Bit operations

C can be used to operate on bits. This is useful for low-level programming though the operations are also used when writing ${\cal X}$ graphics applications.

Setting a bit :-

Suppose you wanted to set bit 6 of i (a long, say) to 1. First you need to create a mask that has a 1 in the 6th bit and 0 elsewhere by doing `1L<<6' which shifts all the bits of the long 1 left 6 bits. Then you need to do a bit-wise OR using `i = i | (1L<<6)'.

Unsetting a bit :-

Suppose you wanted to set bit 6 of i (a long, say) to 0. First you need to create a mask that has a 0 in the 6th bit and 1 elsewhere by doing `1L<<6' then inverting the bits using the ~ operator. Then you need to do a bit-wise AND using the & operator. The whole operation is `i =i & ~(1<<6)' which can be contracted to `i &= ~(1<<6)'.

Creating a mask for an ${\cal X}$ call :-

In ${\cal X}$ graphics, masks are often created each of whose bits represent a option that is to be selected in some way. Each bit can be referred to using an alias that has been set up in an include file. E.g. a mask which could be used in a call to make a window sensitive to key presses and buttonpresses could be set up by doing

unsigned int mask = KeyPressMask | ButtonPressMask;