What does "static" mean in C?

1. A static variable inside a function keeps its value between invocations.
2. A static global variable or a function is "seen" only in the file it's declared in

(1) is the more foreign topic if you're a newbie, so here's an example:

#include <stdio.h>

void foo()
{
    int a = 10;
    static int sa = 10;

    a += 5;
    sa += 5;

    printf("a = %d, sa = %d\n", a, sa);
}


int main()
{
    int i;

    for (i = 0; i < 10; ++i)
        foo();
}

This prints:

a = 15, sa = 15
a = 15, sa = 20
a = 15, sa = 25
a = 15, sa = 30
a = 15, sa = 35
a = 15, sa = 40
a = 15, sa = 45
a = 15, sa = 50
a = 15, sa = 55
a = 15, sa = 60

This is useful for cases where a function needs to keep some state between invocations, and you don't want to use global variables. Beware, however, this feature should be used very sparingly - it makes your code not thread-safe and harder to understand.

(2) Is used widely as an "access control" feature. If you have a .c file implementing some functionality, it usually exposes only a few "public" functions to users. The rest of its functions should be made static, so that the user won't be able to access them. This is encapsulation, a good practice.

Quoting Wikipedia:

In the C programming language, static is used with global variables and functions to set their scope to the containing file. In local variables, static is used to store the variable in the statically allocated memory instead of the automatically allocated memory. While the language does not dictate the implementation of either type of memory, statically allocated memory is typically reserved in data segment of the program at compile time, while the automatically allocated memory is normally implemented as a transient call stack.

And to answer your second question, it's not like in C#.

In C++, however, static is also used to define class attributes (shared between all objects of the same class) and methods. In C there are no classes, so this feature is irrelevant.

Short answer ... it depends.

1. Static defined local variables do not lose their value between function calls. In other words they are global variables, but scoped to the local function they are defined in.

2. Static global variables are not visible outside of the C file they are defined in.

3. Static functions are not visible outside of the C file they are defined in.

It depends:

int foo()
{
   static int x;
   return ++x;
}

The function would return 1, 2, 3, etc. --- the variable is not on the stack.

a.c:

static int foo()
{
}

It means that this function has scope only in this file. So a.c and b.c can have different foo()s, and foo is not exposed to shared objects. So if you defined foo in a.c you couldn't access it from b.c or from any other places.

In most C libraries all "private" functions are static and most "public" are not.

static means different things in different contexts.

1. You can declare a static variable in a C function. This variable is only visible in the function however it behaves like a global in that it is only initialized once and it retains its value. In this example, everytime you call foo() it will print an increasing number. The static variable is initialized only once.

   void foo ()
   {
   static int i = 0;
   printf("%d", i); i++
   }
   

2. Another use of static is when you implement a function or global variable in a .c file but don't want its symbol to be visible outside of the .obj generated by the file. e.g.

   static void foo() { ... }
   

From Wikipedia:

In the C programming language, static is used with global variables and functions to set their scope to the containing file. In local variables, static is used to store the variable in the statically allocated memory instead of the automatically allocated memory. While the language does not dictate the implementation of either type of memory, statically allocated memory is typically reserved in data segment of the program at compile time, while the automatically allocated memory is normally implemented as a transient call stack.

If you declare a variable in a function static, its value will not be stored on the function call stack and will still be available when you call the function again.

If you declare a global variable static, its scope will be restricted to within the file in which you declared it. This is slightly safer than a regular global which can be read and modified throughout your entire program.

In C, static has two meanings, depending on scope of its use. In the global scope, when an object is declared at the file level, it means that that object is only visible within that file.

At any other scope it declares an object that will retain its value between the different times that the particular scope is entered. For example, if an int is delcared within a procedure:

void procedure(void)
{
   static int i = 0;

   i++;
}

the value of 'i' is initialized to zero on the first call to the procedure, and the value is retained each subsequent time the procedure is called. if 'i' were printed it would output a sequence of 0, 1, 2, 3, ...

If you declare this in a mytest.c file:

static int my_variable;

Then this variable can only be seen from this file. The variable cannot be exported anywhere else.

If you declare inside a function the value of the variable will keep its value each time the function is called.

A static function cannot be exported from outside the file. So in a *.c file, you are hiding the functions and the variables if you declare them static.

There are 2 cases:

(1) Local variables declared static: Allocated in data segment instead of stack. Its value retains when you call the function again.

(2) Global variables or functions declared static: Invisible outside compilation unit (i.e. are local symbols in symbol table during linking).

Share what I learned about this point.

In C static is a declaration specifier, which falls into three categories:

• storage classes: there are four classes: auto, static, extern and register.
• type qualifiers: like keywords: const, volatile, etc.
• type specifiers: like keywords: void, char, short, int, etc.

So static is a storage classes. It will determine the following three properties of each variable in a C program.

• storage duration: means when memory is allocated for the variable and when the memory is released. A variable with static storage duration stays at the same memory location as long as the program is running.
• scope: means the portion of the program text in which the variable can be accessed. A static variable has a file scope instead of a block scope.
• linkage: means the extent to which the variable can be shared by different parts(or files) of a program. If a static variable is declared inside a block then it has no linkage. If a static variable is declared outside blocks, then it has internal linkage. Internal linkage makes it accessible in a single file.

The static storage class has a different effect on a variable depending on it is declared outside a block or inside a block. Need to consider case by case.

Static variables have a property of preserving their value even after they are out of their scope!Hence, static variables preserve their previous value in their previous scope and are not initialized again in the new scope.

Look at this for example - A static int variable remains in memory while the program is running. A normal or auto variable is destroyed when a function call where the variable was declared is over.

#include<stdio.h> 
int fun() 
{ 
  static int count = 0; 
  count++; 
  return count; 
} 

int main() 
{ 
  printf("%d ", fun()); 
  printf("%d ", fun()); 
  return 0; 
}

This will output: 1 2

As 1 stays in the memory as it was declared static

Static variables (like global variables) are initialized as 0 if not initialized explicitly. For example in the below program, value of x is printed as 0, while value of y is something garbage. See this for more details.

#include <stdio.h> 
int main() 
{ 
    static int x; 
    int y; 
    printf("%d \n %d", x, y); 
}

This will output : 0 [some_garbage_value]

These are the major ones I found that weren't explained above for a newbie!

In C programming, static is a reserved keyword which controls both lifetime as well as visibility. If we declare a variable as static inside a function then it will only visible throughout that function. In this usage, this static variable's lifetime will start when a function call and it will destroy after the execution of that function. you can see the following example:

#include<stdio.h> 
int counterFunction() 
{ 
  static int count = 0; 
  count++; 
  return count; 
} 

int main() 
{ 
  printf("First Counter Output = %d\n", counterFunction()); 
  printf("Second Counter Output = %d ", counterFunction()); 
  return 0; 
}

Above program will give us this Output:

First Counter Output = 1 
Second Counter Output = 1 

Because as soon as we call the function it will initialize the count = 0. And while we execute the counterFunction it will destroy the count variable.