How to tell a C project to link using a C++ compiler?

[1480c1]

Hi,

Some files to help illustrate what I'm trying to do.

a/a.cc

#include <iostream>
extern "C" void hello(void) {
    std::cout << "Hello World\n";
}

a/a.h

#ifndef A_H
#define A_H

#ifdef __cplusplus
extern "C" {
#endif

void hello(void);

#ifdef __cplusplus
}
#endif

#endif

a/meson.build

project(
  'a',
  'cpp',
  version: '0.1',
  default_options: ['warning_level=3', 'cpp_std=c++14'],
)

pkg = import('pkgconfig')

a = library('a', 'a.cc', 'a.h', install: true)
install_headers('a.h')

pkg.generate(a)

b/b.c

#include "a.h"

int main() {
    hello();
}

b/meson.build

project('b', 'c', version: '0.1', default_options: ['warning_level=3'])

a = dependency('a', version: '>=0.1')

executable('b', 'b.c', install: true, dependencies: [a])

I'm essentially compiling two projects, One is a C++ project that isn't using meson, but generates a .pc file.
The other is a C project that use meson, and depends on the C++ project, and uses the .pc file generated by the C++ project.

Both projects are not owned/developed/maintained by me, I'm simply just trying to compile them.

"Normally," project a's .pc file contains Libs.private: -lstdc++, which would work fine with CC=gcc LDFLAGS="-static-libgcc -static-libstdc++", but that doesn't work with clang+lld+libc++. To be more specific, when clang is passed both -static-libstdc++ and -lstdc++, it will convert lstdc++ to the appropriate c++ library when passing to lld, and convert -static-libstdc++ to -Bstatic -lc++.

In my case, this results in lld being called with ld.lld ... -Bdynamic ... -lc++ -Bstatic -lc++ -Bdynamic ..., which causes lld to try to link both the static and dynamic versions of libc++, which cases a duplicate symbol error. Thus, for my compilations, I've been having to remove any mentions of -lstdc++ from the .pc files that get generated, and have instead resorted to using clang++ as the linker (like passing --ld=$CXX to FFmpeg's configure for example).

For meson, I could not find any equivalent way to set the linker for a 'c' project to $CXX so clang will implicitly choose the right c++ library to link against. I tried a few other things such as modifying the CC_LD=clang++ and LD=clang++, but the first only seemed to result in clang -fuse-ld=clang++ (which doesn't work), and the second didn't do anything.

Of course, if I had infinite patience and time, I could either modify the meson.build and change c to cpp or do what was mentioned in #11603 and move one of the .c files to .cc. However, doing that for every meson project would be a massive pain. I'm hoping there's a better way to do this.

tldr

Trying to find a way to link a C meson project to a C++ library without specifying -lstdc++ in the linking process, as that causes errors.

[Andarwinux]

Try to add --driver-mode=g++ to LDFLAGS?

[1480c1]

That seems to work

cddeg@ccom-laptop CLANG64 /d/b
$ clang -static-libstdc++ b.c -I /local64/include/ -L /local64/lib/ -la
clang: warning: argument unused during compilation: '-static-libstdc++' [-Wunused-command-line-argument]
ld.lld: error: undefined symbol: __cxa_begin_catch
>>> referenced by C:/msys64/clang64/include/c++/v1/ostream:685
>>>               liba.a(a.cc.obj):(std::__1::basic_ostream<char, std::__1::char_traits<char>>& std::__1::__put_character_sequence[abi:se180100]<char, std::__1::char_traits<char>>(std::__1::basic_ostream<char, std::__1::char_traits<char>>&, char const*, unsigned long long))
>>> referenced by C:/msys64/clang64/include/c++/v1/ostream:330
>>>               liba.a(a.cc.obj):(std::__1::basic_ostream<char, std::__1::char_traits<char>>::sentry::~sentry())
>>> referenced by liba.a(a.cc.obj):(__clang_call_terminate)
...

cddeg@ccom-laptop CLANG64 /d/b
$ clang --driver-mode=g++ -static-libstdc++ b.c -I /local64/include/ -L /local64/lib/ -la
clang: warning: treating 'c' input as 'c++' when in C++ mode, this behavior is deprecated [-Wdeprecated]

cddeg@ccom-laptop CLANG64 /d/b
$ $?
bash: 0: command not found

cddeg@ccom-laptop CLANG64 /d/b
$ PATH=/local64/bin:$PATH ldd a.exe
        ntdll.dll => /c/Windows/SYSTEM32/ntdll.dll (0x7ff8c6570000)
        KERNEL32.DLL => /c/Windows/System32/KERNEL32.DLL (0x7ff8c46f0000)
        KERNELBASE.dll => /c/Windows/System32/KERNELBASE.dll (0x7ff8c3b40000)
        ucrtbase.dll => /c/Windows/System32/ucrtbase.dll (0x7ff8c3ef0000)

Do you know if clang or LLVM have any documentation online about what --driver-mode= actually does? DuckDuckGo and Google isn't bringing up much except a gist that contains the output of clang --help-hidden and a few others, but no documentations. (Seems like a lot of the answers for LLVM is to just look at the code within LLVM)

[Andarwinux]

This looks undocumented, I stumbled across the relevant definition in source code.