language lawyer - Which object declarations in C cause storage to be reserved (i.e. are definitions)? -


c11 specifies in section 6.7 declarations definitions:

a definition of identifier declaration identifier that:
— object, causes storage reserved object;
[...]

i didn't find comprehensive list of object declarations cause storage reserved. intuitively clear me, wasn't able information out of c11 standard.

there's no definitive list because standard describes definitions , it's not in single place. i'll try sum here. i'll use type int here without qualifiers (like const) consistency.

  • if add initializer declaration, it's definition:

    int x = 1;

  • without initializer, following definitions when they're in function scope:

    int x; auto int x;        // auto default anyways register int x;    // register hint, "storage" static int x;      // reserves storage, static duration

  • in file scope, rules bit more complicated; following tentative definitions:

    int x; static int x;

    the wording of standard (§6.9.2p2) is:

    a declaration of identifier object has file scope without initializer, , without storage-class specifier or storage-class specifier static, constitutes tentative definition. if translation unit contains 1 or more tentative definitions identifier, , translation unit contains no external definition identifier, behavior if translation unit contains file scope declaration of identifier, composite type of end of translation unit, initializer equal 0

    so means "become definitions" if not found refer definition.

  • with storage class extern , without initializer, don't have definition:

    extern int x;     // <- not definition

afaik, should complete set of rules. please feel free edit/comment if forgot something.


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