Static Functions

Table of contents

1. Static Functions
   1. Private helper functions
   2. Public and private functions
   3. The two meanings of static

We previously used static with a local variable:

void count_calls(void)
{
    static int count = 0;

    count = count + 1;
}

Here, static means that count keeps its value between function calls.

The same keyword has a different meaning when it is placed before a function:

static void helper_function(void)
{
    // ...
}

A static function can only be called from within the .c file where it is defined.

It is a bit annoying that C uses the same keyword for both of these!

Private helper functions

Static functions are normally used for helper functions that are only needed inside one source file.

For example:

#include <stdio.h>

static int double_value(int value);

int main(void)
{
    int result = double_value(5) + 10;

    printf("Result: %d\n", result);

    return 0;
}

static int double_value(int value)
{
    return value * 2;
}

The function double_value() can be called from anywhere inside this source file.

It cannot be called from another .c file because it is declared as static.

The prototype also includes static:

static int double_value(int value);

This must match the function definition:

static int double_value(int value)
{
    return value * 2;
}

Public and private functions

Functions that other source files need to call are declared in a header file:

// calculations.h

int calculate_result(int value);

A source file can contain both the public function and any private helpers it needs:

// calculations.c

#include "calculations.h"

static int double_value(int value);

int calculate_result(int value)
{
    return double_value(value) + 10;
}

static int double_value(int value)
{
    return value * 2;
}

Here:

• calculate_result() is public and is declared in calculations.h
• double_value() is a private helper and is declared as static
• double_value() does not appear in the header file

Another source file can call:

calculate_result(5);

but it cannot call:

double_value(5);

The two meanings of static

1. Inside a function:

void count_calls(void)
{
    static int count = 0;
}

In this case static means:

Keep the value of this variable between function calls.

1. Before a function:

static void helper_function(void)
{
}

Here static means:

Keep this function private to the current source file.

When a helper function is only used in one .c file, declare it as static and do not put it in the header file.