C Preprocessor #if string

Question

Is there a way of optionally including code using #define like this?

#define TIMR_TO_USE TIM14

#if TIMR_TO_USE == TIM14
    // Do some stuff.
#elif TIMR_TO_USE == TIM16
    // Do some other stuff.
#endif

I feel like this ought to work but doesn't. Is there a way round it?

For info I want to optionally choose which timer to use on an STM32. I have lots of places where I have code like

TIMR_TO_USE->ARR = 1000;

But there are other places where I need different lines of code.

Better do #define USE_TIM14 and then #ifdef USE_TIM14 .... #elif defined(USE_TIM16) ... #endif. But why would you want something "around it"? — Eugene Sh.
– Eugene Sh., Commented Jan 11, 2023 at 16:04
"where I need different lines of code:" it's unclear what you mean, show more sample code instead of describing it. — Jabberwocky
– Jabberwocky, Commented Jan 11, 2023 at 16:13
You need to define TIM14 and TIM16 to different integer values, e.g. 1 and 2. — Lindydancer
– Lindydancer, Commented Jan 11, 2023 at 16:36

Bodo · Accepted Answer · 2023-01-11 16:46:53Z

2

Assuming TIM14 and TIM16 are defined as pointers you cannot define these as integers as proposed in a comment but you could use separate preprocessor symbols to select the implementation.

#define USE_TIM14
//#define USE_TIM16

#ifdef USE_TIM14
#    define TIMR_TO_USE TIM14
#elif defined(USE_TIM16)
#    define TIMR_TO_USE TIM16
#else
#    error "no timer selected"
#endif

void func(void)
{
#ifdef USE_TIM14
    // Do some stuff.
#elif defined(USE_TIM16)
    // Do some other stuff.
#endif

    TIMR_TO_USE->ARR = 1000;
}

The error handling could be improved to also detect if multiple USE_TIM* are defined.

answered Jan 11, 2023 at 16:46

Bodo

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1 Comment

Jabberwocky Over a year ago

Good guess. That's probably what they're actually trying to achieve.

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