In C++, I tried to reference a constant in flash defined in another file using this declaration:
extern const int count;
In another C++ file I had the definition:
const int count = 1;
(The original test case had more to it, but I boiled it down to this simple scenario.)
The linker gives the error "undefined reference to 'count'". I am using ARM GCC 9.3.1 on a Cortex-M0+ MCU.
I found if I took off the "const" in both places, thus moving it from flash to RAM, there was no problem. I also eventually figured out that it all linked (and lived in flash) if I added 'extern "C"' to both. This is an adequate work-around, but I'd like to know what the problem is. Is it a bug? Is there something missing from my declarations?
DosDaddy said:I took off the "const" in both places, thus moving it from flash to RAM
This is one of those places where C++ differs from C.
definitely creates an object in memory.
But in C++, it doesn't (necessarily); in C++, it could be just like a #define (but with type & scope).
Is there something missing from my declarations?
No, there is nothing missing from your declarations. What you need to change is your definition.
"extern const int count = 1" should "fix" the issue you are seeing.
In C++, const global variable definitions have internal linkage by default
In C++. non-const global variable definitions have external linkage by default
This means to share a constant global, both the declaration AND the definition needs to be extern.
To share a variable global, only the declaration needs the extern in front of it. The definition is by default considered external linkage.
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