Bitwise or between 2 sets of bytes in c

Question

Let's say we have

int *set1 = malloc(n);

int *set2 = malloc(n);

Then we fill these 2 sets with data. My goal is to perform bitwise or between set1 and set2.

I suppose I can't do something like

for (i=0; i<n; i++) {
    set1[i] = set1[i] | set2[i];
}

because n is the number of bytes and not the number of cells of the array.

Any ideas? Thanks in advance

Answer 1

set1 and set2 are pointers to integers. You try to process bytes. This is not correct.

Try to picture the memory layout:

malloc(1) would allocate one byte, i.e. 8 bits. int *set1 declares a pointer to an int, which typically is 4 bytes, i.e. 32 bits.

A char typically represents 1 byte, i.e. 8 bits. So why not go with that, char *set1 and char *set2 ?

Answer 2

size_t elements = n / sizeof *set; 

for ( i = 0; i < elements; i++ )
  set1[i] = set1[i] | set2[i];

Notes:

• The expression *set1 has type int; thus the expression sizeof *set1 is the same as sizeof (int). I prefer using expressions like this instead of explicitly naming the type; it makes things easier if the type of set1 ever needs to change.

• If n is the number of bytes, n / sizeof *set1 is the number of int elements in the arrays. Remember that in C integer division yields an integer result - if n is not a multiple of sizeof (int), then the result will be the floor of n / sizeof (int) - i.e., 7/4 == 1, 11/4 == 2, etc. It also means that you won’t access those last few bytes as part of that loop; since they don’t make up a ”complete" int object, that’s not a bad thing.