Question

Id like to make a simple function, that return value of two Strings, basically:

java

public native String getAppendedString(String name);

c

jstring Java_com_example_hellojni_HelloJni_getAppendedString(JNIEnv* env,   jobject thiz, jstring s) {
    jstring sx = (*env)->GetStringUTFChars(env, s, NULL);

    return ((*env)->NewStringUTF(env, "asd ")+sx);
}

It returns:

jni/hello-jni.c:32: warning: initialization discards qualifiers from pointer target type
jni/hello-jni.c:34: error: invalid operands to binary + (have 'char *' and 'char *')

The retval will be: "asd qwer", how can I do this?

jstring s1  = (*env)->NewStringUTF(env, "456");
jstring s2  = (*env)->NewStringUTF(env, "123");
jstring sall=strcat(s1, s2);

return sall;

Only returns "456"

Answer 1

There are a few issues here:

1. GetStringUTFChars returns a jbyte * (a null-terminated C string), not a jstring. You need this C string to do string manipulation in C.

2. You need to call ReleaseStringUTFChars when you’re done with it.

3. You need to allocate enough memory to hold the concatenated string, using malloc.

4. As ethan mentioned, you need to concatenate your two C strings with strcat. (You cannot do this with the + operator. When applied to a pointer, + returns the pointer from the offset of the original pointer.)

5. Remember to free the memory you allocated after you’re done with it (ie, after it’s been interned as a Java string.)

You should do something along the lines of:

char *concatenated;
const jbyte *sx;
jstring retval;

/* Get the UTF-8 characters that represent our java string */
sx = (*env)->GetStringUTFChars(env, s, NULL);

/* Concatenate the two strings. */
concatenated = malloc(strlen("asd ") + strlen(sx) + 1);
strcpy(concatenated, "asd ");
strcat(concatenated, sx);

/* Create java string from our concatenated C string */
retval = (*env)->NewStringUTF(env, concatenated);

/* Free the memory in sx */
(*env)->ReleaseStringUTFChars(env, s, sx);

/* Free the memory in concatenated */
free(concatenated);

return retval;