Summarize the distinction between an external variable definition and an external variable declaration.

When we have ''declared'' a variable, we have meant that we have told the compiler about the variable; i.e. its type and its name, as well as allocated a memory cell for the variable (either locally or globally). This latter action of the compiler, allocation of storage, is more properly called the definition of the variable. The stricter definition of declaration is simply to describe information ''about'' the variable.  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.

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.

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