Should i free memory for android bitmap?

Issue

I'm passing window buffer from native side in android app to java side.

AndroidBitmapInfo info;

void saveBufferToBitmap(JNIEnv *env, ANativeWindow_Buffer *buffer, jobject bitmap) {
    void *pixels;

    LOGI(10, "saving buffer to bitmap");

    if (AndroidBitmap_getInfo(env, bitmap, &info) < 0) {
        LOGE(10, "Failed to get bitmap info");
            return;
    }

    if (AndroidBitmap_lockPixels(env, bitmap, &pixels) < 0) {
        LOGE(10, "Failed to lock pixles for bitmap");
        return;
    }

    int i, scan_length;
    scan_length = buffer->width * 4;

    memcpy(pixels, buffer->bits, buffer->width * buffer->height * 4); // 4 = (rgba)

    AndroidBitmap_unlockPixels(env, bitmap);

    //free(pixels); // here
}

Should i free pixels buffer in // here ? Does AndroidBitmap_lockPixels/AndroidBitmap_unlockPixels copy buffer to bitmap?

Solution

As a general rule you generally should never free a pointer that you did not create with new yourself. The library call you get the pointer from may use whatever allocation or just pass out a pointer to an internal data structure. The second is very likely in this case.

Looking at the documentation from the source:

/**
 * Given a java bitmap object, attempt to lock the pixel address.
 * Locking will ensure that the memory for the pixels will not move
 * until the unlockPixels call, and ensure that, if the pixels had been
 * previously purged, they will have been restored.
 *
 * If this call succeeds, it must be balanced by a call to
 * AndroidBitmap_unlockPixels, after which time the address of the pixels should
 * no longer be used.
 *
 * If this succeeds, *addrPtr will be set to the pixel address. If the call
 * fails, addrPtr will be ignored.
 */
 int AndroidBitmap_lockPixels(JNIEnv* env, jobject jbitmap, void** addrPtr);

Source: https://android.googlesource.com/platform/frameworks/native/+/master/include/android/bitmap.h

That tells us not do anything with pixels after AndroidBitmap_unlockPixels and definitely not free it.

Answered By - HHK