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

That's part of the macro definition.

It allows you to concatenate strings inside the macro.

In your case, you can use bt from 7 to 0 like this:

REGISTER_BIT(myreg, 0)

and it will be expanded as:

((volatile _io_reg*)&myreg)->bit0

Without this, you'd have to define the bit part of the macro as one of the macro's arguments:

#define REGISTER_BIT(rg,bt) ((volatile _io_reg*)&rg)->bt

where the usage would be:

REGISTER_BIT(myreg, bit0)

which is more cumbersome.

This also allows you to build new names.

Assume you have these macros:

#define AAA_POS 1
#define AAA_MASK (1 << AAA_POS)
#define BBB_POS 2
#define BBB_MASK (1 << BBB_POS)

and you want a macro that extracts AAA from a bit vector. You can write it like this:

#define EXTRACT(bv, field) ((bv & field##_MASK) >> field##_POS)

and then you use it like this:

EXTRACT(my_bitvector, AAA)