If the same object is declared more than once externally, the declarations must agree in type and linkage. If one or more of the declarations incompletely specify the object's type, and there exists one declaration of the object with completed type, all the declarations are taken to be in agreement with the completed type. The scope of external declarations persist to the end of the file in which they are declared, while internal declarations persist only to the end of the block in which they were declared. Data objects to be used within only one block should be declared in that block. The syntax for external definitions is the same as for all definitions.
Function definitions can only occur at the external level.
Externally declared auto and register objects are not permitted. Internally declared auto and register objects are not automatically initialized and, if not explicitly initialized, have the irrelevant value previously stored at their address. All static objects are automatically initialized to 0, if not explicitly initialized. An external function can be called without previously declaring it in C, but this construction is not recommended because of the loss of type checking and subsequent susceptibility to bugs. If such a function call is made, the compiler will treat the function as if an external declaration of type int appeared in the block containing the call.