CMakeLists.txt: allow compilation without CXX support

[ThomasDevoogdt]

e.g. fluent-bit is a c-only library, so allow compilation without cxx

• Issue: Can't compile with C only toolchain. #4364
• Logs: http://autobuild.buildroot.org/?reason=fluent-bit-2.1.7

[cla-assistant]

All committers have signed the CLA.

[ThomasDevoogdt]

@emasab, a kind remember

[ThomasDevoogdt]

@emasab, @NZKoz, @ctrochalakis, @thijsc, @janmejay, @milindl

Hi all,

This PR has been open for quite some time, is it possible to review and/or approve?
I'm more than happy to change whatever is needed to get this merged.

This commit solves this issue:

• Can't compile with C only toolchain. #4364

Also solves this issue in fluent-bit:

• Kafka support needs a toolchain w/ C++ fluent/fluent-bit#7741
• build: fix Kafka support needs a toolchain w/ C++ fluent/fluent-bit#7765

Which in turn would supersede a patch in buildroot:

• buildroot/buildroot@a70e226

Kr,

Thomas Devoogdt

[ThomasDevoogdt]

@emasab, @NZKoz, @ctrochalakis, @thijsc, @janmejay, @milindl

Hi all,

This PR has been open for quite some time, is it possible to review and/or approve? I'm more than happy to change whatever is needed to get this merged.

This commit solves this issue:

• Can't compile with C only toolchain. #4364

Also solves this issue in fluent-bit:

• Kafka support needs a toolchain w/ C++ fluent/fluent-bit#7741
• build: fix Kafka support needs a toolchain w/ C++ fluent/fluent-bit#7765

Which in turn would supersede a patch in buildroot:

• buildroot/buildroot@a70e226

Kr,

Thomas Devoogdt

@emasab, @NZKoz, @ctrochalakis, @thijsc, @janmejay, @milindl

Please, can anyone review this PR? Or just being polite and reject it with a small comment?

[ThomasDevoogdt]

@emasab can you consider this PR?

[ThomasDevoogdt]

@emasab can you have a look at this PR? With some luck, it can be merged just within 2 years.

[emasab]

@ThomasDevoogdt sorry for the late reply but as you can imagine we've a schedule of features to add to stay on parity with the Java client and about the community issues we give priority to the critical ones. In this case librdkafka has a C++ client too so it's fair it's requiring a C++ compiler.
It's an enhancement to be able to compile without it, if you need only the C library. I see if we can merge this after much time.

[ThomasDevoogdt]

@ThomasDevoogdt sorry for the late reply but as you can imagine we've a schedule of features to add to stay on parity with the Java client and about the community issues we give priority to the critical ones. In this case librdkafka has a C++ client too so it's fair it's requiring a C++ compiler. It's an enhancement to be able to compile without it, if you need only the C library. I see if we can merge this after much time.

Hi, thx for the response. I know that this PR looks like an "enhancement", but I do consider it more a bug that the librdkafka C library requires C++. Not all targets do have a C++ compiler out of the box. I'm working on a PR in fluent-bit fluent/fluent-bit#9277 to check if compilation succeeds without a C++ compiler. But they are a bit waiting on this to be merged. Yes, I can help myself to apply patches on my build targets, but it would help me seriously if this patch doesn't have to wait "for much time" (Which it has been doing for 2 years). Beware that this PR doesn't really change anything if C++ is just available, so there is no real regression possible here.

[emasab]

It cannot be considered a bug as librdkafka never allowed to compile without a C++ compiler, if it was allowed an it was not working it would be a bug.

When changing it for CMake it should be allowed to compile without C++ with mklove configuration too. Tests should be built and runnable, only the C++ tests should be disabled in that case.

Disabling C++ should only be done on purpose like passing a --disable-cxx configure argument, not only when the compiler isn't present, to avoid unintended effects.

[ThomasDevoogdt]

It cannot be considered a bug as librdkafka never allowed to compile without a C++ compiler, if it was allowed an it was not working it would be a bug.

I know, C++ support was indeed always expected, but that doesn't mean it should be. Sometimes you just don't need it.

When changing it for CMake it should be allowed to compile without C++ with mklove configuration too. Tests should be built and runnable, only the C++ tests should be disabled in that case.

I don't see any mention of mklove in any CMakeLists.txt file, not sure what you mean. How to enable it, so that I can check if compilation works with/without C++? About the tests, that is exactly what is done, only the C++ tests are disabled in this commit. (UPDATE: Now I see, the examples were handled correctly, but not the tests, will fix that!)

Disabling C++ should only be done on purpose like passing a --disable-cxx configure argument, not only when the compiler isn't present, to avoid unintended effects.

The current way of doing it on purpose is CXX=/bin/false, I don't see the added value of having to pass e.g. -DDISABLE_CXX=ON.
CFR: buildroot/buildroot@b34e0d2

See this output:

With C++

$ cmake -GNinja ../
-- The C compiler identification is GNU 13.3.0
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Check for working C compiler: /usr/bin/cc - skipped
-- Detecting C compile features
-- Detecting C compile features - done
-- Looking for a CXX compiler
-- Looking for a CXX compiler - /usr/bin/c++
-- The CXX compiler identification is GNU 13.3.0
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Check for working CXX compiler: /usr/bin/c++ - skipped
-- Detecting CXX compile features
-- Detecting CXX compile features - done
$ ninja
$ ls -al src-cpp/librdkafka++.so*
lrwxrwxrwx 1 thomas thomas     17 aug  7 22:16 src-cpp/librdkafka++.so -> librdkafka++.so.1
-rwxrwxr-x 1 thomas thomas 430600 aug  7 22:16 src-cpp/librdkafka++.so.1

Without C++

$ CXX=/bin/false cmake -GNinja ../
-- The C compiler identification is GNU 13.3.0
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Check for working C compiler: /usr/bin/cc - skipped
-- Detecting C compile features
-- Detecting C compile features - done
-- Looking for a CXX compiler
-- Looking for a CXX compiler - NOTFOUND
-- C++ compiler not found, skipping C++ support
$ ninja
$ ls -al src/librdkafka.so* 
lrwxrwxrwx 1 thomas thomas      15 aug  7 22:14 src/librdkafka.so -> librdkafka.so.1
-rwxrwxr-x 1 thomas thomas 4346816 aug  7 22:14 src/librdkafka.so.1

I think that it's quite clear printed that C++ support has been skipped, you can't miss it either, since librdkafka++.so won't be there. So not sure how there could be an unintended effect.

[ThomasDevoogdt]

@emasab I updated the test part.

[ThomasDevoogdt]

@emasab its again a bit silent here. I assume that you don't really like the pull request update? Is e.g. adding a CMakeLists.txt option better? Which is more explicit to do...