6.9.2 External object definitions

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1832 If the declaration of an identifier for an object has file scope and an initializer, the declaration is an external definition for the identifier.

1833 A declaration of an identifier for an object that has file scope without an initializer, and without a storage-class specifier or with the storage-class specifier static, constitutes a tentative definition.

1834 If a translation unit contains one or more tentative definitions for an identifier, and the translation unit contains no external definition for that identifier, then the behavior is exactly as if the translation unit contains a file scope declaration of that identifier, with the composite type as of the end of the translation unit, with an initializer equal to 0.

1835 If the declaration of an identifier for an object is a tentative definition and has internal linkage, the declared type shall not be an incomplete type.

1836 EXAMPLE 1


        int i1 = 1;         // definition, external linkage
        static int i2 = 2;  // definition, internal linkage
        extern int i3 = 3;  // definition, external linkage
        int i4;             // tentative definition, external linkage
        static int i5;      // tentative definition, internal linkage

        int i1;             // valid tentative definition, refers to previous
        int i2;             // 6.2.2 renders undefined, linkage disagreement
        int i3;             // valid tentative definition, refers to previous
        int i4;             // valid tentative definition, refers to previous
        int i5;             // 6.2.2 renders undefined, linkage disagreement

        extern int i1;      // refers to previous, whose linkage is external
        extern int i2;      // refers to previous, whose linkage is internal
        extern int i3;      // refers to previous, whose linkage is external
        extern int i4;      // refers to previous, whose linkage is external
        extern int i5;      // refers to previous, whose linkage is internal

1837 EXAMPLE 2 If at the end of the translation unit containing


        int i[];

the array i still has incomplete type, the implicit initializer causes it to have one element, which is set to zero on program startup.