The ## operator in C

Question

What does ## do in C?

Example:

typedef struct
{
    unsigned int bit0:1;
    unsigned int bit1:1;
    unsigned int bit2:1;
    unsigned          


        

Answer 1

It's called the pasting operator; it concatenates the text in bt with the text bit. So for example, if your macro invocation was

REGISTER_BIT(x, 4)

It would expand to

((volatile _io_reg*)&x)->bit4

Without it, you couldn't put a macro argument directly beside text in the macro body, because then the text would touch the argument name and become part of the same token, and it'd become a different name.