[boost-filesystem] Error linking with boost-filesystem whilst using Emscripten

[uniformization]

Describe the bug
When compiling my application, the linking process fails because boost-filesystem was not compiled with -pthread on Emscripten. The error is as follows:

wasm-ld: error: --shared-memory is disallowed by exception.cpp.o because it was not compiled with 'atomics' or 'bulk-memory' features.

Environment

• OS: Windows
• Compiler: emcc (Emscripten gcc/clang-like replacement + linker emulating GNU ld) 3.1.61 (67fa4c16496b157a7fc3377afd69ee0445e8a6e3)

To Reproduce
Steps to reproduce the behavior:

1. Clone the demo I made for this issue: git clone https://github.com/uniformization/boost-error-vcpkg-test.git.
2. Run the preset then build it: cmake --preset vcpkg-emscripten && cmake --build --preset emscripten.
3. See error.

Expected behavior
The program should link successfully.

Failure logs
build.log

Additional context
I created a quick demo so any reviewer can see the setup I'm using: https://github.com/uniformization/boost-error-vcpkg-test/.

[Neumann-A]

Not really an issue with vcpkg but your assumption about the emscripten triplet.
vcpkg just includes include("${EMSCRIPTEN_ROOT}/cmake/Modules/Platform/Emscripten.cmake") you however want to also have

string(APPEND CMAKE_CXX_FLAGS " -sWASM=1 -sUSE_PTHREADS -pthread")
string(APPEND CMAKE_C_FLAGS " -sWASM=1 -sUSE_PTHREADS -pthread")

So write your own emscripten triplet/toolchain and add the required flags? (Or adjust CFLAGS and CXXFLAGS in your environment accordingly.)

[dg0yt]

string(APPEND CMAKE_CXX_FLAGS " -sWASM=1 -sUSE_PTHREADS -pthread")
string(APPEND CMAKE_C_FLAGS " -sWASM=1 -sUSE_PTHREADS -pthread")

AFAIU -pthread is all what is needed today:
https://emscripten.org/docs/porting/pthreads.html#pthreads-support

Why should this be handled in the triplet/toolchain for emscripten? This is normally detected/added by the packages, for other platforms.

Or adjust CFLAGS and CXXFLAGS in your environment accordingly.

Doesn't this have an effect on building host dependencies?

[Neumann-A]

Why should this be handled in the triplet/toolchain for emscripten? This is normally detected/added by the packages, for other platforms.

If you read https://emscripten.org/docs/porting/pthreads.html#pthreads-support carefully:

Pass the compiler flag -pthread when compiling any .c/.cpp files, AND when linking to generate the final output .js file.

which basically means it is an all or nothing flag which also is inline with my experience with it while testing microsoft/vcpkg-tool#1315 with wasm. You will just get errors that abc wasn't build with thread support.

Doesn't this have an effect on building host dependencies?

Hmm yes. So setting CFLAGS and CXXFLAGS in the triplet is required, e.g.:

set(ENV{CXXFLAGS} " -sWASM=1 -sUSE_PTHREADS -pthread")
set(ENV{CFLAGS}  " -sWASM=1 -sUSE_PTHREADS -pthread")

[uniformization]

So write your own emscripten triplet/toolchain and add the required flags? (Or adjust CFLAGS and CXXFLAGS in your environment accordingly.)

This worked perfectly, thanks!