Warning: while trying to convert java byte[] to C unsigned char*

Question

I am writing a JNI. In that, my Java program takes an Image byte array using ByteOutputStream() and then this array is used to call a function in C that converts byte array to unsigned char*. Here is the code:

JNIEXPORT void JNICALL Java_ImageConversion_covertBytes(JNIEnv *env, jobject obj, jbyteArray array) 
 {
    unsigned char* flag = (*env)->GetByteArrayElements(env, array, NULL);
    jsize size = (*env)->GetArrayLength(env, array);

    for(int i = 0; i < size; i++) {
    printf("%c", flag[i]);}
 }

In this I keep getting a warning when I compile:

warning: initializing 'unsigned char *' with an expression of type 'jbyte *' (aka 'signed char *') converts between pointers to integer types with different sign [-Wpointer-sign]

unsigned char* flag = (*env)->GetByteArrayElements(env, array, NULL);

How can I remove this warning? I want to print the all characters.

Answer 1

The warning exists because the sign change might be important. In JNI the jbyte corresponds to Java byte which is a signed 8-bit integer; in C it is explicitly signed char.

However, it is OK to access any object with any character pointer, so you can cast to unsigned char explicitly:

unsigned char* flag = (unsigned char*)(*env)->GetByteArrayElements(env, array, NULL);

Alternatively, you can declare flag as signed char:

signed char* flag = (*env)->GetByteArrayElements(env, array, NULL);

This is fine for printf("%c\n", flag[i]); because %c requires that the argument be an integer; the integer is then converted to unsigned char so both signed and unsigned char will do.

However 3rd option would be to use neither - if you just want to write them to the terminal, use a void * pointer and fwrite:

JNIEXPORT void JNICALL 
Java_ImageConversion_covertBytes(JNIEnv *env, jobject obj, jbyteArray array) 
{
    void *flag = (*env)->GetByteArrayElements(env, array, NULL);
    jsize size = (*env)->GetArrayLength(env, array);
    fwrite(flag, 1, size, stdout);     
}

and let fwrite worry about the looping.