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Re: [Qemu-devel] [PATCH v10 5/8] module: implement module loading

From: Alex Bligh
Subject: Re: [Qemu-devel] [PATCH v10 5/8] module: implement module loading
Date: Mon, 16 Sep 2013 12:00:47 +0100

On 16 Sep 2013, at 11:38, Paolo Bonzini wrote:

> No, librbd does need to be there for the other symbols that are not weak
> (e.g. rbd_aio_read).  This approach cannot be "taken to the limit", i.e.
> removing the librbd dependency altogether.  For example:
> xx.c:
> int f(void)
> {
>       return 42;
> }
> yy.c:
> #pragma weak f
> extern int f(void);
> int main()
> {
>       printf("%p %d", f, f ? f(): 67);
> }
> $ gcc xx.c -shared -o xx.so
> $ LD_RUN_PATH=$PWD gcc yy.c xx.so -o yy
> $ ./yy
> 0x4005b0 42
> $ rm xx.so
> $ ./yy
> ./yy: error while loading shared libraries: xx.so: cannot open shared
> object file: No such file or directory

I think you need to wrap f, i.e. take g as a pointer to f(), and
call g().

> Also, the code _is_ ugly.  Do it once and it's perhaps acceptable.  Do
> it for libiscsi, librbd, libcurl, libssh2, SPICE, GTK+, SDL etc. and it
> becomes unmaintainable.

I agree it's ugly. However, it's pretty much the only way to cope
with different versions of libraries.

However, even if you don't use weak symbols, we could simply dlopen()
a fixed list of modules known at compile time from a single directory
(because we also know at compile which executable needs what, e.g.
that qemu-img doesn't need spice or whatever).

Alex Bligh

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