Global variables in C are static or not?

c static global-variables extern
Mishthi picture Mishthi · Nov 21, 2010 · Viewed 26.2k times · Source

Are global variables in C static or extern by default?
If global variables are by default static then it means that we would be able to access them in a single file, but we can use global variables in different files as well.
Does this imply that they have extern storage by default?

Answer

Adam Rosenfield picture Adam Rosenfield · Nov 21, 2010

If you do not specify a storage class (that is, the extern or static keywords), then by default global variables have external linkage. From the C99 standard:

§6.2.2 Linkages of identifiers

3) If the declaration of a file scope identifier for an object or a function contains the storage-class specifier static, the identifier has internal linkage.

5) If the declaration of an identifier for a function has no storage-class specifier, its linkage is determined exactly as if it were declared with the storage-class specifier extern. If the declaration of an identifier for an object has file scope and no storage-class specifier, its linkage is external.

So even if you don't specify the extern keyword, globals can still be accessed by other source files (so-called translation units), because they can still have an extern declaration for the same variable. If you use the static keyword to specify internal linkage, then even in the presence of an extern declaration for the same variable name in another source file, it will refer to a different variable.

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