cbitmemcpytwos-complement

How to initialize a buffer of large size to all bits on?

I am trying to initialize a bitmap to all bits on. My idea was to initialize an array of the buffer size and then use memcpy; however:

#include <stdio.h>

#define SIZE 5

int main () {
  int i[SIZE] = { ~0 };
  for (int j = 0 ; j < SIZE ; j++) {
    printf("%d\n", i[j]);
  }
  return 0;
}

results in:

-1
0
0
0
0

my expectation was that I would see all -1. Why does this not work, and how can I accomplish my goal?

Solution

  • This:

    int i[SIZE] = { ~0 };

    Explicitly initializes only the first element of the array to ~0. The rest are implicitly set to 0.

    If you want all bits to be set, you need to use memset:

    memset(i, ~0, sizeof i);