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Re: [Qemu-devel] [QUESTION] SDL 1.2 support

From: Eric Blake
Subject: Re: [Qemu-devel] [QUESTION] SDL 1.2 support
Date: Wed, 17 Jul 2019 14:57:16 -0500
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.7.0

On 7/17/19 2:20 PM, Aleksandar Markovic wrote:

> But, does "configure" list somewhere unmet soft dependencies? (the
> question is general, not looking at SDL only) Is there any other way for
> an end user to have info on unmet dependencies (whether soft or hard),
> other than see QEMU is not building, or something is not working in
> QEMU run-time?

Yes - at the end of ./configure, the output includes a list similar to:

profiler          no
static build      no
SDL support       yes (2.0.9)
SDL image support no
GTK support       yes (3.24.1)
GTK GL support    yes
VTE support       yes (0.54.4)
TLS priority      NORMAL
GNUTLS support    yes
libgcrypt         no
nettle            yes (3.4.1)

so on my system, I have SDL 2.0.9 (auto-detected, so it is in use);
whereas a system with no SDL or only SDL 1.2 would probably state "SDL
support no". You can similarly observe how I lack libgcrypt but have
libnettle. Basically, if configure exited with status 0, anything with a
'no' status was a soft dependency and you are building without that
feature; but if you want to ensure that a feature is used, you could use
'./configure --enable-sdl=yes' to force configure to make SDL a hard
dependency (fail it if was not found, instead of the default of probing
if it is available and then deciding yes or no based on the probe
results).  Conversely, you can use './configure --disable-sdl' (also
spelled './configure --enable-sdl=no') to forcefully build without SDL
(and prove that it was a soft dependency, and to see what gets omitted
from the build) even when the probe would otherwise have automatically
built against SDL 2.0.

> Daniel,
> We had message "SDL 1.2 is going to be deprecated" in QEMU 3.0
> "configure" and, if I remember well, in QEMU 3.1 as well. And now,
> when we finally deprecated it, is it true that there is no message
> whatsoever on systems with SDL 1.2 only?

You'll get the 'SDL supported no' message, but unless you were explicit
at the configure command line, the default behavior is that it relies on
system probing with silent fallback; you have to supply an
--enable/--disable command-line option if you want better behavior than
an implicit default.  In fact, if you ever look at downstream packages,
you'll notice that the packagers have a very long list of
--enable/--disable to prove that their build is reproducible with the
intended components instead of arbitrary based on whatever probing
happened to find on the system at the time.

Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org

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