Re: New to musl and C++ compiling

[Szabolcs Nagy <nsz@port70.net>]

* 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