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Some file-scope declarations allocate space for the variable, and some don’t.
A file-scope declaration with an initial value must allocate space for the variable; if there are two of such declarations for the same variable, even in different compilation modules, they conflict.
An extern declaration never allocates space for the variable.
If all the top-level declarations of a certain variable are
extern, the variable never gets memory space. If that variable
is used anywhere in the program, the use will be reported as an error,
saying that the variable is not defined.
A file-scope declaration without an initial value is called a tentative definition. This is a strange hybrid: it can allocate space for the variable, but does not insist. So it causes no conflict, no error, if the variable has another declaration that allocates space for it, perhaps in another compilation module. But if nothing else allocates space for the variable, the tentative definition will do it. Any number of compilation modules can declare the same variable in this way, and that is sufficient for all of them to use the variable.
In programs that are very large or have many contributors, it may be wise to adopt the convention of never using tentative definitions. You can use the compilation option -fno-common to make them an error, or --warn-common to warn about them.
If a file-scope variable gets its space through a tentative definition, it starts out containing all zeros.