How Long Do Static Variables in C Functions Live?

Lifetime of Static Variables in C Functions

Declaring a variable as static within a function's scope ensures its initialization only once, maintaining its value across function invocations. Understanding its precise lifetime is crucial.

When Are Static Variables Created and Destroyed?

Static variables in a function have a lifetime that spans from the first encounter with their declaration to program termination. This means that:

• The constructor is called during the initial declaration.
• The destructor is called upon program termination or when the variable goes out of scope (e.g., function exit).

Tracking Construction/Destruction Order

Determining the order of construction and destruction of static variables is essential, especially in multi-threaded environments. The standard dictates that destructors of static objects are executed in the reverse order of construction completion.

Implementation Example

Consider the following code snippet:

struct emitter {
    string str;
    emitter(const string& s) : str(s) { cout << "Created " << str << endl; }
    ~emitter() { cout << "Destroyed " << str << endl; }
};

void foo(bool skip_first) {
    if (!skip_first)
        static emitter a("in if");
    static emitter b("in foo");
}

int main(int argc, char*[])
{
    foo(argc != 2);
    if (argc == 3)
        foo(false);
}

Output:

C:>sample.exe
Created in foo
Destroyed in foo
C:>sample.exe 1
Created in if
Created in foo
Destroyed in foo
Destroyed in if
C:>sample.exe 1 2
Created in foo
Created in if
Destroyed in if
Destroyed in foo

This demonstrates the lifetime and construction/destruction order of static variables.

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