Origin
In the predecessors of C, including BCPL and B, there was already a concept of static storage. [1] [2] , which meant a storage which is always in existence. However, In B, there wasn't a static keyword, but there was an extrn keyword to specify external storage (external to all functions and must be defined outside a function), which is always static, in contrast with auto keyword, which declared an variable with auto storage - one appears whenever the function is invoked, and disappears when the function is left. [2] . All variables must be declared, as one of auto, extrn, or implicitly as function arguments.
C was developed as a successor of B, and the static and register keyword were added as storage class specifiers, alongside with auto and extern , which kept their meaning from B. However, in C, the concept of linkage for variables outside functions was introduced. A C program can be formed by multiple compilation units and linked together to form a complete program, which a variable or a function can be either specified as having internal linkage (visible to its own compilation unit only), or external linkage (visible to the whole program). These keywords specify both the storage duration and linkage as follows [3] :
| Storage class | Duration | Linkage |
|---|
extern | static (program execution) | external (whole program) |
static | static (program execution) | internal (translation unit only) for top-level identifier
none for block-scope variables |
auto, register | function execution | none |
Every variable and function has one of these storage classes; if a declaration does not specify the storage class, a context-dependent default is used:
extern for all top-level declarations in a source file,auto for variables declared in function bodies.
So, in C, although the static keyword, when used on variables, always declare a variable with static storage duration, there are two distinct meanings of the keyword, depending on where it is used:
- Top-level declaration: to declare an identifier with internal instead of external linkage, applicable to both variables and functions.
- Block-scoped declaration: to declare a variable with static instead of automatic storage duration. C doesn't support block-scoped functions.
Therefore, in C, the term "static variable" has two meanings which are easy to confuse:
- A variable with the same lifetime as the program, as described above (language-independent); or
- (C-family-specific) A variable declared with storage class
static.
Variables with storage class extern, which include variables declared at top level without an explicit storage class, are static in the first meaning but not the second.
In C++ (not C), the static keyword can also be used to declare a member variable in a class to have static storage duration as well, independent from the storage duration of the class object itself, and such a variable must be defined outside the class. The effect is that the variable is shared among all class instances, becoming a class member instead of an instance member. When applied to a member function (method), it specifies that the member function operates independently of an instance, which means it can't access any non-static members of the class nor the this pointer.
As a storage duration specifier
The static keyword is used in many programming languages to specify a local variable to have a lifetime of the whole program, preserved between function invocations, instead of having its own copy for each function invocation as in automatic storage duration, inherited from the usage in C.
Examples of programming languages which support the usage of static keyword for local variables to be kept across invocation include such as C, C++, Objective-C, C#, PHP.
The following programming languages with C-like syntax do not support static local variables as in C: Java, JavaScript, Dart. In these languages, a variable which is kept for the whole program execution needs to be declared outside functions.
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register storage class specifier, which similarly provides an optimization hint. - The second purpose of
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