[libcxx-dev] [llvm-dev] Shipping custom libc++ on MacOS

[Louis Dionne via libcxx-dev]

> On Feb 1, 2021, at 15:16, Louis Dionne via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> 
> 
> 
>> On Feb 1, 2021, at 02:37, Tobias Hieta via libcxx-dev <libcxx-dev at lists.llvm.org> wrote:
>> 
>> Hello,
>> 
>> We are working on an application where we want to have full support
>> for C++17 but still ship on older versions of macOS (10.9/10.10 in
>> this case). Our plan was to build our own libc++ from llvm and ship it
>> with the application and then pass "-D_LIBCPP_DISABLE_AVAILABILITY"
>> 
>> This seemed to work fine in internal testing - but then we ran into a
>> similar issue as the one described here:
>> https://reviews.llvm.org/D74489#2497044
>> 
>> Now I can patch my copy of libc++ with the upstream fix for this - but
>> the comment below made me think otherwise:
>> https://reviews.llvm.org/D74489#2505156
>> 
>> We have run into problem where if you ship your own libc++ the system
>> frameworks loads the system libc++ and some strange errors happen - we
>> thought we could work around these issues - but maybe this is just us
>> being naive.
>> 
>> So I guess my question is - is it possible to ship a custom libc++ in
>> a .app archive for older versions of macOS in order to use C++17? Or
>> is my only hope to raise the minimum version of our app and always
>> link to the system libc++?
> 
> Generally, I would say this is a tricky thing to do. As you mention, the problem is that if you link against any other library that does link against the system-provided libc++.dylib, you'll have ODR violations because some symbols will be found in both libraries. As a result, I believe you could also end up in a situation where you end up using globals from one library when the globals from the other library has been initialized. So basically, nothing good happens.
> 
> If you can ensure that your application doesn't pull in the system libc++.dylib (even transitively), then this would in theory be safe to do. Due to the brittleness of the solution, my immediate reaction would be to not recommend that. The reason why some parts of C++17 aren't available on older platforms is that we put the vtables required for exception classes in the dylib. That is a code size optimization that we think is worth the trouble. You can generally use the parts of C++17 APIs that can't throw exceptions even on older platforms because they don't require those vtable symbols - perhaps that is something you can consider?

Also, Chrome is known to do something similar to what you're inquiring about. They build libc++ statically with hidden visibility, and then they link it into their application. I assume they also make sure to not export any symbol from libc++ that would be created by using the libc++ headers (e.g. by instantiating some function template). I don't know their setup very well, but I assume they make sure to not link against any other system library that does require libc++.dylib from the system, or else things get trickier as I explained above. But as long as you satisfy that requirements, I believe things have worked fairly well for them so far. You just have to be able and willing to walk that fine line.

Louis

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