Doubt related to extern keyword usage?

That's valid syntax, there is even an essentially identical example in the C99 standard. (See §6.9.2-4. ).

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AFAIK, extern keyword should be used for declaration and no value can be associated with the variable being declared with extern keyword. But supposing I write a statement like extern int I = 10; Should the compiler flag an error for the same? I have seen some compilers being tolerant and ignoring this?

Why is this so? What does the 'C' standard says about this? EDIT: @All, Thanks for the answers.

I have a doubt still though. Suppose I have the definition for this variable without the extern linkage in another file say a. C and I add this statement in b.c.

Still is it Ok for the compiler not to flag an error? Does it come under redefintion? C extern link|improve this question edited Mar 1 '10 at 7:23 asked Feb 27 '10 at 19:46Jay3,208827 87% accept rate.

No, because this is in fact a definition not a declaration. – Clifford Feb 27 '10 at 23:50.

That's valid syntax, there is even an essentially identical example in the C99 standard. (See §6.9.2-4. ) It's true that the examples are not normative but I believe it was intended to be legal syntax.

The compiler will often output a warning, because it doesn't really accomplish anything. 4 EXAMPLE 1 int i1 = 1; // definition, external linkage static int i2 = 2; // definition, internal linkage extern int i3 = 3; // definition, external linkage int i4; // tentative definition, external linkage static int i5; // tentative definition, internal linkage int i1; // valid tentative definition, refers to previous int i2; // 6.2.2 renders undefined, linkage disagreement int i3; // valid tentative definition, refers to previous int i4; // valid tentative definition, refers to previous int i5; // 6.2.2 renders undefined, linkage disagreement extern int i1; // refers to previous, whose linkage is external extern int i2; // refers to previous, whose linkage is internal extern int i3; // refers to previous, whose linkage is external extern int i4; // refers to previous, whose linkage is external extern int i5; // refers to previous, whose linkage is internal.

– Jay Feb 27 '10 at 20:01 Lots of keywords and syntax happen to be the default interpretation in any given place. For example, inside a function int i; and auto int i; are the same definition. While adding auto doesn't accomplish anything specific, allowing it provides a consistent use of the storage class specifier.

– DigitalRoss Feb 27 '10 at 20:07.

The following code ; extern int I ; declares a variable i, but does not instantiate it. If it is not also defined in the same compilation unit, the linker will attempt to resolve it from the object files and libraries that comprise the final executable. However your example: extern int I = 10 ; initialises the object, and therefore must also instantiate it.

In this case the extern keyword is redundant because the object is initialised in the same compilation unit (in fact the same statment). It is equivalent to: extern int I ; // redundant int I = 10 ; Although in this last example the extern keyword is redundant, it is exactly equivalent to what you have when a global variable is declared in a header file, and instantiated in a source file that also includes that header (as it should, to allow the compiler to perform type checking). You can test this as follows: extern int I ; int main() { I = 10 ; } The above will cause a linker error for unresolved variable i.

Whereas: extern int I = 10 ; int main() { I = 10 ; } will link without problem.

The extern keyword indicates that the given variable is allocated in a different module. It has nothing to do with access to that variable. It's perfectly legal to assign to assign to an extern variable.

Firstly, the variable in the OP is allocated in this module, since the declaration in the OP is actually a definition. Extern doesn't say that something is "in a different module". It just gives something external linkage.

Secondly, there's no assignment in the OP. It is an initialization, not assignment. – AndreyT Feb 27 '10 at 20:11 Note that this is and example initialisation not assignment.

– Clifford Feb 27 '10 at 23:30.

The purpose of extern keyword is to give the entity external linkage. Whether it is used in a declaration in a or definition makes no difference. There's absolutely no error in the code you posted.

If you prefer to think about it in terms of "export vs. import", then extern keyword applied to a non-defining declaration means that we are importing an entity defined in some other translation unit. When extern keyword applied to a definition, it means that we are exporting this entity to be used by other translation units. (Although it is worth noting that "export vs. import" is not exactly a standard way of thinking about the concept of C linkage.

) The reason you won't see the keyword used in definitions very often is because in C file-scope definitions have external linkage by default. So writing extern int I = 10; is valid, but redundant, since it is equivalent to plain int I = 10; Yet, from time to time in the actual code you might see people using this keyword with function declarations and definitions, even though it is superfluous there as well extern void foo(int i); /* `extern` is superfluous */ ... extern void foo(int i) /* `extern` is superfluous */ { /* whatever */ }.

I down-voted this in error, and was too late to correct it. To redress the balance I found another answer you gave I liked and upped it. Not that it looks like you need it ;) Apologies.

– Clifford Feb 27 '10 at 23:48.

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