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cmathbinarybit-manipulation

How can I check my byte flag, verifying that a specific bit is at 1 or 0?

I use a byte to store some flag like 10101010, and I would like to know how to verify that a specific bit is at 1 or 0.

Solution

  • Here's a function that can be used to test any bit:

    bool is_bit_set(unsigned value, unsigned bitindex)
{
    return (value & (1 << bitindex)) != 0;
}

    Explanation:

    The left shift operator << creates a bitmask. To illustrate:

    • (1 << 0) equals 00000001
    • (1 << 1) equals 00000010
    • (1 << 3) equals 00001000

    So a shift of 0 tests the rightmost bit. A shift of 31 would be the leftmost bit of a 32-bit value.

    The bitwise-and operator (&) gives a result where all the bits that are 1 on both sides are set. Examples:

    • 1111 & 0001 equals 0001
    • 1111 & 0010 equals 0010
    • 0000 & 0001 equals 0000.

    So, the expression:

    (value & (1 << bitindex))

    will return the bitmask if the associated bit (bitindex) contains a 1 in that position, or else it will return 0 (meaning it does not contain a 1 at the assoicated bitindex).

    To simplify, the expression tests if the result is greater than zero.

    • If Result > 0 returns true, meaning the byte has a 1 in the tested bitindex position.
    • All else returns false meaning the result was zero, which means there's a 0 in tested bitindex position.

    Note the != 0 is not required in the statement since it's a bool, but I like to make it explicit.