C preprocessor: concatenation of another define

Question

I'm coding for a ARM with GCC and need to concatenate (##) a name with the a definition, like this:

#define LCD_E_PORT GPIOC 
...
#define RCC_PORT(x) (RCC_APB2Periph_ ## (x)) // ???
...

so that afterRCC_PORT(LCD_E_PORT) I would get RCC_APB2Periph_GPIOC. It is important to say, that the LCD_E_PORT and the RCC_APB2Periph_GPIOC are NOT strings but some low-level system defined pointers (accessing them processor's memory map).

The point of all this is to have one macro, which handles multiple port definitions.

Is there a solution for this?

Removed parantheses
With parentheses

I'm using arm-none-eabi-gcc.

What happens if you try #define RCC_PORT(x) RCC_APB2Periph_##x ? — Paul R
– Paul R, Commented Feb 26, 2015 at 11:03
For example: RCC_PORT(LCD_E_PORT) will result in RCC_APB2Periph_LCD_E_PORT so it is wrong. I've also tried double definition, I mean: #define RCC_PORT1(x) RCC_PORT(x) but after RCC_PORT1(LCD_E_PORT) I get an error that RCC_APB2_Periph_ and "(" don't give a valid token because, like I said, the GPIOx is a (u32 *) ptr. It is that or it doesn't expand macros, or it expands macros to the final definition... All what I need is the definition in the middle. — Grzegorz Grzeda
– Grzegorz Grzeda, Commented Feb 26, 2015 at 11:35

Community · Accepted Answer · 2017-05-23 10:09:53Z

4

The kind of thing you need required more indirection through the pre-processor:

This (defines.c) :

#define TOKENPASTE( x, y ) x ## y
#define TOKENPASTE2( x, y ) TOKENPASTE( x, y )

#define LCD_E_PORT GPIOC
#define RCC_PORT(x)  TOKENPASTE2( RCC_APB2Periph_, x )

#define STRING(x) #x

int main( int argc, char* argv[] )
{
    RCC_PORT(LCD_E_PORT);
}

results in the expected output through gcc -E defines.c we get:

# 1 "defines.c"
# 1 "<built-in>"
# 1 "<command-line>"
# 1 "defines.c"
# 10 "defines.c"
int main( int argc, char* argv[] )
{
    RCC_APB2Periph_GPIOC;
}

See this SO question for more information.

edited May 23, 2017 at 10:09

CommunityBot

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answered Feb 26, 2015 at 12:16

Brian Sidebotham

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