13

I initially used a function to return a char* to java as UTF-8 string, but since I kept getting errors, I wrote the following function to return a char* as a Java byte[], so that I could try to convert the array into a String in java side: 

jbyteArray Java_com_vektor_amapper_util_InputDeviceManager_getDevNameBytes(JNIEnv* env, jobject thiz, jint index) {
    if(pDevs[index].device_name == NULL) return NULL;
    int n=0;
    while(pDevs[index].device_name){
        n++;
    } if (n==0) return NULL;
    jbyteArray arr = (*env)->NewByteArray(env, n);
    (*env)->SetByteArrayRegion(env,arr,0,n, (jbyte*)pDevs[index].device_name);
    return arr;
}

But when I call it my application crashes. Am I missing something?

Update: The condition was missing a ++ and this caused an infinite loop. But now with the following code:

jbyteArray Java_com_vektor_amapper_util_InputDeviceManager_getDevNameBytes(JNIEnv* env, jobject thiz, jint index) {
    int n=0;
    if(pDevs[index].device_name == NULL) return NULL;
    while(pDevs[index].device_name++){
        n++;
    } if(n==0) return NULL;
        jbyteArray arr = (*env)->NewByteArray(env, n);
        (*env)->SetByteArrayRegion(env,arr,0,n, (jbyte*)pDevs[index].device_name);
        return arr;
}

I get this weird JNI warning:

06-15 22:40:02.303: W/dalvikvm(7616): JNI WARNING: negative jsize (NewByteArray)

How can it be since I am only increasing the value of n?

Update 2: the following code works:

jbyteArray Java_com_vektor_amapper_util_InputDeviceManager_getDevNameBytes(
        JNIEnv* env, jobject thiz, jint index) {

    if(pDevs[index].device_name == NULL) return NULL;
    int n=0;
    char* p = pDevs[index].device_name;
    while(*p++){
        n++;
    } if(n<=0) return NULL;
    jbyteArray arr = (*env)->NewByteArray(env, n);
    (*env)->SetByteArrayRegion(env,arr,0,n, (jbyte*)pDevs[index].device_name);

    return arr;
}
Vektor88
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    If reached this `int n=0; while(pDevs[index].device_name){ n++; }`looks like an endless loop. – alk Jun 15 '13 at 14:34
  • @alk You were right, i forgot the ++ in the while condition, but the code doesn't work yet. I put the updated code. – Vektor88 Jun 15 '13 at 14:41
  • FYI, the hello-jni sample in the Android NDK distribution shows you how to return a UTF string. – Chris Stratton Jun 15 '13 at 19:50
  • @ChrisStratton Yes, I was able to create the string, but sometimes it gave me errors, so I preferred converting a byte array to string in Java side. Still no errors with this new method. – Vektor88 Jun 16 '13 at 12:21
  • Couldn't you simply do `int n = strlen(pDevs[index].device_name)` ? – Larphoid Sep 08 '14 at 10:03

1 Answers1

9

Shouldn't be this ? 

char* p = pDevs[index].device_name;
while( *p++) {
...
}
xqterry
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