Re: [PATCH] Makefile: Compute libraries for libqemuutil.a and libvhost-user.a

[Stefan Hajnoczi <stefanha@gmail.com>]

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On Tue, Jun 16, 2020 at 06:18:14PM +0200, Christophe de Dinechin wrote:
> In util/Makefile.objs, there is a setting for dbus.o-libs.
> Trying to copy-paste that to add a library for module.o that was was
> not otherwise linked yields link errors on a number of binaries,
> e.g. qemu-ga, with unsatisfied symbols in libqemuutil.a(module.o).
> The reason is that library dependencies are not propagated to the .a
> files automatically.
> 
> Adding a call to extract-libs to get the libraries for the two .a
> files that are used elsewhere.
> 
> Signed-off-by: Christophe de Dinechin <dinechin@redhat.com>
> ---
>  Makefile | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/Makefile b/Makefile
> index 2e93068894..5fb0c78a0b 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -598,6 +598,8 @@ Makefile: $(version-obj-y)
>  ######################################################################
>  # Build libraries
>  
> +libqemuutil.a-libs += $(call extract-libs, $(util-obj-y) $(trace-obj-y) $(stub-obj-y))
> +libvhost-user.a-libs += $(call extract-libs, $(libvhost-user-obj-y) $(util-obj-y) $(stub-obj-y))
>  libqemuutil.a: $(util-obj-y) $(trace-obj-y) $(stub-obj-y)
>  libvhost-user.a: $(libvhost-user-obj-y) $(util-obj-y) $(stub-obj-y)

I wonder if there is GNU Make syntax to avoid duplicating the
dependencies of libqemuutil.a and libvhost-user.a?

Another thing I wonder about: the purpose of the .a files is to compile
all object files and only link those .o files needed by the program
(i.e. a subset of the .a file).

Now that libqemuutil.a-libs is defined, do programs using libqemuutil.a
link libs required by .o files that aren't being used?

For example, say we had util/mp3encoder.o which depends on an MP3
encoder library. A utility like qemu-img does not depend on mp3encoder.o
from libqemuutil.a. Therefore -lmp3enc or whatever shouldn't be on
qemu-img's linker command-line.

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