While learning about extern and static variables in C/C++, I came across this answer.
Maybe I'm missing some point, but this answer raised doubts about a code of mine.
Suppose I have the following files:
header.h
static int global_foo = -1;
void doSomething(void);
source1.c
#include "header.h"
void doSomething(void) {
global_foo = 1;
}
main.c
#include "header.h"
int main(void) {
doSomething();
printf("%d\n", global_foo);
}
What exactly is the output of the printf in the main function? My interpretation is that since global_foo is included two times, there will be two distinct global_foo, and therefore one such change will affect only the global_foo of the file that it belongs to.
Your assessment is correct.
Because global_foo is declared static, each source file has its own distinct variable by that same name, and a change to one does not affect the other.
Because of this, the program will print -1, since the global_foo in main.c is unchanged.
Global variables have static storage duration anyway so there's no need to include the static qualifier to explicitly state its storage duration. When you declare a global variable as static within a translation unit you are just saying that it has internal linkage within that translation unit. This means it can only be identified by its name within the translation unit.
So, if you declare a variable as static in a header file, every translation unit that includes it gets its own copy of the variable that is different from all the others.
If you have a function that returns the address of the variable, i.e.
int *getStaticAddress ()
{
return &static_var;
}
You can use that to access the variable outside the translation unit.
Related
I recently came across the question like how to access a variable which declared static in file1.c to another file2.c?
Is it possible to access static variable?
My understanding about static keyword in C is,
static is "internal linkage", so they are accessible only from one compilation unit - the one where they were defined. Objects declared with internal linkage are private to single module.
As one of my friend suggest me below solution.
In file1.c
#include <stdio.h>
int main()
{
int b=foo();
printf("%d",b);
return 0;
}
in file2.c
static int a=25;
int foo()
{
return a;
}
compiled by gcc file1.c file2.c -o file
If I do above I can access the variable.
So my questions are:
Does the above program violate static variable rules?
If not, why is this so, and is there any other way to access static variable except including file (#include <…>) not like this.
How am I able to access a static variable from another file?
In C, how do I restrict the scope of a global variable to the file in which it's declared?
Correct me if I'm wrong with static variable concept and if any better solutions are available to access static variable?
1) does the above program violate static variable rules?
No you are not vailoting any rules. Here foo function create copy of value of that static variable and used in other file. Its fine.
2) If not why is this so, and is there any other way to access static variable except including file (#include<>) not like this How am I able to access a static variable from another file?
Static variable are only mean to use in that file only.
You can not use that variable making them extern in other files.
Another dirty hack is to get pointer of that static variable and make that as global pointer and making that as extern in another file you can use that static variable.
file1.c
#include<stdio.h>
static int a=25;
int* ptr = &a;
file2.c
#include<stdio.h>
extern int *ptr;
int main()
{
printf("%d",*ptr);
return 0;
}
Correct me if I'm wrong with static variable concept and if any better solutions are available?
A static variable has a lifetime extends across the entire run of the program
If you do not initialize static variable with some value then its default value would be 0.
A static variable has scope limited to its file only. You can not access it by name from a different file.
You have temp1.c and temp2.c both are getting compiled together then also you can have static variable of same name in both files — and they are separate variables.
In C, how do I restrict the scope of a global variable to the file in which it's declared?
By making that global variable as static.
What we commonly call a variable in C is actually two things: an object, the memory allocated for the variable interpreted with a certain type, and an identifier, one way to access that object.
There is no problem in accessing a static object or its value from another compilation unit. Your function foo promotes the value to another unit, that is fine, but it could even promote the address of a without problems.
Having internal linkage only concerns the identifer, the name a. This one is only visible inside file2.c.
With the static int a=25; the variable a will have internal linkage; meaning the linker cannot see a anywhere outside of the file2.c TU.
When you're calling foo() in file2.c, you get a copy of a, it's the copy that you print; but this doesn't mean you have access to the actual a defined in file2.c When you need such an access where the same variable is visible across different TUs, you could do this
Defining file
This file both declares and defines the variable; additionally initializes it to 1 too, without which it'll be default initialized to 0.
// (non-static) global variable with external linkage and thus visible across TUs
int var_across = 0;
void use()
{
var_across = 1;
}
Using file
// just a declaration to tell that it's defined elsewhere; not a definition
extern int var_across;
void use_here()
{
var_across = 2;
}
Assigning address of static variable to pointer will make static variable available to subfiles.
In subfiles we have to use extern keyword to the pointer.
But it is not necessary to do that.
I have two C-files, each having defined a static int variable sharing the same name.
My understanding is that static variables declared at top-level should be limited to usage within the same file.
However, when I run my program it is obvious that these files affect the value of one another's static variable.
Have I misunderstood how the static keyword works and is there another way to obtain this file-based separation of scopes?
*Edit: Added source code to demonstrate problem. This code is from 3 separate files, as indicated by the comments.
//file 1
static int buffer;
void setter_1(int *input) {
buffer = *input;
}
void getter_1(int *output) {
*output = buffer;
}
//file 2
static int buffer;
void setter_2(int *input) {
buffer = *input;
}
void getter_2(int *output) {
*output = buffer;
}
//main
#include <stdio.h>
#include "buffer_1.c"
#include "buffer_2.c"
int main() {
int int1 = 1;
int int2 = 2;
setter_1(&int1);
setter_2(&int2);
getter_1(&int1);
getter_2(&int2);
printf("%i, %i\n", int1, int2);
return 0;
}
We expected to get two different numbers ("1, 2"), but got two identical numbers ("2, 2").
Thanks in advance
/Frisch
Even though we often talk about the structure of C program in terms of "files", most of the time what is really meant by "file" is translation unit - a source file together with everything that is #included into it.
Now, static variable in C means a variable with internal linkage, i.e. a variable that is not linkable by name between different translation units. Each translation unit is such case gets its own, completely independent variable. Having multiple translation units in this case is absolutely critical: the separation in question is only possible, again, between different translation units.
In your example you have only one translation unit: you included your .c files into a single main.c file, i.e you merged all of your translation units into one translation unit. The title of your question refers to static variable "accessed from another file". In reality there's no "another file" in your example. You have only one "file".
Since you merged everything into a single translation unit, your static variable declarations became repetitive declarations of the same variable inside one translation unit.
Note that your static variable declarations happen to be definitions at the same time. In C++ such repetitive definitions of the same variable would trigger a "multiple definition" error. In C such definitions are treated as tentative definitions (a C-specific feature), which allows them to slip through. But if you add explicit initializers to your static variables (e.g. static int buffer = 0;) the definitions will no longer be tentative and the code will fail to compile even in C.
If you want to maintain different, independent variables in this case, stop including your .c files into your main.c file. Translate each .c file independently, as a separate translation unit, and then link them together into the final program.
One way this can happen is when pointers to those static variables are passed between functions in the two files:
file1.c:
static int i1;
...
foo(&i1);
file2.c:
void foo(int *ip)
{
*ip = 42;
}
Calling foo in file1.c would modify i1 from a function outside file1.c
I suppose by inclusions you have effectively declared the same static global variable twice, which is why you no longer have two separate variables, but one.
The way you did it, by including file 1 and file 2 in main, you effectively have only one "buffer" variable.
Here's my code:
File DataTypes.h
static int count=0;
File TreeOps.h
#include"DataTypes.h"
void printTree(Tree* ptr)
File TreeOps.c
#include"TreeOps.h"
void printTree(pointer){
count++; // incrementing the count;
printf("%d",counter);
}
File TreeMain.c
#include"TreeOps.h"
printTree(pointer); // all the necessary declarations are done.
printf("%d",count);
If in printTree function the printf gives count=1; while in main function it gives me 0.
Why?
static variable in this context means: every c file has its own variable instance. Remove static definition in h-file:
extern int count;
and add this to one of c files:
int count = 0;
extern means: this is forward declaration. By defining a variable as extern, you tell to compiler that count has int type, and this variable is created somewhere. Actually, this variable is created in one and only one c file. You can use it in any c file where DataTypes.h is included. In the file where this variable is created, compiler uses it. In all other file this variable becomes external reference, which is resolved later by linker.
First off, defining data or functions in header files is a bad practice in C programming. In DataTypes.h you don't just declare the count variable, but you define it.
What actually happens is that the count is defined separately in each translation unit and you end up with two variables after linking. The linker doesn't merge them because they are marked static, that means they should be local to the translation unit.
If you want the count variable to be shared between the TreeOps.c and TreeMain.c translation units, you must use extern in the header file which only declares it:
extern int count;
And then define it globally as int count in either of TreeOps.c or TreeMain.c.
You don't have a "global static int" in your program. Entities declared as static cannot possibly be "global". The whole point of declaring something static is to make it local to a specific translation unit. This is exactly what you've done: you have declared two completely independent static variables in two different translation units. Each variable is local to its own translation unit. Then you are modifying one of these variables and printing the other. No wonder that the other remains unchanged.
In this case you have to decide what it is exactly you want. You can either have your variable as a global variable or as a static variable, but not both at the same time. "Global variable" and "static variable" are mutually exclusive concepts. So, what is it you want: global or static?
What is the difference between static and extern in C?
From http://wiki.answers.com/Q/What_is_the_difference_between_static_and_extern:
The static storage class is used to declare an identifier that is a local variable either to a function or a file and that exists and retains its value after control passes from where it was declared. This storage class has a duration that is permanent. A variable declared of this class retains its value from one call of the function to the next. The scope is local. A variable is known only by the function it is declared within or if declared globally in a file, it is known or seen only by the functions within that file. This storage class guarantees that declaration of the variable also initializes the variable to zero or all bits off.
The extern storage class is used to declare a global variable that will be known to the functions in a file and capable of being known to all functions in a program. This storage class has a duration that is permanent. Any variable of this class retains its value until changed by another assignment. The scope is global. A variable can be known or seen by all functions within a program.
static means a variable will be globally known only in this file. extern means a global variable defined in another file will also be known in this file, and is also used for accessing functions defined in other files.
A local variable defined in a function can also be declared as static. This causes the same behaviour as if it was defined as a global variable, but is only visible inside the function. This means you get a local variable whose storage is permanent and thus retain its value between calls to that function.
I'm no C expert so I might be wrong about this, but that's how I've understood static and extern. Hopefully someone more knowledgable will be able to provide you with a better answer.
EDIT: Corrected answer according to comment provided by JeremyP.
You can apply static to both variables and functions. There are two answers that discuss the behaviour of static and extern with respect to variables, but neither really covers functions. This is an attempt to rectify that deficiency.
TL;DR
Use static functions whenever possible.
Only declare external functions in headers.
Use the headers where the functions are defined and where the functions are used.
Don't declare functions inside other functions.
Don't exploit the GCC extension with function definitions nested inside other functions.
External functions
By default, functions in C are visible outside the translation unit (TU — basically the C source file and included headers) in which they are defined. Such functions can be called by name from any code that notifies the compiler that the function exists — usually by a declaration in a header.
For example, the header <stdio.h> makes visible declarations of functions such as printf(), fprintf(), scanf(), fscanf(), fopen(), fclose(), and so on. If a source file includes the header, it can call the functions. When the program is linked, the correct library must be specified to satisfy the function definition. Fortunately, the C compiler automatically provides the library that provides (most of) the functions in the standard C library (and it usually provides a lot more functions than just those). The 'most of' caveat applies because on many systems (Linux, for instance, but not macOS), if you use functions declared in the <math.h> header, you need to link with the maths library ('math' library if you're American), which usually is indicated by the option -lm on the linker command line.
Note that external functions should be declared in headers. Each external function should be declared in one header, but one header may declare many functions. The header should be used both in the TU where each function is defined and in each TU that uses the function. You should never need to write a declaration for a global function in a source file (as opposed to a header file) — there should be a header to declare the function and you should use that header to declare it.
Static functions
As an alternative to generally visible functions, you can make your own functions static. This means that the function cannot be called by name from outside the TU in which it is defined. It is a hidden function.
The primary advantage of static functions is hiding details which the outside world doesn't need to know about. It is a basic but powerful information hiding technique. You also know that if a function is static, you do not need to look for uses of the function outside the current TU, which can greatly simplify the search. However, if the functions are static, there can be multiple TUs which each contain a definition of a function with the same name — each TU has its own function, which may or may not do the same thing as a function with the same name in a different TU.
In my code, I qualify all functions except main() with the keyword static by default — unless there's a header that declares the function. If I subsequently need to use the function from elsewhere, it can be added to the appropriate header and the keyword static removed from its definition.
Declaring functions inside other functions
It is possible, but very inadvisable, to declare a function inside the scope of another function. Such declarations fly in the face of Agile Development maxims such as SPOT (Single Point of Truth) and DRY (Don't Repeat Yourself). They're also a maintenance liability.
However, you can, if you so desire, write code such as:
extern int processor(int x);
int processor(int x)
{
extern int subprocess(int);
int sum = 0;
for (int i = 0; i < x; i++)
sum += subprocess((x + 3) % 7);
return sum;
}
extern int subprocess(int y);
int subprocess(int y)
{
return (y * 13) % 37;
}
The declaration in processor() suffices for it to use subprocess(), but is otherwise unsatisfactory. The extern declaration before the definition is necessary if you use GCC compiler options such as:
$ gcc -O3 -g -std=c11 -Wall -Wextra -Werror -Wmissing-prototypes -Wstrict-prototypes \
> -c process.c
process.c:12:5: error: no previous prototype for ‘subprocess’ [-Werror=missing-prototypes]
int subprocess(int y)
^~~~~~~~~~
cc1: all warnings being treated as errors
$
This is, I find, a good discipline, similar to what C++ enforces. It's another reason I make most functions static, and define the functions before they're used. The alternative is to declare static functions at the top of the file and then define them in whatever order seems appropriate. There are some merits to both techniques; I prefer to avoid the need to declare and define the same function in the file by defining before use.
Note that you cannot declare a static function within another function, and if you attempt to define a function such as subprocess() as a static function, the compiler gives an error:
process.c:12:16: error: static declaration of ‘subprocess’ follows non-static declaration
static int subprocess(int y)
^~~~~~~~~~
process.c:5:20: note: previous declaration of ‘subprocess’ was here
extern int subprocess(int);
^~~~~~~~~~
Since functions that are externally visible should be declared in a header, there is no need to declare them inside a function, so you should never run into this as a problem.
Again, the extern is not necessary in the function declaration inside the function; if omitted, it is assumed. This can lead to unexpected behaviour in novice programs here on SO — you sometimes find a function declaration where a call was intended.
With GCC, the option -Wnested-externs identifies nested extern declarations.
Called by name vs called by pointer
If you have a nervous disposition, stop reading now. This gets hairy!
The 'called by name' comment means that if you have a declaration such as:
extern int function(void);
you can write in your code:
int i = function();
and the compiler and linker will sort things out so that the function is called and the result used. The extern in the declaration of the function is optional but explicit. I normally use it in a header file to match the declaration of those rare global variables — where the extern is not optional but mandatory. Many people disagree with me on this; do as you wish (or must).
Now what about static functions?
Suppose the TU reveal.c defines a function static void hidden_function(int) { … }.
Then, in another TU openness.c, you cannot write :
hidden_function(i);
Only the TU that defines the hidden function can use it directly. However, if there's a function in reveal.c that returns a function pointer to the hidden_function(), then the code openness.c can call that other function (by name) to get a pointer to the hidden function.
reveal1.h
extern void (*(revealer(void)))(int);
Obviously, that's a function that takes no arguments and returns a pointer to a function that takes an int argument and returns no value. No; it isn't pretty. One of the times it makes sense to use typedef on pointers is with pointers to functions (reveal2.h):
typedef void (*HiddenFunctionType)(int);
extern HiddenFunctionType revealer(void);
There: much simpler to understand.
See Is it a good idea to typedef pointers for a general discussion on the subject of typedef and pointers; the short summary is "it isn't a good idea except perhaps with function pointers".
reveal1.c
#include <stdio.h>
#include "reveal1.h"
static void hidden_function(int x)
{
printf("%s:%s(): %d\n", __FILE__, __func__, x);
}
extern void (*(revealer(void)))(int)
{
return hidden_function;
}
Yes, it is legitimate (but very unusual) to define the function with an explicit extern — I very, very seldom do it, but here it emphasizes the role of extern and contrasts it with static. The hidden_function() can be returned by revealer(), and could be called by code inside reveal.c. You can remove the extern without changing the meaning of the program.
openness1.c
#include <stdio.h>
#include "reveal1.h"
int main(void)
{
void (*revelation)(int) = revealer();
printf("%s:%s: %d\n", __FILE__, __func__, __LINE__);
(*revelation)(37);
return 0;
}
This file cannot usefully contain a direct call by name to hidden_function() because it is hidden in the other TU. However, the revealer() function declared in reveal.h can be called by name and it returns a pointer to the hidden function, which can then be used.
reveal2.c
#include <stdio.h>
#include "reveal2.h"
static void hidden_function(int x)
{
printf("%s:%s(): %d\n", __FILE__, __func__, x);
}
extern HiddenFunctionType revealer(void)
{
return hidden_function;
}
openness2.c
#include <stdio.h>
#include "reveal2.h"
int main(void)
{
HiddenFunctionType revelation = revealer();
printf("%s:%s: %d\n", __FILE__, __func__, __LINE__);
(*revelation)(37);
return 0;
}
Sample outputs
Not the most exciting output in the world!
$ openness1
openness1.c:main: 7
reveal1.c:hidden_function(): 37
$ openness2
openness2.c:main: 7
reveal2.c:hidden_function(): 37
$
Both of these modifiers have something to do with memory allocation and linking of your code. The C standard[3] refers to them as storage-class specifiers. Using those allows you to specify when to allocate memory for your object and/or how to link it with the rest of the code. Let’s have look on what exactly is there to specify first.
Linking in C
There are three types of linkage – external, internal and none. Each declared object in your program (i.e. variable or function) has some kind of linkage – usually specified by the circumstances of the declaration. Linkage of an object says how is the object propagated through the whole program. Linkage can be modified by both keywords extern and static .
External Linkage
Objects with external linkage can be seen (and accessed) through the whole program across the modules. Anything you declare at file (or global) scope has external linkage by default. All global variables and all functions have external linkage by default.
Internal Linkage
Variables and functions with internal linkage are accessible only from one compilation unit – the one they were defined in. Objects with internal linkage are private to a single module.
None Linkage
None linkage makes the objects completely private to the scope they were defined in. As the name suggests, no linking is done. This applies to all local variables and function parameters, that are only accessible from within the function body, nowhere else.
Storage duration
Another area affected by these keywords is storage duration, i.e. the lifetime of the object through the program run time. There are two types of storage duration in C – static and automatic.
Objects with static storage duration are initialized on program startup and remain available through the whole runtime. All objects with external and internal linkage have also static storage duration. Automatic storage duration is default for objects with no linkage. These objects are allocated upon entry to the block in which they were defined and removed when the execution of the block is ended. Storage duration can be modified by the keyword static .
Static
There are two different uses of this keyword in the C language. In the first case, static modifies linkage of a variable or function. The ANSI standard states:
If the declaration of an identifier for an object or a function has
file scope and contains the storage-class specifier static , the
identifier has internal linkage.
This means if you use the static keyword on a file level (i.e. not in a function), it will change the object’s linkage to internal, making it private only for the file or more precisely, compilation unit.
/* This is file scope */
int one; /* External linkage. */
static int two; /* Internal linkage. */
/* External linkage. */
int f_one()
{
return one;
}
/* Internal linkage. */
static void f_two()
{
two = 2;
}
int main(void)
{
int three = 0; /* No linkage. */
one = 1;
f_two();
three = f_one() + two;
return 0;
}
The variable and function() will have internal linkage and won’t be visible from any other module.
The other use of static keyword in C is to specify storage duration. The keyword can be used to change automatic storage duration to static. A static variable inside a function is allocated only once (at program startup) and therefore it keeps its value between invocations
#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();
}
The output will look like this:
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
Extern
The extern keyword denotes, that “this identifier is declared here, but is defined elsewhere”. In other words, you tell the compiler that some variable will be available, but its memory is allocated somewhere else. The thing is, where? Let’s have a look at the difference between declaration and definition of some object first. By declaring a variable, you say what type the variable is and what name it goes by later in your program. For instance you can do the following:
extern int i; /* Declaration. */
extern int i; /* Another declaration. */
The variable virtually doesn’t exist until you define it (i.e. allocate memory for it). The definition of a variable looks like this:
int i = 0; /* Definition. */
You can put as many declaration as you want into your program, but only one definition within one scope. Here is an example that comes from the C standard:
/* definition, external linkage */
int i1 = 1;
/* definition, internal linkage */
static int i2 = 2;
/* tentative definition, external linkage */
int i3;
/* valid tentative definition, refers to previous */
int i1;
/* valid tenative definition, refers to previous */
static int i2;
/* valid tentative definition, refers to previous */
int i3 = 3;
/* refers to previous, whose linkage is external */
extern int i1;
/* refers to previous, whose linkage is internal */
extern int i2;
/* refers to previous, whose linkage is external */
extern int i4;
int main(void) { return 0; }
This will compile without errors.
Summary
Remember that static – the storage-class specifier and static storage duration are two different things. Storage duration is a attribute of objects that in some cases can be modified by static , but the keyword has multiple uses.
Also the extern keyword and external linkage represent two different areas of interest. External linkage is an object attribute saying that it can be accessed from anywhere in the program. The keyword on the other hand denotes, that the object declared is not defined here, but someplace else.
Static
The static variables declared with the keyword static. The static variable initial value is 0. The static variables has block file scope scope.
Extern
A program in C, particularly when it is large, can be broken up into smaller programs. After compiling these, each program file can be joined together to form the large program. These small programs modules that combine together may need some variable that is used by all of them. In C, such a provision can be made by specifying these variables, accessible to all the small program modules, as an external storage class variable. These variables are global to all the small program modules that are formed as separate files. The keyword for declaring such global variables is extern.
Such a global variable is declared like any other variable in one of the program modules while the declaration of these variables is preceded with the keyword extern in all other combining program modules.
The program modules may also be a function or a block. These variables remain in existence as long as the program is in execution and their existence does not terminate upon the exit of a function or block or a program module from its state of execution. These variables are stored in the primary memory and their default value is zero.
Storage classes in C
Is the static keyword in C used only for limiting the scope of a variable to a single file?
I need to know if I understood this right. Please assume the following 3 files,
file1.c
int a;
file2.c
int b;
file3.c
static int c;
Now, if the 3 files are compiled together, then the variables "a" & "b" should have a global scope and can be accessed from any of the 3 files. But, variable "c" being static, can only be accessed from file3.c, right?
Does static have any other use in C ? (other than to limit the scope of a variable as shown above?)
The static keyword serves two distinct purposes in C, what I call duration (the lifetime of an object) and visibility (where you can use an object from). Keep in mind the C standard actually uses different words for these two concepts but I've found in teaching the language that it's best to use everyday terms to begin with.
When used at file level (outside of any function), it controls visibility. The duration of variables defined at file level are already defined as being the entire duration of the program so you don't need static for that.
Static variables at file level are invisible to anything outside the translation unit (the linker can't see it).
When used at function level (inside a function), it controls duration. That's because the visibility is already defined as being local to that function.
In that case, the duration of the variable is the entire duration of the program and the value is maintained between invocations of the function.
You are misusing the term "scope". static in C has absolutely nothing to do with scope.
Scope is the region where the name of an entity (variable, function, typename etc.) is visible. In C language "file scope" is the largest scope ever. For that reason, there's no point in limiting anything to a single file: there's simply nothing larger to limit. There's no such thing as "global scope" in C. The term "global scope" is sometimes used informally, but in that case it has the same meaning as "file scope".
Again, static in C has absolutely nothing to do with scope. static in C affects storage duration of an object and linkage of an identifier. When used with objects (variables) static gives the object static storage duration (i.e. the object exists as long as the program runs). And, when used with identifiers of non-local objects or functions, it gives them internal linkage, meaning that the same identifier refers to the same entity within a single translation unit (where the entity is defined), but not in other translation units.
static is also used within a function definition to define a variable which keeps its value between function calls. I found an example here. In contrast, variables which are created anew with each function call are called automatic.
An example to augment Kinopiko’s answer:
#include <stdio.h>
int foo() {
static int foo = 0;
return ++foo;
}
int main() {
printf("%i\n", foo()); // 1
printf("%i\n", foo()); // 2
}
This can be used for example to return a safe pointer to a local function variable. Or in Objective-C it’s sometimes used to guard against repeated class initialization:
- (void) initialize
{
static BOOL initialized = NO;
if (initialized)
return;
// …perform initialization…
initialized = YES;
}
A variable may have three kinds of storage:
In program's Static Area
On stack (during function call)
On Heap (when you allocate using new/malloc)
Global variables are always stored in static area. But to store a local variable in static area, you need the keyword static. As a static variable is not allocated on stack, you can access the variable on subsequent calls.
Also static keyword at global scope gives a variable internal linkage.Consequently the variable cannot be accessed from some other file using the extern qualifier.
You are correct, this is called "static linkage": The symbol declared as static is only available in the compilation unit where it is defined.
The other use of static would be inside a function:
void f() {
static int counter = 0;
counter++;
// ...
}
In this case the variable is only initialized once and keeps it's value through different calls of that function, like it would be a global variable. In this example the counter variable counts the number of times the function was called.
internal linkage vs external linkage by example
//file1.c
#include <stdio.h>
int glb_var=3;//global variable
int func(); //prototype of function
int main()
{
func();
func();
func();
return 0;
}
int func()
{
static int counter=0;//static varible
printf("val of counter=%d",counter);
counter+=5;
return 0;
}
when we will compile this program and run this program then os will load this program in memory.then below things will happened:
glb_var identifier will be stored in initialized data segment.
counter identifier will be stored in uninitialized data segment called ".bss".
static variable initialized once and the values persists during function calls.because static variable is stored in data segment not in stack so static variable persist during function calls.
So output of the program will be:
0 5 10
one important thing about static variable is that it has internal linkage.so we can access this variable to a particular file.In which they are defined (not in other file).
We can access global variable glb_var in other file by using extern keyword.
for eg:
//file2.c
#include <stdio.h>
extern glb_var; //for declaration of this variable
int main()
{
if(glb_var)
{
printf("glb_var=%d",glb_var);
}
}
output: 3
this is called external linkage.