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If in "normal" C, I define a global constant in one module:
const int iTabCount= 5;
and then use this constant in another module (module2.c) with extern:
extern const int iTabCount;
Then "normal" C will do this without any problems.
But in C++ files this fails, even if I encapsulate these definitions in extern "C" { ... };
If I use C++, in the first module, I get
warning #177-D: variable "iTabCount" was declared but never referenced
(This is correct, it is really not referenced in this module - but as this variable is NOT defined as static, there should be no problem ...).
And on linking I get the following error:
Error: L6218E: Undefined symbol iTabCount (referred from module2.c)
All works fine, if I remove the "const" qualifier. But I would like to keep it - especially for some larger const table, where it would be quite inefficient to reserve RW data space for such a code table.
I think this is some C++ compiler problem? Not possible to use const data over several modules in C++?
Your problem starts right here:
You should basically never write "extern" in any *.c file. Extern declarations belong into *.h files, which you then #include into *.c files (and that includes the *.c file with the definition).
This would probably not have happened had you #included that *.h file into the module defining the variable.
I think this is some C++ compiler problem?
No. It's a problem with your assumption that C and C++ were basically the same language, so you could just compile some arbitrary piece of C as C++ and get away with it. Well, that hasn't been the case since basically day 1 of the existence of C++ as a practical programming language. So, to be blunt: C and C++ are different languages, get used to it. And yes, the precise meaning of the 'const' qualifier is one of the key differences between them. C uses it to signify write-protected variables, C++ for actual constants.
Ups - I am surprised. You are right - if I insert the
extern const int iAnzParTab;
into the header file (included into both modules), then all works fine.
Thank's a lot! (I tried it also with my table - I can also declare the "unknown size" table in this header file, and then give the specific size only in module2.cpp - that's exactly what I needed:)
extern const TAB aTab[];