Re: New to musl and C++ compiling

[Michele Portolan <michele.portolan@grenoble-inp.fr>]

Thanks for the information.

You say "use a cross compiler that is built for musl" ... how do I do this?

Binaries are useful, but if I can also get the build process right it is 
more powerful, and easier to pass to students (I am an academic).

Thanks,


Michele

On 21/01/2019 13:02, Szabolcs Nagy wrote:
> * Szabolcs Nagy <nsz@port70.net> [2019-01-21 12:53:13 +0100]:
>
>> * Michele Portolan <michele.portolan@grenoble-inp.fr> [2019-01-21 11:24:12 +0100]:
>>> Hello,
>>>
>>> I just installed MUSL because I have a C++ multithreaded application that
>>> uses threads heavily and I would like to make it independent from an OS. I
>>> was able to easily install and run MUSL for C targets, but when I try a
>>> simple C++ Hello world I get an error for the standard libs.
>>>
>>> My file is the simplest possible (no multithreading to start with):
>>>
>>>   #include <iostream>
>>>
>>>   int main() {
>>>            std::cout << "Hello, World" << std::endl;
>>>            return 0;
>>> }
>>>
>>> Here is my output for standard and musl-based compilation.
>>>
>>> portolan@noumea:~/musl/examples$ g++ -o test_cpp test_cpp.cpp
>>> portolan@noumea:~/musl/examples$ ./test_cpp
>>> Hello, World
>>> portolan@noumea:~/musl/examples$ g++ -o test_cpp test_cpp.cpp -specs
>>> "/home/portolan/musl/install/lib/musl-gcc.specs"
>> for c++ the recommended practice is to use a cross compiler that
>> is built for musl, instead of a glibc based native compiler with
>> a specs file or other wrapping mechanism, because c++ headers are
>> difficult to get right: in this case the specs file disabled all
>> c++ header paths, you need to add those back manually, see
>>
>> g++ -v -E -xc++ - </dev/null
>>
>> but there may be still issues
>> - the header ordering matters as libstdc++ uses include_next and
>> - some headers are installed based on the libc found at configure
>>    time of gcc, so the abi is slightly different depending on what
>>    libc you built your compiler for,
>> - e.g. with static linking (which you need if you want a portable
>>    executable) one issue is that libstdc++ has a broken way to
>>    detect multi-threadedness and all locks become nops (unless your
>>    binary has a definition for the 'pthread_cancel' symbol).
>>    if gcc is configured for *-musl* this is fixed.
>>
>> in short: use a cross compiler targetting *-linux-musl, there are
>> prebuilt ones at http://musl.cc/
>> (note that you will have to build and install all your application
>> dependencies into a path where the cross compiler can find them)
> oh and if you have many dependencies then the simplest way is of
> course to use a musl based distro (alpine, void, adelie,..) then
> you can use all the prebuilt packages and the native toolchain
> with g++ -static and you get a portable executable.
> (setting up a chroot or docker with whatever distro should not be
> too much work).
>
>>> test_cpp.cpp:1:11: fatal error: iostream: No such file or directory
>>>    #include <iostream>
>>>             ^~~~~~~~~~
>>> compilation terminated.
>>>
>>> I am probably missing something REALLY basic, at least I hope so!
>>>
>>> Best regards,
>>>
>>>
>>> Michele