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Mar 28th, 2007

Working with header files ???

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Hi All

I have a header file link.h , in which i declare a variable itemp.

When i assign this variable a value in the C file link.C i get an error during compilation:

cxx: Error: link.C, line 5: this declaration has no storage class or type
specifier
itemp=0;
^
cxx: Error: link.C, line 5: declaration has already been defined by variable
"itemp" (declared at line 3 of "/home/team/.shared/link.h")
itemp=0;

Plz help

Thnx

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gpta_varun

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Mar 28th, 2007

Re: Working with header files ???

Seems like you've declared the variable like this:
itemp=0;

If that's the case it's wrong and it should be something like:
int itemp ;

If that's not the case please post the exact declaration of this variable from link.h

thekashyap

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Mar 28th, 2007

Re: Working with header files ???

Hi

Thnx 4 the reply ....

Actually I was not initializing this variable within a function in my C file. Does That means that the variables declared in a header file are visible in a function only ??

Else they are considered global for that C file if defined outside any function...and need to be declared before definition.

gpta_varun

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Mar 28th, 2007

Re: Working with header files ???

Hi,
I would like to add here, that to use the variable in the file link.c , try to declare it again as follows

C Syntax (Toggle Plain Text)

 extern int itemp;

This 'll probably resolve the issue..

bala24

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Mar 28th, 2007

Re: Working with header files ???

you should never actually declare variables in header files because if you have two or more *.c files that include that header file the compiler linker will issue duplicate objects errors (or something like that).

To get around that problem, in header files use the extern keyword, as in bala's example:
extern int itemp;

Then in ONE and only ONE *.c or *.cpp file declare it again but without the extern keyword. This will actually create an instance of the variable.

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Mar 28th, 2007

Re: Working with header files ???

Hi

I tried making two static libraries each containing a single object file (abc1.C and abc2.C).
and libs: lib1.a (with abc1.o) and lib2.a (with abc2.o). Both the C files include the same header file and access the same variable "global".

I access this variable from both the files and find that a single copy of the variable is used , since a increment to variable "global" in one is seen in other file also.

I have declared the variable global as
int global;

Now everything is working perfectly.

Still do i need to declare this variable as an extern ??? Why am i not getting the multiple declaration error ???

Thnx

gpta_varun

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Re: Working with header files ???

Click to Expand / Collapse Quote originally posted by bala24 ...

Hi,
I would like to add here, that to use the variable in the file link.c , try to declare it again as follows
c Syntax (Toggle Plain Text)
 extern int itemp;
extern int itemp;
This 'll probably resolve the issue..

and why would you do this???
If variable is declared in link.h (which of course isn't a good idea) and link.h is #included in link.C there is no logic in doing this.

If link.h is not included in link.C (which I very much doubt looking at their names) then one should do what you suggest.
But then again see the errors he reported. To me at least it seems clear that link.h is included in link.C:
cxx: Error: link.C, line 5: this declaration has no storage class or type
specifier
itemp=0;
^
cxx: Error: link.C, line 5: declaration has already been defined by variable
"itemp" (declared at line 3 of "/home/team/.shared/link.h")
itemp=0;

@@gpta_varun
I don't think your code have any issues with scope etc. It's seems like a simple "wrong declaration".
Did you actually try changing declaration in link.h to "int itemp;" ??
Can you post the declaration from link.h ??

Last edited by thekashyap; Mar 28th, 2007 at 8:27 am.

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Mar 28th, 2007

Re: Working with header files ???

Click to Expand / Collapse Quote originally posted by gpta_varun ...

Hi

I tried making two static libraries each containing a single object file (abc1.C and abc2.C).
and libs: lib1.a (with abc1.o) and lib2.a (with abc2.o). Both the C files include the same header file and access the same variable "global".

I access this variable from both the files and find that a single copy of the variable is used , since a increment to variable "global" in one is seen in other file also.

I have declared the variable global as
int global;

Now everything is working perfectly.

Still do i need to declare this variable as an extern ??? Why am i not getting the multiple declaration error ???

Thnx

If you declare variables with the same variable names in two or more program units and the variables are of the same type, some compiler will combine them as if one was declared with the extern keyword. If you don't want that behavior then use the static keyword and each program unit will have its own copy of the variables.

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Mar 29th, 2007

Re: Working with header files ???

Click to Expand / Collapse Quote originally posted by Ancient Dragon ...

If you declare variables with the same variable names in two or more program units and the variables are of the same type, some compiler will combine them as if one was declared with the extern keyword. If you don't want that behavior then use the static keyword and each program unit will have its own copy of the variables.

Declaring this variable as an extern give an error

ld:
Unresolved:
global

while compiling the 2 C files .

thnx

gpta_varun

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Mar 29th, 2007

Re: Working with header files ???

Either you forgot to put the actual variable declaration (the one without the extern keyword) in one of the source files, or you forgot to feed that file to the linker...

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