C++ signal handler can't notify Java side

Question

I have the following CPP code. What I want to do is when an error occurs in my native side, I will notify Java about the error. I used How can I catch SIGSEGV (segmentation fault) and get a stack trace under JNI on Android? as reference.

static JavaVM* g_JVM = NULL;
static jobject  g_thejavaobject  = NULL;

void InitializeNativeSide(JNIEnv *env, jclass, jobject object)
{
    env->GetJavaVM(&g_JVM);
    g_thejavaobject = env->NewGlobalRef(object);
}

// this executes in another thread running in parallel with UI thread
void StartExecuting(JNIEnv *_env, jclass) {
    struct sigaction sa;
    memset(&sa, 0, sizeof(struct sigaction));
    sigemptyset(&sa.sa_mask);
    sa.sa_sigaction = SignalErrorHandler;
    sa.sa_flags   = SA_SIGINFO;
    sigaction(SIGSEGV, &sa, NULL);

    // native starts executing here. after a while, a SEGFAULT is encountered
    // triggering SignalErrorHandler()
    ...
}

void SignalErrorHandler(int signal, siginfo_t *si, void *arg)
{
    JNIEnv *env;
    g_JVM->GetEnv((void**)&env, JNI_VERSION_1_6);

    jclass myClass = env->FindClass("com/company/MyClass");
    jmethodID myMethod = env->GetMethodID(myClass, "nativeCrashed", "()V" );
    env->CallVoidMethod(g_thejavaobject, myMethod);

    env->DeleteLocalRef(myClass);
}

Everything works fine but the call to myClass.nativeCrashed() does not work. What am I doing wrong?

Do you really think it's safe to make Java calls in a signal handler? That is not going to work. The **only** function calls that can be safely made from a signal handler are async-signal-safe one listed at http://pubs.opengroup.org/onlinepubs/9699919799/functions/V2_chap02.html#tag_15_04 — Andrew Henle, Aug 14 '18 at 13:30

score 2 · Accepted Answer · answered Aug 14 '18 at 13:40

2

You can't do this:

void SignalErrorHandler(int signal, siginfo_t *si, void *arg)
{
    JNIEnv *env;
    g_JVM->GetEnv((void**)&env, JNI_VERSION_1_6);

    jclass myClass = env->FindClass("com/company/MyClass");
    jmethodID myMethod = env->GetMethodID(myClass, "nativeCrashed", "()V" );
    env->CallVoidMethod(g_thejavaobject, myMethod);

    env->DeleteLocalRef(myClass);
}

That will not work for at least two fundamental reasons.

First, only async-signal-safe functions may be called from within a signal handler. The POSIX-specified list can be found at http://pubs.opengroup.org/onlinepubs/9699919799/functions/V2_chap02.html#tag_15_04_03.

No Java JNI call is async-signal-safe.

Second, the Java JVM uses SIGSEGV internally - getting a SIGSEGV is not necessarily fatal:

Signals Used in Oracle Solaris, Linux, and macOS

...

SIGSEGV, SIGBUS, SIGFPE, SIGPIPE, SIGILL These signals are used in the implementation for implicit null check, and so forth.

answered Aug 14 '18 at 13:40

Andrew Henle

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Thanks for the link. I appreciate it. By the way, the link in my post claims that he was able to call java from his signal handler. Now im confused. – user1506104 Aug 14 '18 at 14:51
@user1506104 Read the POSIX documentation I linked: "when a signal interrupts an unsafe function or equivalent ... and the signal-catching function calls an unsafe function, **the behavior is undefined**." The problems are only specified to happen at certain times, and then the results are "undefined behavior". In short, it can *appear* to work - until it fails. It's similar to overrunning an array - it might "work", but it's still wrong. And it can fail. – Andrew Henle Aug 14 '18 at 14:58
1

You are awesome! Thanks! – user1506104 Aug 14 '18 at 15:01
@user1506104 Thanks for that. I had to learn this the hard way, too. This question is also relevant, as the questioner was doing something similar: https://stackoverflow.com/questions/34547199/art-prevents-any-java-calls-from-jni-during-native-signal-handling/34553070#34553070 – Andrew Henle Aug 14 '18 at 15:02

C++ signal handler can't notify Java side

1 Answers1