Question

I just started working with the Android NDK but I keep getting SIGSEGV when I have this call in my C code:

jobjectArray someStringArray;
someStringArray = (*env)->NewObjectArray(env, 10, 
(*env)->FindClass(env,"java/lang/String"),(*env)->NewStringUTF(env, ""));

Base on all the example I can find, the above code is correct but I keep getting SIGSERGV and everything is ok if the NewObjectArray line is commented out. Any idea what could cause such a problem?

Answer 1

that looks right, so i m guessing you ve done something else wrong. i assume you re running with checkjni on? you might want to break that up into multiple lines: do the FindClass and check the return value, do the NewStringUTF and check the return value, and then call NewObjectArray.

btw, you might want to pass NULL as the final argument; this pattern of using the empty string as the default value for each element of the array is commonly used (i think it s copy & pasted from some Sun documentation and has spread from there) but it s rarely useful, and it s slightly wasteful. (and it doesn t match the behavior of "new String[10]" in Java.)

Answer 2

I guess one of the possible causes is that in a long-run JNI method, the VM aborts when running out of the per-method-invocation local reference slots (normally 512 slots in Android).

Since FindClass() and NewStringUTF() functions would allocate local references, if you stay in a JNI method for a long time, the VM do not know whether a specific local reference should be recycled or not. So you should explicitly call DeleteLocalRef() to release the acquired local references when not required anymore. If you don t do this, the "zombie" local references will occupy slots in VM, and the VM aborts while running out of all the local reference slots.

In short-run JNI method, this may not be a problem due to all the local references would be recycled when exiting from a JNI method.

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