Question

I am trying to pass a variable by reference into a void pointer in order to update the original value. When I try it, the old value is never updated. Any help will be appreciated.
gst_filter_chain is like the main function (Gstreamer)

void update_value(void *oldValue, void *newValue)
{
    oldValue = newValue;
}

void update_struct(myStruct *oldStruct, myStruct newStruct)
{
    update_value((void *)&oldStruct->a, (void *)&newStruct.a)
}

static GstFlowReturn
gst_filter_chain (GstPad * pad, GstBuffer * buf)
{
    GstFilter *filter= GST_FILTER (gst_pad_get_parent (pad));
    myStruct temp_data;

    int buf_size = GST_BUFFER_SIZE(buf);     
    if(buf_size > 1) //if buffer is not empty
    {
        if(!filter->is_init)
        {
            memcpy(&filter->data, GST_BUFFER_DATA(buf), sizeof(myStruct));
            filter->is_init = TRUE;
        }
        else
        {
            memcpy(&temp_data, GST_BUFFER_DATA(buf), sizeof(myStruct));
            update_struct(&filter->data, temp_data);
        }
    }   


    gst_buffer_unref(buf);
    return GST_FLOW_OK;
}

Answer 1

void update_value(void *oldValue, void *newValue)
{
    oldValue = newValue;
}

In this snippet, you are only updating the pointer variable, not the value it points to.
You have to de-reference the pointer to access the actual data. If you want a really generic setter solution, you could use memcpy, but you’d need to pass to update_value a size parameter, to tell memcpy what’s the size of the datatype pointed by oldValue and newValue.

void update_value(void *oldValue, void *newValue, size_t dataSize)
{
    memcpy(oldValue, newValue, dataSize);
}