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ufolyah opened this issue Jul 12, 2023 · 6 comments · May be fixed by #24322
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embind cannot be initialized correctly in a thread created in static constructor #19849

ufolyah opened this issue Jul 12, 2023 · 6 comments · May be fixed by #24322
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@ufolyah
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ufolyah commented Jul 12, 2023 •
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I found this bug because I created a thread in wasmfs_before_preload(), which has __attribute__((init_priority(100))).

When the thread runs, embind has not been fully initialized in main thread. So Module['__embind_initialize_bindings'] did not register the types for me, but set initializedJS to true.

I did not use embind in thread created in wasmfs_before_preload, but the worker is reused in another thread, and in that thread embind does not work, caused a crash in my app.

I hope we can have a way to ensure wasm_init_ctors are completed, or embind inited, when spawnthread get called.
@ufolyah ufolyah changed the title embind cannot initialized correctly in a thread created in static constructor embind cannot be initialized correctly in a thread created in static constructor Jul 12, 2023
@sbc100
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sbc100 commented Jul 12, 2023

@brendandahl

@sbc100
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sbc100 commented Jul 12, 2023

When the thread runs, embind has not been fully initialized in main thread. So Module['__embind_initialize_bindings'] did not register the types for me, but set initializedJS to true.

I'm curious about this statement. Why does running __embind_initialize_bindings in a new thread depend on it having already run on the main thread? Doesn't embind_initialize_bindings work independently on each thread that calls it?

@ufolyah
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ufolyah commented Jul 12, 2023

I'm curious about this statement. Why does running __embind_initialize_bindings in a new thread depend on it having already run on the main thread? Doesn't embind_initialize_bindings work independently on each thread that calls it?

From my investigation, EMSCRIPTEN_BINDINGS macro works based on static constructors. when it contructs, the embind declare function are registered to init_func, which get called in other thread.

@brendandahl
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A small example the reproduces the issue would be helpful.

@sbc100
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sbc100 commented Jul 12, 2023

I'm curious about this statement. Why does running __embind_initialize_bindings in a new thread depend on it having already run on the main thread? Doesn't embind_initialize_bindings work independently on each thread that calls it?

From my investigation, EMSCRIPTEN_BINDINGS macro works based on static constructors. when it contructs, the embind declare function are registered to init_func, which get called in other thread.

Ah yes, embind_initialize_bindings must be called only after all the EMSCRIPTEN_BINDINGS static constructors have run.

@ufolyah
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ufolyah commented Jul 12, 2023 •
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//Build with: emcc example.cpp -lembind -sWASMFS=1 -sUSE_PTHREADS=1 -g -O0 -s PTHREAD_POOL_SIZE=2 -o example.html

#include <emscripten.h>
#include <emscripten/wasmfs.h>
#include <emscripten/bind.h>
#include <thread>

void EMSCRIPTEN_KEEPALIVE wasmfs_before_preload(void) {
    // remove next 3 lines to work.
    std::thread([](){
        emscripten_log(EM_LOG_CONSOLE| EM_LOG_ERROR, "thread 1");
    }).detach();
    
    //block the wasm_init_ctors for a while.
    auto start = emscripten_get_now();
    while (emscripten_get_now() - start < 500) {}
}

using namespace emscripten;

class Foo{
public:
    int onInfo(std::string a) {
        auto object = val::object();
        object.set("a", a);
        EM_ASM({
            console.log(Emval.toValue($0));
        }, object.as_handle());
        return 0;
    }
};

int EMSCRIPTEN_KEEPALIVE main() {
    auto start = emscripten_get_now();
    while (emscripten_get_now() - start < 1000) {}
    std::thread([](){
        std::thread([](){
            Foo f;
            f.onInfo("thread 2");
        }).detach();
    }).detach();
    return 0;
}

@brendandahl brendandahl linked a pull request May 14, 2025 that will close this issue
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[embind] Initialize embind on workers after main thread ctors. brendandahl/emscripten
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@sbc100 @brendandahl @ufolyah