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?
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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