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  1. #1
    ignacio is offline Member
    Join Date
    Oct 2009
    Rep Power

    Default JNI, pthreads and ucontext in Ubuntu

    Hi everyone,

    I'm facing some trouble with JNI and ucontext under Linux Ubuntu 7.04. My JVM version is:

    Java(TM) SE Runtime Environment (build 1.6.0_16-b01)
    Java HotSpot(TM) Client VM (build 14.2-b01, mixed mode, sharing)

    I have a C program that successfully starts the JVM (in main method) using the following code, getting in "JavaVM *jvm" variable the JVM instance:

    Java Code:
    static JavaVM *jvm; // Reference to JVM
    int main(void)
    [INDENT]JNIEnv *env;
    JavaVMOption *options;
    JavaVMInitArgs vm_args;
    JNIEnv *env;
    jint result;
    // launch pthread, not shown here.[/INDENT]
    After that, I launch a pthread in order to instantiate a precompiled java class named TestJNI using the already launched JVM. This class does nothing but printing "Hello!" inside it's default constructor. I instantiante pthread in main method this way:

    Java Code:
    pthread_t tid;
    int thr_id;
    // native_thread is main thread method
    In order to make JVM accessible inside a thread method, I must obtain the JNIEnv reference to JVM. This is done with AttachCurrentThread. After that, I instantiate successfully my TestJNI class. Code for "native_thread" method is:

    Java Code:
    void *native_thread(void *arg){
    JNIEnv *env; // cannot be cached, each thread must attach to JVM and acquire it's own JNIEnv
    // jvm has it's value set on first code snippet.
    // Get class reference (located on classpath ".")
    // Get reference to default constructor
    // Instantiate object (successfully)
    All code described above runs successfully.

    However, my programs needs using <ucontext.h> capability of creating contexts to switch between them. Ucontext is used to implement, for example, coroutines. So my intention is to execute the same method "native_thread" after having switched the same thread to another context.

    So, I have 2 ucontext_t variables.

    Java Code:
    ucontext_t new_coroutine;
    ucontext_t thread_coroutine;
    new_coroutine is the coroutine I switch to where I wan't to access JVM.
    Initialization on "thread_coroutine" is done this way:

    Java Code:
    void prepare_context()
    new_coroutine.uc_link = NULL;
    // assign stack where coroutine can execute. 1MB stack.
    new_coroutine.uc_stack.ss_sp = malloc(1024*1024);
    new_coroutine.uc_stack.ss_size = (1024*1024);
    new_coroutine.uc_stack.ss_flags = 0;
    makecontext(&new_coroutine,(void *)my_coroutine_method,1,NULL);[/INDENT]
    This way, my coroutine will execute "my_coroutine_method" which is a literal copy of already show "native_thread" method. This way, my "native_thread" method becomes:

    Java Code:
    void *native_thread(void *arg){
    // swap to new_coroutine to run "my_coroutine_method".
    swapcontext(&(thread_coroutine), &(new_coroutine));
    When coroutine reaches AttachCurrentThread, JVM generates a fatal error:

    Java Code:
    #  Internal Error (threadLocalStorage.cpp:40), pid=4797, tid=3035433872
    #  Error: guarantee(get_thread() == thread,"must be the same thread, quickly")
    threadLocalStorage.cpp::set_thread method is:

    Java Code:
    void ThreadLocalStorage::set_thread(Thread* thread) {
      // The following ensure that any optimization tricks we have tried
      // did not backfire on us:
      guarantee(get_thread()      == thread, "must be the same thread, quickly");
      guarantee(get_thread_slow() == thread, "must be the same thread, slowly");
    This error makes me suspect that JVM tries to check that thread changed and exits. However, thread didn't change, only the stack where it executes, by means of ucontent_t.

    I also tried executing AttachCurrentThread on main stack thread BEFORE switching to coroutine. In that case, AttachCurrentThread succeded inside coroutine. However, when i try to get reference to the class TestJNI (by means of FindClass method) i get a segmentation fault error:

    Java Code:
    siginfo:si_signo=SIGSEGV: si_errno=0, si_code=1 (SEGV_MAPERR), si_addr=0x00000094
    Did anyone face this problem before?

    Sorry for this mega-description of my situation, hope it isn't difficult to understand.

    Many thanks in advance


  2. #2
    r035198x is offline Senior Member
    Join Date
    Aug 2009
    Rep Power


    You are better off asking in a C forum because any solutions do not involve changing any Java code at all.

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