I am new to C and I experience some confusion between the declaration and definition of a variable. Another thing I would like to know is if the following is true:
"Declaration appears many times and definition comes once."
Also:
int x;
Is this a declaration only? Since memory is allocated for x then why isn't this a definition instead of a declaration?
Simply writing int x; at either global scope or local scope, is both a declaration and definition. Typically, the declaration tells the compiler "This variable will exist, at some point, under this name, so you can use it." The definition tells the compiler to actually arrange for the variable to be created - obviously this can only happen once.
Typically the way you'll use this is by putting in a header file:
// Foo.h
#ifndef FOO_H
#define FOO_H // make sure structs aren't redefined
extern int bar; // Declare a variable bar
#endif
And in a single source file
#include "foo.h"
int bar; // Define bar
If you were to define bar in multiple files, you would get an error; you can't create the variable twice. But you do have to tell the compiler about it in every source file you use bar in. Hence the extern declaration.
The precise semantics are defined in §6.9.2 of the C standard and can be summarized as follows:
When a variable is declared at file scope with an initializer, it is an external definition. (§6.9.2/1)
When a variable is declared at file scope without an initializer, and without a storage-class specifier or with the static storage-class specifier, it is a tentative definition. If the translation unit (file) has one or more tentative definitions and no external definition, the compiler automatically adds a true file scope declaration at the end of the translation unit, with a zero initializer. (§6.9.2/2)
What this means is that, strictly speaking, int x; is not a definition; but it automatically creates a definition if and only if there is no other definition with an initializer, and no static definition (this third case is undefined behavior due to linkage disagreement per §6.2.2/7)
Note that extern int x; is not an external definition. It is a declaration with an extern storage class specifier. As such, extern int x; alone does not cause a definition to be created, but if you have both:
extern int x;
int x;
Then you will end up having a definition created at some point in the file.
It is also, technically speaking, legal to do this:
extern int x;
int x;
int x = 42;
In this case, the int x; in the middle is superfluous and has no effect. That said, this is poor form, as it's confusing in this case where the actual definition is.
This isn't something you see too much in C, but it works like this:
In a header file, you can have a line like this:
extern int x; //declaration
Because of the extern modifier, this tells the compiler that there is an int named x somewhere. The compiler doesn't allocate space for it - it just adds int x to the list of variables you can use. It'll only allocate space for x when it sees a line like this:
int x; //definition
You can see that because only the int x; line changes your executable, you can have as many extern int x; lines as you feel like. As long as there's only int x; line, everything will work like you want it to - having multiple declarations doesn't change a thing.
A better example comes from C++ (sorry if this is a C-only question - this applies to structs as well, but I don't know the syntax off the top of my head):
class Pineapple; //declaration
Pineapple* ptr; //this works
Pineapple pine; //this DOES NOT work
This declaration tells the compiler that there's a class called "Pineapple". It doesn't tell us anything about the class (how big it is, what its members are). We can use pointers to Pineapples now, but we can't yet have instances - we don't know what makes up a Pineapple, so we don't know how much space an instance takes up.
class Pineapple
{
public:
int ounces;
char* name;
}; //definition
Pineapple* ptr; //still works
Pineapple pine; //this works now too!
//we can even get at member variables, 'cause we know what they are now:
pine.ounces = 17;
After a definition, we know everything about the class, so we can have instances, too. And like the C example, you can have multiple declarations, but only one definition.
Hope this helps!
Related
#include <stdio.h>
extern int x;
int main()
{
int x;
x = 1;
printf("%d", x);
}
#include <stdio.h>
int main()
{
extern int x;
int x;
x = 1;
printf("%d", x);
}
I am learning about extern keyword and I was told that the word extern is used to declare variable and compiler will not allocate for this variable.
But when I write 2 codes, the first code run normally and the second has an error.
Please help me explain why it has this difference. Thanks.
Conflicting Declarations in Same Scope
When extern int x; appears outside a function, it declares x at file scope. Then, when int x; appears inside the function, it declares a new instance of x that is unrelated to the earlier extern int x;. This is allowed by the C language, so the compiler does not complain about it.
When extern int x; appears inside the function, it declares x at block scope. Then, when int x; appears after it, it attempts to declare a different x in the same scope. This is not allowed by the C standard, so the compiler reports an error.
The extern keyword is not particularly relevant here—the error is caused by the fact that there are two conflicting declarations of the same identifier. For example:
char c;
int main(void)
{
char d;
int c; // Allowed, new declaration in new scope.
int d; // Not allowed, conflicting declaration in same scope.
}
Rules About Declarations
The rules about C declarations have some irregularities due to the history of C development. Consider these declarations at file scope:
extern int x;
int x;
The first declaration does (or does not do) several things:
It says x is an identifier for an int.
It says x has external linkage, meaning it can be made (during linking of the object modules) to refer to an object named x declared somewhere else.
It does not define an int.
For the second declaration:
It says x is an identifier for an int.
It says x has external linkage (because external is the default for declarations for objects without a storage class specifier like static outside functions).
It defines an int. (It is actually a tentative definition, but we will not deal with that here.)
Both of these declarations say x is an identifier for an int and has external linkage. The difference between them is the first does not define an object (it merely says x is a name for an object defined somewhere else) and the second does define an int (so it is the somewhere else). So these declarations do not conflict and are allowed.
On other hand, consider these same declarations inside a function. Then they are at block scope.
Then extern int x; has the same meaning as above: x is an identifier with external linkage for an object defined elsewhere.
But int x; has a different meaning. Instead of saying x has external (or internal) linkage, it says x has no linkage, because no linkage is the default for declarations in block scope. This creates a conflict because C 2018 6.7 3 says an identifier with no linkage shall not be declared more than once in the same scope (and name space, not addressed here) except for typedef names and tags with certain conditions.
Scopes
C has four kinds of scopes:
File scope is for declarations outside of functions and lasts to the end of the source file being compiled.
Block scope is for declarations inside functions and lasts until the end of a block (discussed below).
Function prototype scope is for declarations in the parameters of function prototypes. (For example, in void foo(int n, float a[n][n]);, n and a have function prototype scope.)
Function scope is for labels to be used in goto statements.
A compound statement is a list of declarations and statements inside { and }. Each compound statement is a block, which creates a new scope for declarations. The main body of a function is a compound statement that is a block, and it may have additional compound statements inside it, each starting a new scope.
Blocks are also created by switch, if, do, while, and for statements, but they are largely unimportant for the first four of those, as only the for statement provides an opportunity for further declarations. For example, in a for statement, you can write for (int x = 3; x < 20; ++x), and that creates a new instance of x because the for statement starts a new block.
In the following statements first three are definitions and the last one is the declaration:
auto int i;
static int j;
register int k;
extern int l;
What's the reason for the same?
In first three(int i, static int j, register int k) is a definition. It denotes the space for the integer to be in this translation unit and advices the linker to link all references to i against this entity. If you have more or less than exactly one of these definitions, the linker will complain.
But in last extern int l, is a declaration, since it just introduces/specifies l, no new memory address/space is allocated. You can have as many extern int l in each compilation unit as you want.
A declaration introduces names into a translation unit or redeclares names introduced by previous declarations.
I assume the question is about the terms declaration and definition in C.
A declaration tells the compiler name and type of "something".
A definition is a declaration, but additionally "creates" the "something" that is declared. E.g. for a variable, this would introduce some storage space for this variable.
In your first three examples, the variables are actually created. The storage classes auto, static and register all just specify a storage duration. In contrast, the storage class extern tells the compiler that this variable is known, but it might exist in a different translation unit.
Maybe an example comparing the declaration and definition of functions will make the concept easier to understand:
// function declaration:
int foo(int x);
// (now we know a function foo should be "somewhere", but it doesn't exist yet)
// function definition:
int foo(int x) {
return x+1;
}
This question already has answers here:
What is the difference between a definition and a declaration?
(27 answers)
Closed 5 years ago.
Can you please someone explain me the flow of below problem,
#include <stdio.h>
int main(){
extern int a;
printf("%d\n",a);
return 0;
}
int a = 20;
and the output is 20. I am not sure where is the variable a getting defined and where is it getting declared?
The variable a is declared and defined as a global variable in the line:
int a = 20;
The extern line just tells the main() function scope that a is defined in another place.
In this case, the use of extern is not really necessary. You could just declare and define a before the main() function, and then main() would be familiar with it.
Usually, you would use extern when you want to use a variable or a function that was defined in another source file (and not just later in the same source file).
The C programming language has been designed to be one-pass, so that the compiler could process each line only once from top to bottom. So considering your program:
#include <stdio.h>
int main(){
extern int a;
printf("%d\n",a);
return 0;
}
int a = 20;
The identifier a is declared twice, and defined once.
Before the 4th line extern int a;, the compiler doesn't know anything about the identifier a. The declaration extern int a; has the block scope within the function main, and it declares the identifier a as an int and that its storage duration is static and linkage is external. So the compiler can write code that access a global identifier by name a as an int variable that could be defined in another module (external linkage). This is what the compiler does on line 5 when it is used in printf.
Finally at line 9, int a = 20; is another declaration and definition. This declares and defines a as a int with static storage duration, and external linkage.
If you'd put the int a = 20; before the main, the declaration extern int a; would be useless, because it doesn't add anything. I tend to put my main and other depending functions last in my source code, so that minimal amount of extra declarations are needed.
extern is syntactically a "storage class" keyword. But there is no such storage class. C has "static storage", "dynamic storage (malloc, etc) and "automatic storage" (local variables, usually represented using a stack).
If an identifier is declared extern inside a block scope, it means that the declaration refers to an external definition. If the entity being declared is an object, then it has static storage, simply because external objects have static storage. It can be a function too; functions aren't said to have storage.
In C, there is a concept called "linkage". Objects declared outside of any function at file scope, and functions, can have "external" or "internal" linkage.
If we have extern in a block scope, as you have in the example program, there can be a prior declaration of the same name at file scope, or in a nested scope, like this:
static int x;
/* ... */
{
extern int x;
}
Here, the inner x refers to the outer x, and, in spite of being "extern", it has internal linkage because of the "static".
In a nutshell, extern usually means "refer to the earlier declaration, and if there isn't one, declare this as an identifier with external linkage".
The word "external" refers to two separate concepts: the aforementioned "external linkage" and also to the meaning "outside of any function", as in "external declaration". Confusingly, "external declarations", like the static int x above, can have "internal linkage"!
In your program, things are correct because the block scope extern declaration of a and the later int a = 20, which are in separate scopes, happen to independently agree with each other.
The int a = 20; is an external declaration, which is also an external definition (because of the initializer). Since in that scope, no prior declaration of a is visible, it gets external linkage.
So, where is a defined? It is defined as an object with external linkage, in the entire translation unit as a whole. That translation unit is what defines a. a is declared in every place of the program where a declaration appears; and its definition is also a declaration. It is declared in main and also in the last line of the translation unit's source code.
A "declaration" is syntax which makes a name known in some scope. It's a concept that is active during the translation of a program. A "definition" is the fact that some object or function is provided in some translation unit. Translated units still provide definitions, but need not retain information about declarations. (Which is why when we make libraries, we provide header files with declarations in them!)
From the point of view of your main function, that function doesn't "care" where a is defined. It has declared a in such a way that if a is used, then an external definition of a, with external linkage, must exist. That definition could come from anywhere: it could be in the same translation unit, or in another translation unit.
When ever you declare a variable as extern then It means that Variable is declared as global and you cannot initialize the variable there.Because no memory is allocated for that variable It is just declared as a Variable
you can define it some where in your code.
Let us take an example ..consider the code
int main()
{
extern int i;
i=10;
printf("%d",sizeof(i));
}
here you get an error that int 'i' is not defined
therefore you need to write it as:
int main()
{
extern int i;
int i=10;
printf("%d",sizeof(i));
}
In case of your code:
This is declaration
extern int a;
This is definition:
int a = 20;
The storage class extern specifies storage duration and linkage of the object the identifier refers to:
The storage duration is set to static, which means the variable is alive for the whole time the program runs. As you declare that variable inside a function scope, this matters in your example, because in function scope, the default storage duration would be automatic.
The linkage is set to external, this just means different translation units of the same program can share the object. It has the side effect that a definition in another (file) scope is acceptable, as shown in your example. This is a logical consequence of the shared nature, you would typically declare a variable with external linkage in all translation units using it, but define it only in one.
This answer confused me.
If we have two lines in same .c file:
extern int c;
int c;
How is the first line of code a declaration and second a definition?
Aren't both declarations?
How these two lines differ?
The extern keyword is what makes the first line a declaration. It say "this variable exists somewhere". A line like that can appear in a header file.
The second line is a definition because the extern keyword is not present. If you were to have this line in a header file, two source files that include that header will both define that variable and linking those two files will result in a variable redefinition error.
When the program you're writing consists of multiple source files linked together, where some of the variables defined, for example, in source file file1.c need to be referenced in other source files, so this is the reason why using extern.
About your question how these lines differ:
extern int c;
int c;
A variable is defined when the compiler allocates the storage for the
variable while
A variable is declared when the compiler is informed that a variable
exists (and this is its type); it does not allocate the storage for
the variable at that point.
so only int c; is defined while extern int c; is declared .
A definition creates space for a variable:
int c;
Wherever you put this line, either local, global, this says that a new variable c of type int shall come to life.
extern int c;
A declaration says that there is somewhere else some variable c of type int. By using extern, you say that c is defined somewhere else. If you put only an extern declaration without a definition somewhere else, you will have a link error. Using extern is the equivalent of a forward declaration of a function:
/* declaration */
int f(int x);
vs.
/* definition */
int f(int x) {
return x*x;
}
The first means that there is somewhere a function f returning an int and accepting an int as parameter. The latter is the actual function, its code, which also works both as a declaration and a definition.
IMO, this declaration-vs-definition naming is confusing. I hardly remember which one is what, and I usually need to think about it. You should, however, understand the what extern means and what a forward declaration is.
Long story short, defining something means providing all of the necessary information to create that thing in its entirety. However, declaring something means providing only enough information for the computer to know it exists.
Edit: To be clearer: A definition both defines and declares, a declaration ONLY declares. When you are using the extern keyword by definition you are not defining anything. Your confusion stems from the understanding of extern.
I have the following source code which interests me.
#include <stdio.h>
extern int foo;
int foo = 32;
int main()
{
printf("%d", foo);
}
This a perfectly normal piece of code, and when I compile it with
gcc -Wall -Wextra -pedantic foo.c
I get no warnings.
And it seems weird, because a variable is defined both as external, and also global in the same file.
I'm quite sure that it's easy to the linker to find the reference for the external variable in the same file, but doesn't it look like a coding error? And if so, why doesn't the compiler warn about this?
There's nothing weird. You first made a declaration of a variable (you promised the compiler that it exist) and then you actually defined it. There's no problem in that.
Also, by default, all variables that aren't local to functions and aren't defined as static are extern.
You seem to misunderstand what extern does. extern simply makes your declaration just a declaration instead of a definition.
int i; //definition of i
extern int i; //declaration of i
It is perfectly normal to have multiple declarations of the same variable, but only one definition should be present in the whole program. Compare this with a function
void f(void); //declaration
void f(void) //definition(and redeclaration)
{
} //definition
In order to use a variable or function, you only need its declaration. Its definition may appear anywhere in the program (the linker will find it). Anywhere can be the same file, another file, or even an external library.
And it's seems weired, because a variable is defined both as external, and also global in the same file.
extern int foo;
says: it declares without defining an object of type int named foo.
int foo = 32;
it declares and defines an object of type int named foo with external linkage.
There is no contradiction and it is valid C code.
The difference is that the former is a declaration -> extern declares a variable and says it will be available somewhere around. You can have as many declarations as you want and the latter is a definition which must be there exactly once.
So there should be no warning and no error.
extern is a way to provide visibility to a variable that is defined elsewhere...
extern is like a promise...
in example.h
extern int g;// promises that this will be in the object code to anything that includes example.h
example. c
int g;