How to create mask with least significat bits set to 1 in C

Question

Can someone please explain this function to me?

A mask with the least significant n bits set to 1.

Ex:

n = 6 --> 0x2F, n = 17 --> 0x1FFFF // I don\'t ge

Answer 1

I believe your first example should be 0x3f.

0x3f is hexadecimal notation for the number 63 which is 111111 in binary, so that last 6 bits (the least significant 6 bits) are set to 1.

The following little C program will calculate the correct mask:

#include 
#include 

int mask_for_n_bits(int n)
{
    int mask = 0;

    for (int i = 0; i < n; ++i)
        mask |= 1 << i;

    return mask;
}

int main (int argc, char const *argv[])
{
    printf("6: 0x%x\n17: 0x%x\n", mask_for_n_bits(6), mask_for_n_bits(17));
    return 0;
}