CODEWITHSUNDEEP
c++bit-manipulationbitwise-operators
Mike Irish
Mike Irish

Reputation: 960

Can someone explain this bit wise operation

I am trying to understand what is happening in this bitwise operation that is used to set the size of a char array but I do not fully understand it.

unsigned long maxParams = 2;// or some other value, I just chose 2 arbitrarily

unsigned char m_uParamBitArray[(((maxParams) + ((8)-1)) & ~((8)-1))/8];

What size is this array set to based on the value of maxParams?

Upvotes: 0

Views: 71

Answers (2)

Jeffrey
Jeffrey

Reputation: 11430

This counts the number of 8-bit chars needed to fit maxParams bits.

+7 is to round up to the next multiple of 8.

0 -> 7 will round to 0 char

1 -> 8 will round to 1 char

After division by 8. Which is done by /8

The bitwise and is to discard the last three bits before dividing by 8.

Upvotes: 2

Sergey Kalinichenko
Sergey Kalinichenko

Reputation: 726987

When maxParams is not divisible by eight, the first part of the formula formula rounds it up to the next multiple of eight; otherwise, it leaves the number unchanged. The second part of the formula divides the result by eight, which it can always do without getting a remainder.

In general, if you want to round up to N without going over it, you can add N-1 before the division:

(x + (N - 1)) / N

You can safely drop the & ~((8)-1) part of the expression:

(maxParams + (8-1))/8

Upvotes: 2

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