If we do the following
static int x; // in file1.c
and
extern int x; // in file2.c
wouldn't the linker complain when the 2 object files are linked to generate the .o for the module? Shouldn't specifying some variable as static hide that variable from other source files. In the above case, I agree that x will not be exported, but will the module even build successfully?
It will. but will fail during loading!
I guess it will fail during loading, but only if the variable x is not defined anywhere in the rest of the kernel globally. Is my assumption correct?
Now I understand why the module will build successfully. The
static int x
declaration is specific to that file(file1.c) and the
extern int x
declaration in the other file(file2.c) will look for an x defined globally in the kernel at the time of loading.
This means the solution is still incorrect because if x is defined in the kernel, then the module will build but the references to x in file2.c will be to the x defined in the kernel.
And if x is not defined in the rest of the kernel, then the module will fail to load.
Can someone please explain this?
-Vishwas.
Mike DeKoker wrote:
Yes, you'll end up having a local symbol for the static declaration and an undefined exported symbol for the extern declaration. The local symbol is not exported by definition. The undefined exported symbol cannot be exported because there is no object (i.e. variable or function) to which it is bound.
-Mike DeKoker
--- "Sponsel Carsten (ext)" <carsten.sponsel@erln.gepas.de> wrote:
But then I've two symbols when i view my module with nm...
-----Original Message----- From: Mike DeKoker [mailto:mdekoker@yahoo.com] Sent: Freitag, 13. Februar 2004 18:20 To: Sponsel Carsten (ext) Subject: Re: global Variable in multiple files
Just define your variable as a module-level static and define an external reference to it in the other source files. This works for functions also.
e.g.
/* File1.c */ static int x;
/* File2.c */ extern int x;
This assumes that File1.o and File2.o are being relocatably linked into a single object file. (Since you're coding for a module they should be.)
-Mike DeKoker
--- "Sponsel Carsten (ext)" <carsten.sponsel@erln.gepas.de> wrote:
Hi friends,
it might be a simple question:
What's the best way to get (to define) a global variable which can be used in multiple files of a kernel module in Linux Kernel 2.6? The variable should not be exported as a symbol. In my special problem I need an array of a struct which is
useable
in all sourcefiles which form my kernel module.
thanks, Carsten
snip
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-- -- Vishwas Raman Software Engineer, Eternal Systems, Inc, 5290 Overpass Rd, Bldg D, Santa Barbara. CA 93111 Email: vishwas@eternal-systems.com Tel: (805) 696-9051 x246 Fax: (805) 696-9083 URL: http://www.eternal-systems.com/
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