C Use variable defined in file being included from

Question

I am currently googling this, but I don't know how to word it. I suspect someone will point me to a duplicate question. But... if I have a file like so

parent.h:

...
include "child.h";
int foo;
...

and the following source file, which is called above via its header file:

child.c:

int display ()
{
    printf ( "%d\n", foo );
}

Now this is a simplified example, currently I am using C++ and I am creating an object, and that object calls a method of another object, like foo declared in the parent file. Naturally I get the following error:

error: ‘foo’ was not declared in this scope

Is there any way to get around this scope issue, or must I pass foo down as a parameter?

score 3 · Accepted Answer · edited May 23 '17 at 10:32

3

I believe you are looking for extern.

EDIT:

Adding some initial linkage.

edited May 23 '17 at 10:32

Community

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answered Dec 08 '13 at 01:24

codnodder

1,674
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For a function you just need the prototype `int display();`, which is often put into a header file. – codnodder Dec 08 '13 at 01:35

score 1 · Answer 2 · answered Dec 08 '13 at 03:52

1

Usually, variables live in the source files...

parent.c

int foo = 0;

...and are externed in the header files...

parent.h

extern int foo;

In this way, whomever #includes parent.h will extern foo.

answered Dec 08 '13 at 03:52

Fiddling Bits

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I think you made a mistake. Shouldn't `parent.c` be `parent.h` (without initialization) and `parent.h` should be `child.h`, correct? – puk Dec 08 '13 at 20:21
@Puk14 No mistake. :-D But, there's many ways to accomplish the same goal. – Fiddling Bits Dec 08 '13 at 20:38

C Use variable defined in file being included from

2 Answers2