static and const in c

[shariefbe]

Dear friends,
can anyone tell me what is diff between

static int a=10;
const int a=10;

[gerard4143]

I justed googled C const variable and C static variable and this is the results

Static variable - Wikipedia, the free encyclopedia

const-correctness - Wikipedia, the free encyclopedia

[shariefbe]

yes i too saw this link previously. still i have some doubt. it seems same in some condition. i want what is the exact difference between these two.

[Cabhan]

So just in case there's confusion, "static" and "const" do completely different things.

Let's start with const because it's easy. const means that the value is a constant, so it cannot change. For instance:

const int PI = 3.14;

int circle_area(double radius)
{
    return PI * radius * radius;
}

We declare PI as a constant because its value will never change. This would be an error:

const int PI = 3.14;

int circle_area(double radius)
{
    PI = 3.15; /* will not compile */
    return PI * radius * radius;
}

You also frequently see something like:

void foo(const int bar)
{
    printf("You gave me the value %d\n", bar);
}

Here the paramter is declared as "const" to ensure that it is always the same value that was passed in.

For pointers, const is a bit different:

char *a = malloc(4);
a[0] = 'f';
a[1] = 'o';
a[2] = 'o';
a[3] = '\0';

This is obviously allowed. This, however, is not:

const char *a = malloc(4);
a[0] = 'f';
a[1] = 'o';
a[2] = 'o';
a[3] = '\0';

Here, "a" is a pointer to "const char"s. Therefore, the characters that a points to cannot be changed. However:

const char *a = "hello";
a = "goodbye";

This is allowed, because a is not itself constant. Only the characters that a points to are constant. Therefore, we would have to do something like this:

const char * const a = "hello";
a = "goodbye"; /* will not compile! */

Now a is a constant pointer to constant characters, and a cannot be modified.


For "static", there's really only one reason to use static: to declare something local to a file. By default in C, anything declared in one file is visible to everything else. Therefore, if I declare a function foo() in file1.c, then in file2.c, I can write:

void bar()
{
    foo();
}

However, if foo() was declared as "static" in file1.c, file2.c would not compile (the error would be that I never declared a function foo()).

Technically, "static" has other meanings in C related to persistant storage of local variables in functions across function calls. However, I barely understand it myself, and I have _never_ seen it used. In object-oriented languages, "static" does have a very important meaning (it turns instance variables into class variables), but that's not in C, obviously.

Does this help?

[Rubberman]

const int a = 10;

The variable 'a' is a constant. It cannot be written to.

static int a = 10;

If the declaration is in a header file, it is a local read-write variable in every translation unit (non-header source file) than includes that header either directly or indirectly. If it is declared in a source (not header) file but not in a function, then it is a local variable in that translation unit only. If it is inside a function, then it is a persistent variable in that function and will maintain its value, if changed, between calls to that function.