What is '##' in C?

Question

I'm using USART in Keil and I saw this;

#define USART_DRV_NUM 6

#define USART_BAUDRATE 9600

#define _USART_Driver_(n) Driver_USART##n
#define USART_Driver_(n) _USART_Driver_(n)

extern ARM_DRIVER_USART USART_Driver_(USART_DRV_NUM);
#define ptrUSART (&USART_Driver_(USART_DRV_NUM))

Here is the thing;

#define _USART_Driver_(n) Driver_USART##n

I couldn't find any examples on the Internet. Does anyone know something about this thing?

Iharob Al Asimi · Answer 1 · 2016-02-08T21:45:48.670

1

It's called Token Pasting Operator. It generates an identifier for example

#define _USART_Driver_(n) Driver_USART##n

int _USART_Driver_(_EXAMPLE);

is translated to

int Driver_USART_EXAMPLE;

Note that, n is not a variable. It's replaced literally before actual compilation in the prepropcessing stage.

edited Feb 08 '16 at 21:45

answered Feb 08 '16 at 21:43

Iharob Al Asimi

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Thanks for answer. Can I use this in a loop? For example; while(i--){ husart.instance = _USART_Driver_(i) } – Arif Ahmet Balık Feb 08 '16 at 21:44
No, you can't. It's replaced at compile time, `n` cannot be a variable. – Iharob Al Asimi Feb 08 '16 at 21:45
Is there anything like that I can use in a loop? – Arif Ahmet Balık Feb 08 '16 at 21:46
Yes an array. What are you trying to do? – Iharob Al Asimi Feb 08 '16 at 21:47
I dont want to use array. This token pasting operator seems very clean. An array takes more memory than this macro. – Arif Ahmet Balık Feb 08 '16 at 21:50
What are you trying to do? This may not be the right option for you. – Iharob Al Asimi Feb 08 '16 at 21:51
I have an input. For example : "init -usart 6" and I need to init USART6 :) – Arif Ahmet Balık Feb 08 '16 at 21:56
Can I use enum for this macro? – Arif Ahmet Balık Feb 08 '16 at 21:56
No, macros are compile time and you need a runtime solution, it will not work with macros at all. – Iharob Al Asimi Feb 08 '16 at 21:57
So, I need to use an array? Is that most efficent method? – Arif Ahmet Balık Feb 08 '16 at 21:58
Perhaps, I can't tell you without seeing the ACTUAL problem. You can send it to me at iharob@gmail.com and I can take a look. BTW: انا اسمي يعرب – Iharob Al Asimi Feb 08 '16 at 21:59

What is '##' in C?

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