Re: New to musl and C++ compiling

[Szabolcs Nagy <nsz@port70.net>]

* 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)


> 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