Parameter checking in C preprocessor

Question

How can I test which identifier is passed as the value of a macro parameter in C at compile time?

I'm writing wrapper macros for a micro-controller in C (GNU).

I have some macros that work like this:

#define _ReadBits(port, mask)      (PORT ## port) & (mask)
#define ReadBits(portmask)         _ReadBits(portmask) 
#define SWITCH D, (1<<7)

This way I can say:

foo = ReadBits(SWITCH);

and I'll get

foo = PORTD & (1<<7);

That works great. I want to expand these to do something like this:

#define _ConfigAnalog(port, mask)  BUILD_BUG_ON(port != B); AD1PCFGCLR = (mask)
#define ConfigAnalog(portmask)    _ConfigAnalog(portmask)

That is, I want a compile-time error to happen if parameter port isn't B (because this micro-controller can configure only port B as analog).

Is there some way to do this in C?

nerdfever.com · Accepted Answer · 2015-08-15 00:14:02Z

4

Figured it out myself:

#define PORT_B_SUPPORTS_ANALOG

#define __CheckPort(p)  PORT_##p##_SUPPORTS_ANALOG
#define __ConfigAnalogBits(mask) AD1PCFGCLR = (mask)

#define _ConfigAnalogBits(port, mask)  __CheckPort(port); __ConfigAnalogBits(mask)
#define ConfigAnalogBits(portmask)     _ConfigAnalogBits(portmask)

#define NO_GOOD C, (1<<3)
#define VOLTAGE B, (1<<3)

void tryit(void)
{
    ConfigAnalogBits(VOLTAGE);
    //ConfigAnalogBits(NO_GOOD);
}

If "port" is B then _CheckPort compiles into a null statement.

If "port" is anything else, a build error happens.

answered Aug 15, 2015 at 0:14

nerdfever.com

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5 Comments

dreamlax Over a year ago

Avoid using identifiers that start with an underscore followed by either another underscore or an uppercase letter (e.g. avoid __this and _This) because such identifiers are reserved for the implementation.

nerdfever.com Over a year ago

@dreamlax: Thanks; will fix.

Jens Over a year ago

This does not test any "string value" of a macro parameter. It tests an identifier name. Should the downvote in my answer be yours, I'd think it fair to reconsider if your question (which I cite) is maybe not stated as clearly as it could.

Aleksi Torhamo Over a year ago

@Jens: Whether the first line uses "string" instead of "identifier" - which is an understandable colloquialism/mistake in my opinion - the rest of the question made it perfectly clear what he meant.

Simon Kissane Over a year ago

@Jens I edited the question to remove the confusing reference to "string value"

Jens · Accepted Answer · 2015-08-15 00:01:39Z

0

How can I test the string value of a macro parameter in C at compile?

You can't.

According to C99 6.10.1 Conditional inclusion, pre-processor conditions must be an integer constant expressions, and may test whether a macro is defined (using #if, #ifdef identifier or #if defined(identifier)).

answered Aug 15, 2015 at 0:01

Jens

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3 Comments

nerdfever.com Over a year ago

I think the C preprocessor is Turing complete. So you always can; the question is just how.

Gene Over a year ago

@nerdfever.com Turing completeness has nothing to do with arbitrary input data manipulation. A language can be Turing complete over moonbeams. That doesn't mean it can make decisions about strings.

nerdfever.com Over a year ago

@Gene: If you can encode strings in moonbeams, then I think you can made decisions about strings. (In principle anyway; not saying it's practical.)

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