR(&r, 7) = *OFFSET_PTR(&i, 0); 
    return r;
}

/*
 Because the bit position is defined in network byte order (big-endian), but the integer 
 represented on i386 is little-endian byte order. Or we can say the bit position is counted
 from the start address.
*/
int main(int, char **) {
    uint64 a = 0x0020000000C00012;
    uint64 b = 0;
    setBit((byte*) &b, 11);
    setBit((byte*) &b, 41);
    setBit((byte*) &b, 42);
    setBit((byte*) &b, 60);
    setBit((byte*) &b, 63);
    uint64 c = htoni64(b);
    assert(a == c);
    return 0;
}