Thread: Doubt in using either extern or including a header file

Hi,

I have 3 files as below

a.c:

Code:

int x(int a, int b)
{
   return a + b;
}

b.c

Code:

extern int x(int,int);
int main()
{
     int res;
     res = add(2,3);
     printf("res= %d",res);
     return 0;
}

In the file b.c, as function add is defined in a.c, I have used extern declaration for the prototype of the function.

But instead of this, if I use a header file lets say a.h which contains the prototype of the function x as below

a.h

Code:

int x(int,int);

then if I include this a.h in b.c, then do I need not place the extern declaration again??

extern declaration tells the compiler that the function x is defined externally in another file.

But if I include the a.h simply in b.c, as it copies the contents of the a.h in b.c, then there will be no extern. So how the compiler knows that function x is defined externally somewhere??

Thanks laserlight!!.... So without the header file, if I put the forward declaration of function add in b.c file as

Code:

 int add(int,int);

without extern, will it work??

You might like to read section 6.2.2 of the C11 Standard ( http://www.open-std.org/JTC1/sc22/wg...docs/n1570.pdf ), particularly paragraph 4

For an identifier declared with the storage-class specifier extern in a scope in which a
prior declaration of that identifier is visible if the prior declaration specifies internal or
external linkage, the linkage of the identifier at the later declaration is the same as the
linkage specified at the prior declaration. If no prior declaration is visible, or if the prior
declaration specifies no linkage, then the identifier has external linkage.