static keyword

static storage duration [keyword]

This keyword can be used to declare a local variable with static storage duration.

The keyword static applied to a class data member in a class definition gives the data member static storage duration.

It is usually seen in two places:

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

1. A static variable inside a function keeps it's value between invocations.

Let's try to print the number of times a function was called using a counter count.

#include <stdio.h>

void myFun(void){
	int count = 0;
	count = count + 1;
	printf("This function executed %d time(s)\n", count);
}

int main(){
    myFunc();    // Function call #1
    myFunc();    // Function call #2
    myFunc();    // Function call #3
    return 0;
}

output

This function executed 1 time(s)
This function executed 1 time(s)
This function executed 1 time(s)

This is happening because each time the function is called, the variable count does not retain it's value.
Now, let's try using the keyword static to make give this variable static storage.

#include <stdio.h>

void myFun(void){
	static int count = 0;
	count = count + 1;
	printf("This function executed %d time(s)\n", count);
}

int main(){
    myFunc();    // Function call #1
    myFunc();    // Function call #2
    myFunc();    // Function call #3
    return 0;
}

output

This function executed 1 time(s)
This function executed 2 time(s)
This function executed 3 time(s)

Now the variable retained it's value!

2. A static global variable or function is "seen" only in the file in which it's declared.

If we want a global variable that it's private to a function but it is also a variable that does not lose its existence even if the execution control goes out of the scope of that variable, we use the keyword static.

If we have a C project with two or more files, we might want to declare a global variable with limited scope, like this:

main.c

#include <stdio.h>

void file1_func(void);

static int mainPrivateData;

int main(){
    mainPrivateData = 100;
    printf("mainPrivateData = %d\n", mainPrivateData);

	file1_func();
	printf("mainPrivateData = %d\n", mainPrivateData);
    return 0;
}

file1.c

extern int mainPrivateData;

void file1_func(void){
	mainPrivateData = 900;
}

output

/tmp/ccwCorz.o: IN function 'file1_func':
file1.c:(.text+0x6): undefined reference to 'mainPrivateData'
collect2: error: 1d returned 1 exit status

Here we get an error because the file1_func( ) function can't access the main.c private variable mainPrivateData and therefore can't change it.