How do I pass compiler/linker arguments to Emscripten?

[ahmedyarub]

I'm creating a cross-platform application that uses Qt downloaded using the official tool in addition to some other libraries that I could successfully build in vcpkg like OpenSSL, libssh, and aws-sdk-cpp. Now I can build the application using the single-threaded Qt 6.5.0 beta and link them to vcpkg libraries. However, I need the Qt Concurrent library which means that I have to build against the multi-threaded version of Qt. Linking to these Qt libraries does work but when I add my vcpkg libraries I get the following error:

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

That error should be resolved by passing -pthread parameter, -matomics, or some other compiler/linker parameters to Emscripten. I did try to pass them like this in a new toolchain file but that still didn't work:

set(VCPKG_CXX_FLAGS "-matomics -s SHARED_MEMORY=1")
set(VCPKG_C_FLAGS "-matomics -s SHARED_MEMORY=1")
set(VCPKG_LINKER_FLAGS "-matomics -s SHARED_MEMORY=1")

Any ideas?

[autoantwort]

VCPKG_CXX_FLAGS et al are triplets variables, you have to set them in a new triplet. https://learn.microsoft.com/en-us/vcpkg/users/triplets#vcpkg_cxx_flags

[ahmedyarub]

I did create a new triplet and added the parameters like this to the end of a new triplet but that didn't change anything:

set(VCPKG_CXX_FLAGS "-pthread")
set(VCPKG_C_FLAGS "-pthread")

I then tried to manually build libssh outside vcpkg passing the parameters as command-line parameters and it worked as expected:
-DCMAKE_CXX_FLAGS="SHELL:-pthread" -DCMAKE_C_FLAGS="-pthread"

Is there a bug in vcpkg or am I doing something wrong?

[ahmedyarub]

I noticed that flag was nowhere to be found in the generated build scripts so I tried adding the lines to the toolchain of Emscripten instead. I could see the flag added to the scripts but the files were still not built for pthreads.

[netcorefan1]

@ahmedyarub I'm also having the same problem. The docs says that set(VCPKG_*_FLAGS .) only works when not using VCPKG_CHAINLOAD_TOOLCHAIN_FILE and according to this article written by @retifrav (#3577), it is expected to work when using any of /path/to/vcpkg/scripts/toolchains/*.cmake. In my case I'm just using the default one (vcpkg.cmake), but the flags are just ignored.

Did you found a workaround?

I then tried to manually build libssh outside vcpkg passing the parameters as command-line parameters and it worked as expected: -DCMAKE_CXX_FLAGS="SHELL:-pthread" -DCMAKE_C_FLAGS="-pthread"

Is there a bug in vcpkg or am I doing something wrong?

Same for me and I'm also wondering if this is a bug that should be reported.

[ahmedyarub]

@ahmedyarub I'm also having the same problem. The docs says that set(VCPKG_*_FLAGS .) only works when not using VCPKG_CHAINLOAD_TOOLCHAIN_FILE and according to this article written by @retifrav (#3577), it is expected to work when using any of /path/to/vcpkg/scripts/toolchains/*.cmake. In my case I'm just using the default one (vcpkg.cmake), but the flags are just ignored.

Did you found a workaround?

I then tried to manually build libssh outside vcpkg passing the parameters as command-line parameters and it worked as expected: -DCMAKE_CXX_FLAGS="SHELL:-pthread" -DCMAKE_C_FLAGS="-pthread"
Is there a bug in vcpkg or am I doing something wrong?

Same for me and I'm also wondering if this is a bug that should be reported.

Unfortunately, the only workaround I could find is modifying the toolchain of Emscripten that resides inside the emsdk folder.
Far from being ideal but it worked for me.

[netcorefan1]

Did you tried to directly deal with VCPKG_CMAKE_CONFIGURE_OPTIONS ? I had success to make CMake recognize my custom flags (although I got a different output than expected).

[fwcd]

I have had success defining a custom triplet with

set(ENV{EMCC_CFLAGS} "$ENV{EMCC_CFLAGS} -pthread")

This is certainly not ideal, but seems to work (you can verify that the generated objects indeed have this enabled by checking wasm-objdump -x <path to object file>, which should include [+] atomics).