String gets a weird form after calling a ndk c++ function from Android Java

Question

This is the function I call from Android:

String entityID = "414";
app.JoinConference(entityID);

And I have this code in my c++ jni files:

JNIEXPORT void Java_com_vidyo_vidyosample_VidyoSampleApplication_JoinConference(jstring eid)
{
FUNCTION_ENTRY;
LOGI("GuiOnOutEvent errorNOT JoinConference() enter\n");
VidyoClientPortalServiceJoinConferenceRequest createSchedRoomReq = {0};
createSchedRoomReq.typeRequest = VIDYO_CLIENT_PRIVATE_SOAP_JOIN_CONFERENCE;
LOGI("GuiOnOutEvent errorNOTJoinConference enter3: %s \n", eid);
//  createSchedRoomReq.entityID = eid;
strlcpy(&createSchedRoomReq.entityID, eid, sizeof(eid));
LOGI("errorNOTJoinConference request is:  %d pin %s + link: %d ", createSchedRoomReq.typeRequest,
     createSchedRoomReq.entityID, VIDYO_CLIENT_PRIVATE_IN_EVENT_VCSOAP);
LOGI("errorNOTJoinConference sizeof : %d", sizeof(VidyoClientPrivateSoapInEventCreateScheduledRoom));
VidyoClientSendEvent( VIDYO_CLIENT_PRIVATE_IN_EVENT_VCSOAP, &createSchedRoomReq, sizeof(VidyoClientPortalServiceJoinConferenceRequest));
LOGI("GuiOnOutEvent errorNOTJoinConference EXIT\n");
FUNCTION_EXIT;
}

What Am I doing wrong? The line with "enter3" which logs out the eid, logs out the following instead of "414":

01-24 18:04:29.310: I/VidyoMobile app/src/main/jni/ndkVidyoSample.c(12976): GuiOnOutEvent errorNOTJoinConference enter3: |9Ǵ 

What should I use instead of jstring in the methods parameters for the ndk function? OR do I need to change it from both the android and ndk to some kind of char array?

Answer 1

You cant directly pass jstring eid as argument to printf like function, you need to first get const char* from it, like:

const char *s = (*env)->GetStringUTFChars(env, eid, 0);

use s as argument to LOGI, and then when s is no longer needed:

(*env)->ReleaseStringUTFChars(env, eid, s);

Also why your jni function have no JNIEnv *env parameter? Static native methods signature should be:

JNIEXPORT jstring JNICALL
....... (JNIEnv *env, jclass type, jstring str);

and for non static:

JNIEXPORT jstring JNICALL
....... (JNIEnv *env, jobject instance, jstring str);

with android studio support for jni code its now super easy, you write in java code your native function for example:

public native void test(String s);

then IDE marks it as red, you click alt+enter on it and choose to generate native function which looks as follows:

JNIEXPORT void JNICALL
Java_com_example_hellojni_HelloJni_test(JNIEnv *env, jobject instance, jstring s_) {
  const char *s = (*env)->GetStringUTFChars(env, s_, 0);

  // TODO

  (*env)->ReleaseStringUTFChars(env, s_, s);
}