Enable multithreading in CMake

Ivan4587

Hello everyone,

i need to build my project with QT for WebAssembly using CMake. But i cannot find the way to enable multithreading.
What i found is the description with that, "Qt on WebAssembly supports multithreading, however support is disabled by default in order to be compatible with as many browsers as possible. Thread support can be enabled by building Qt from source and configuring with the "-feature-thread" flag".
But how could i config it in my CMake file? Any suggestion? Thanks a lot!

Best regards,
Ivan

Mesrine

@Ivan4587
First you need to compile Qt from source enabling multithreading as you said.

Then you need to enable emscripten code generation for pthreads in your Cmake project like this.

I normally put this on my root CMakeLists.txt

if(EMSCRIPTEN)
target_compile_options(target PRIVATE -pthread)
target_link_options(target PRIVATE -pthread -sPTHREAD_POOL_SIZE=8)
endif()

And this work for firefox, but not for Chrome(I think)

JonB

@Ivan4587 said in Enable multithreading in CMake:

by building Qt from source and configuring with the "-feature-thread" flag".

That argument is to be used with the Qt configure tool.

Ivan4587

@JonB understood, so i'm looking for the way in CMake.

jsulm

@Ivan4587 Do you use make, nmake or ninja?
ninja enables parallel build (processes by the way, not threads).
If you use make/nmake then use -j NUMBER_OF_PARALLEL_BUILDS command, like:

make -j 16

Ivan4587

@jsulm i'm using QT Creator with CMake file. So i need to add some parameters in the CMakeLists.txt file to enable multithreading for QT on WebAssembly.

jsulm

@Ivan4587 No, you do not specify the number of parallel builds inside CMakeLists.txt! This should be decided on each machine and/or by each developer depending on available ressources.
Simply specify the parameter in QtCreator: go to Projects (on the left side), then "Build Steps" and edit "Make" entry.

Ivan4587

This post is deleted!

Mesrine

@Ivan4587
First you need to compile Qt from source enabling multithreading as you said.

Then you need to enable emscripten code generation for pthreads in your Cmake project like this.

I normally put this on my root CMakeLists.txt

if(EMSCRIPTEN)
target_compile_options(target PRIVATE -pthread)
target_link_options(target PRIVATE -pthread -sPTHREAD_POOL_SIZE=8)
endif()

And this work for firefox, but not for Chrome(I think)

jsulm

@jsulm Looks like I completelly misunderstood the question. Ignore my posts in this thread :-)

Ivan4587

@jsulm Maybe i didn't describe correctly... I'm not looking for the way to build it parallel, but the way to enable multithreading when i use the WebAssembly.

jsulm

@Ivan4587 No, your description is OK, I just was too fast while reading :-)

Mesrine

@Ivan4587 But it is not clear if you have manage to compile qt from source enabling multithreading.

In that case my previous answer will help you after you do that.

In your cmake project you need to link to the qt wasm libraries compiled with multithreading support(by using the configure flag). You should not add multi threading support to Qt libraries from your CMake project.

JoeCFD

@Ivan4587 said in Enable multithreading in CMake:

-feature-thread

if you have the pro file and are able to build it with the flag, you can find out where it is added in Makefile. From there, you will be able to figure out where to add it in the cmake file.

Ivan4587

@Mesrine

@Mesrine said in Enable multithreading in CMake:

But it is not clear if you have manage to compile qt from source enabling multithreading.

That's the point. I'm using QT 6, as i known, it is not open source anymore.

jsulm

@Ivan4587 said in Enable multithreading in CMake:

as i known, it is not open source anymore

It is open source. You can get Qt6 source code in exact same way as Qt5.

Ivan4587

@jsulm ok... i'll have a try.