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
