Static variables in c

In the c programming language we can use the keyword static to define a static variable. This keyword is part of the storage class specifiers, one of which you may know as extern and maybe the lesser known auto and register.

int
main()
{
    static int x = 5;
    printf("My cool static variable... %d", x);
}

$ My cool static variable... 5

These keywords define how a variable is stored.

According to C programming language standard

The storage-class specifiers determine two independent properties of the names they declare: storage duration and linkage.

Storage and Linkage

All variables have the notion of storage. They may be allocated on the stack (come in and out of existence as functions are called and returned) or on the heap - dynamically.

When we declare an object (variable or function) with the static keyword it's storage duration is that of the entire program. It is allocated once before the main function.

Implication

All variables without a storage class specifier use the auto keyword, meaning that it is allocated in the block in which it is entered and de-allocated when it leaves that block.

void f1()
{
    int x = 5; /* x goes into scope */
 
    /* do some stuff */
 
    return; /* x goes out of scope and is de-allocated and ceases to exist */
}
 
int
main()
{
    f1() /* We have no notion of x in main() */
}

In the previous example, if we declared x as a static variable after f1() exits we could still access, use and modify x even after it exits f1() scope.

Linkage

Linkage refers to an object (variable or function) that can be referenced in different scopes. All variables have external linkage by default. This means that they are visible to other translation units in the same program. However when we use the static keyword in the global scope, that variable is internally linked meaning it is only visible to that translation unit.

Implication

This means that any objects defined are only visible to that translation unit. For instance if we had a variable called buffer in file.c and we want to contain this variable in file.c because it will conflict with other definitions we can name it like so:

static char[SIZE] buffer;
 
int
main()
{
    /* do stuff with buff here */
}

char[SIZE] buffer; /* refers to a different buff than file.c buff */
 
int
main()
{
    /* do stuff with buff here */
}

Hope you enjoyed the read!