Calling native code through the Android NDK crashes with Fatal signal 11 (SIGSEGV) when called from non UI thread

Issue

I am calling into my native C library using the NDK from my Activity. When calling from the main UI thread everything works fine (but the UI is blocked). When calling from a new thread (either created with new Thread() or using AsyncTask) it crashes with a "A/libc(32044): Fatal signal 11 (SIGSEGV) at 0x77f1a000 (code=2), thread 32624 (Thread-56564)" error in logcat.

I make two calls in the library. The first call executes ok. The second call crashes on the GetByteArrayRegion statement:

JNIEXPORT void JNICALL Java_com_mycompany_myproduct_library_process_data(JNIEnv *env, jobject obj, jbyteArray jBuffer) {
    int len = (*env)->GetArrayLength(env, jBuffer);
    unsigned char buffer[len];
    (*env)->GetByteArrayRegion(env, jBuffer, 0, len, buffer);   // crash
    process_buffer(buffer);
}

The reason I am trying to run in a separate thread is to not block the UI during execution and to show a progress bar.

Any help is greatly appreciated.

Solution

As @BitBank pointed out correctly, the declaration of buffer[len] does not allocate the memory and unallocated memory is used for the buffer (probably on the stack). This only became visible to me when using a different thread. Changing it to

unsigned char *buffer;
buffer = malloc(len);
(*env)->GetByteArrayRegion(env, jBuffer, 0, len, buffer);
process_buffer(buffer);
free(buffer);

solved the issue.

Answered By - mvandillen