Way to redefine a macro constant in just one single line using C syntax?

Question

I want to redefine the value of a macro constant on a certain condition in my C source code. I don´t be able to use conditional preprocessing directives for defining LEN_OSG because whether it is redefined with a another value or not is decided at run-time.

What I already have is:

if(condition)                      // proof condition for replacing the value in LEN_OSG.
{
   if(LEN_OSG != CBS_LEN)          // check if LEN_OS already has the value in CBS_LEN.
   {
      #undef LEN_OSG               // undefine macro LEN_OSG by using `#undef` directive.
      #define LEN_OSG CBS_LEN      // recreate the macro LEN_OSG with new value.
   }
}

Does the C syntax allow another way to abbreviate this instead of to using the #undef directive and thereafter recreating the macro with a new value by another #define directive (in just a single line)?

Desired solution would be something like that:

if(LEN_OSG != CBS_LEN)          // check if LEN_OS already has the value in CBS_LEN.
{
   #redefine LEN_OSG CBS_LEN    // redefine LEN_OS with another value in one line. 
}

No. All preprocessor directives (#define, #if, ...) have disappeared when the compiler itself starts to run. — pmg
– pmg, Commented Mar 10, 2020 at 12:24
You are mixing compile-time and run-time stuff here. Your ifs are evaluated at run-time by your program and the target's processor, but the preprocessor lines are evaluated at compile-time by the compiler and the host's processor. -- What do you really want to achieve? Please show us a minimal reproducible example. — the busybee
– the busybee, Commented Mar 10, 2020 at 12:24
It's impossible to redefine a macro at runtime. C scopes do not affect preprocessor directives. — Petr Skocik
– Petr Skocik, Commented Mar 10, 2020 at 12:24
@pmg: Preprocessing is integrated into GCC and Clang, so the preprocessing directives are certainly still present when the compiler starts, unless a user explicitly requests a separate preprocessing execution. It is even possible to get phase 7 (semantic analysis) error messages from code containing an #error directive, thus proving that some traditional compilation is performed before phase 4 (preprocessing) is complete. — Eric Postpischil
– Eric Postpischil, Commented Mar 10, 2020 at 12:40
@thebusybee Idiotic mistake from me. I edited the question, but my issue still remains - I´m asking for a way to abbreviate the redefining of a macro without using #undef and #define. — RobertS supports Monica Cellio
– RobertS supports Monica Cellio, Commented Mar 10, 2020 at 13:01

VLL · Accepted Answer · 2020-03-10 12:24:55Z

5

Your code does not work as you expect. #define is executed at compile time, so you must use compile-time #if with it. Otherwise it is always executed.

Also, there is no #redefine, so two commands are needed.

If you have a runtime condition, use a normal variable and change its value during runtime.

answered Mar 10, 2020 at 12:24

VLL

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2 Comments

RobertS supports Monica Cellio Over a year ago

Yes, I didn´t compiled it before - sorry for being dumb - but the case I´m asking for still remains. I´ve edited my question accordingly.

VLL Over a year ago

@RobertSsupportsMonicaCellio See my answer: "Also, there is no #redefine, so two commands are needed."

Yunnosch · Accepted Answer · 2020-03-10 12:24:30Z

3

You are trying to mix runtime code execution with preprocessor directives, i.e. things which have an effect even before compiling.
That does not work.

answered Mar 10, 2020 at 12:24

Yunnosch

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1 Comment

RobertS supports Monica Cellio Over a year ago

Yes, that was mistake. I apoligize for being so idiotic. I´ve edited the question, so that my intention comes clear and valid.

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