beginner emscripten issue:emscripten_fetch on a background thread is failing

[Scott Watson]

I am having trouble porting our application to asm.js.  We have a class that handles HTTP GETs for us, that I am trying to port using the emscripten_fetch api.

I can only get the asynchronous fetch to work, and only when called on the main thread.  I can't get synchronous to work at all.  Using the sample code from: https://kripken.github.io/emscripten-site/docs/api_reference/fetch.html, I built this.

    // main.cpp

    #include <thread>

    #include <iostream>

    

    #include <emscripten/fetch.h>

    

    void downloadSucceeded(emscripten_fetch_t *fetch) {

      printf("Finished asynchronously downloading %llu bytes from URL %s.\n", fetch->numBytes, fetch->url);

      // The data is now available at fetch->data[0] through fetch->data[fetch->numBytes-1];

      emscripten_fetch_close(fetch); // Free data associated with the fetch.

    }

    

    void downloadFailed(emscripten_fetch_t *fetch) {

      printf("Downloading %s asynchronously failed, HTTP failure status code: %d.\n", fetch->url, fetch->status);

      //emscripten_fetch_close(fetch); // Also free data on failure.

    }

    

    void worker()

    {

      emscripten_fetch_attr_t attr;

    

      emscripten_fetch_attr_init(&attr);

    

      strcpy(attr.requestMethod, "GET");

    

      attr.attributes = EMSCRIPTEN_FETCH_LOAD_TO_MEMORY;

      attr.onsuccess = downloadSucceeded;

      attr.onerror = downloadFailed;

    

      emscripten_fetch(&attr, "http://localhost/VOD/nangu/00000041.ts"); // Blocks here until the operation is complete.

    }

    

    int main()

    {

        // worker();

        std::thread test1(worker);

    }

I am building it using:

    emcc main.cpp -o $(OUTPUT_FOLDER)/thread_test.html -s USE_PTHREADS=1 -s PTHREAD_POOL_SIZE=5 -s FETCH=1

My downloadFailed method is being called with a 'fetch->status' of 0.  If I call the 'worker()' method directly I get the expected behavior.

I am tearing my hair out here.  I have googled everything I can think of to no avail.

[Jukka Jylänki]

First thing is to remember to also compile with -s NO_EXIT_RUNTIME=1,
since once main() exits, it will tear down the application runtime,
along with all threads, so the linker flag will keep it running after
exiting main. I wonder if that already will solve the issue?

Another note is that C++11 threading is known to have some bugs, the
pthread_create() API is robust and unit tested. (If someone knows of a
good open source unit test suite for C++11 threading, I'm all ears)

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[Scott Watson]

First, thank you for the assistance!

-s NO_EXIT_RUNTIME=1 had no effect.  Actually, I think you get that automatically with -s USE_PTHREADS=1 don't you?

I switched to using pthreads directly with no effect either.

> email to emscripten-discuss+unsub...@googlegroups.com.

[Jukka Jylänki]

Hmm, can you try specifying the following attributes for the download?

attr.attributes = EMSCRIPTEN_FETCH_LOAD_TO_MEMORY |
EMSCRIPTEN_FETCH_SYNCHRONOUS | EMSCRIPTEN_FETCH_REPLACE;

Also there is a linker flag -s FETCH_DEBUG=1 that can show more information.

>> > email to emscripten-disc...@googlegroups.com.

>> > For more options, visit https://groups.google.com/d/optout.
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[Scott Watson]

Ok, that’s interesting. The synchronous download worked on the background thread. This works now with -s NO_EXIT_RUNTIME=1. It had no effect on the asynchronous download on a background thread. I still get a call to downloadFailed with a status of 0.

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