Vid Vid - 2 months ago 7x
C Question

c code for valid netmask

I there any way to check that 32 bit netmask is valid or not using bitwise operator?

I have to check from msb side that '1' are in continuous stream or not.
eg 11111111.0.0.0 ( valid
but 11111101.0.0.0 ( is not.


First thing to do is to check for the netmask being non zero (a nasty edge case). Given this is ok, you need to take the bitwise inverse.

uint32_t y = ~x;

Then add one

uint32_t z = y + 1;

Then if x was a proper netmask, there will be at most 1 bit set in this.

To test that, simply and z with z - 1, which happens to be y. The result will be zero if all is OK, non zero otherwise.

valid = (z & y) == 0;