c/c++ single colon [duplicate]

This question already has answers here: Closed 11 years ago.

Possible Duplicate:

What does '开发者_开发问答;unsigned temp:3' means

I'm a newbie to c/c++ and I have this code running on linux and win32

typedef struct tMessageAction
{
    unsigned char ActionId : 4;
    unsigned short ID : 10;
}tMessageAction;

so what are the single colons doing to this variables? are they specifying how many bits are going to be used? is this related to some type of structure alignment attribute?

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The colons are defining a bitfield. They do not change the alignment rules.

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Here's about the C Bit fields.