Question

I am reviewing some C code, and found a header file full of defines of the style:

#define BLABLABLABLA (1 << 2)
#define XXXXXXXXXXXX (1 << 3)
#define YYYYYYYYYYYY (1 << 4)

What do they mean? What do they do?

Answer 1

<< is the shift operator in C.
So you define BLABLABLABLABLA by a zero value with a binary 1 shifted by 2 digits to the left.

The resulting value is then :
…00000100

You would normally do this do mask things.
So, say you have one status byte where every bit is a flag.
And if the 3rd bit is set, this means BLABLABLABLABLA, you would do :

int blablaFlag = statusByte & BLABLABLABLABLA;

If this is greater 0, your flag was set.