Re: [avr-gcc-list] Preprocessor question

[Preston Wilson]

"Thomas D. Dean" wrote:

> I have a question about preprocessor syntax.
> 
> I want to declare som macros that ease controlling port assignments.
> 
> in the include file, I have
> 
> #define GLUE(a, b)     (a##b)
> #define PORT(x)        (GLUE(PORT, x))
> #define PIN(x)         (_BV((x)))
> #define DDR(x)         (GLUE(DDR, x))
> #define SET_INPUT(x)   ( GLUE(x, _DDR)  |= ~GLUE(x, _PIN ) )
> #define SET_OUTPUT(x)  ( GLUE(x, _DDR) |= GLUE(x, _PIN ) )
> #define SET_BIT(p,b)   (p |= b)
> #define CLR_BIT(p,b)   (p &= ~b)
> 
> #define LEFT_MOTOR_DIR_PORT      PORT(C)
> #define LEFT_MOTOR_DIR_PIN       PIN(3)
> #define LEFT_MOTOR_DIR_DDR       DDR(C)
> 
> 
> In the .c file, I have
> 
> #include <avr/io.h>
> #include <macro.h>
> int main() {
>   SET_INPUT(LEFT_MOTOR_DIR);
>   return 0;
> }
> 
> # avr-gcc -mmcu=atmega16 -Wall -Wmissing-prototypes -Os -I.
> -I../../../asus-avr-1.0/include -E -o xx macro.c
> 
> and, in xx
> 
> ...
> int main() {
>   ( ((GLUE(DDR, C))) |= ~(((1 << ((3))))) );
>   return 0;
> }
> 
> 
> The macro is fully expanded on the RHS, but, not the LHS
> 
> What did I do wrong?

If a macro contains its own name (directly or via other macro expansion) it
will not be expanded again.

In the case of LEFT_MOTOR_DIR_PIN there is not another GLUE() "call"

In the case of LEFT_MOTOR_DIR_DDR there is another GLUE() "call", so that
second call is not expanded.

-Preston