A special characteristic that the block declaration has, is that the parameter names can be omitted, and just keep the parameter types.
For automatic and register variables, there is no difference between definition and declaration. The process of declaring an automatic or a register variable defines the variable name and allocates appropriate memory. However, for external variables, these two operations may occur independently. This is important because memory for a variable must be allocated only once, to ensure that access to the variable always refers to the same cell. Thus, all variables must be defined once and only once. If an external variable is to be used in a file other than the one in which it is defined, a mechanism is needed to ``connect'' such a use with the uniquely defined external variable cell allocated for it. This process of connecting the references of the same external variable in different files, is called resolving the references.
As we saw in the previous section, external variables may be defined and declared with a declaration statement outside any function, with no storage class specifier. Such a declaration allocates memory for the variable. A declaration statement may also be used to simply declare a variable name with the extern storage class specifier at the beginning of the declaration. Such a declaration specifies that the variable is defined elsewhere, i.e. memory for this variable is allocated in another file. Thus, access to an external variable in a file other than the one in which it is defined is possible if it is declared with the keyword extern; no new memory is allocated. Such a declaration tells the compiler that the variable is defined elsewhere, and the code is compiled with the external variable left unresolved. The reference to the external variable is resolved during the linking process.