Re: extern

Typically, you have "extern ..." in all but one source file, and just
"..." in one source file where that symbol is defined.

Saying "extern int foo;" means "I expect some other module to have foo."

Saying "int foo;" means "I have foo."

So exactly one module needs to have foo, to avoid linker errors.

There's an exception in C...

If you have an uninitialized variable, it tells the linker to allocate
space for it (it's a "common symbol" instead of a regular symbol).
IMHO this is bad practice, because it lets two modules think they
"own" a variable, perhaps with two different purposes.

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