How is the callback function called in this OpenCV example? - c++

Question

I started reading Learning OpenCV 3 but since I haven't programmed much in c++ I am confused by the way of using a callback function in the following code snippet:
int g_slider_position = 0;
int g_run = 1, g_dontset = 0;
cv::VideoCapture g_cap;
void onTrackBarSlide(int pos, void *)
{
g_cap.set(CV_CAP_PROP_POS_FRAMES, pos);
if (!g_dontset) g_run = 1;
g_dontset = 0;
}
int main(int argc, char** argv)
{
g_cap.open(std::string(argv[1]));
int frames = (int)g_cap.get(CV_CAP_PROP_FRAME_COUNT);
cv::createTrackbar(
"Position",
"Example 2-4",
&g_slider_position,
frames,
onTrackBarSlide
);
cv::Mat frame;
for (;;)
{
if (g_run != 0)
{
g_cap >> frame;
if (frame.empty())
{
break;
}
int current_pos = (int)g_cap.get(CV_CAP_PROP_POS_FRAMES);
g_dontset = 1;
cv::setTrackbarPos("Position", "Example 2-4", current_pos);
cv::imshow("Example 2-4", frame);
g_run -= 1;
}
char c = (char)cv::waitKey(10);
if (c == 's')
{
g_run = 1;
}
if (c == 'r')
{
g_run = -1;
}
}
return 0;
}
What I gather from this is that pressing somewhere on the Trackbar raises an event that calls onTrackBarSlide.
What I'm wondering there is where do the int pos argument come from? I couldn't find it on the function documentation.
When I step through this program onTrackBarSlide is called after setTrackbarPos. This confuses me and makes me think that setTrackbarPos is just raising an event with current_pos as the argument that onTrackBarSlide listens for.
But why couldn't it be done more directly by having setTrackbarPos call g_cap.set directly?