draw icon with transparency in xlib and cairo

Now that I've got the icons I want to draw them correctly. At the moment I'm using Cairo to draw these images on a window. I have a mask and the icon as pixmap.

cairo_surface_t *image;
cairo_surface_t *imask;
cairo_surface_t *surface;
cairo_t *csurface;

surface = cairo_xlib_surface_create(display, d, DefaultVisual(display, screen), 400, 400);
csurface = cairo_create(surface);

Pixmap icon;
Pixmap mask;

//XWM Stuff ...
if(icon != 0)
{
   get_pixmap_geometry(display, icon, &width, &height, &depth);
   image = cairo_xlib_surface_create(display, icon, DefaultVisual(display, screen), width, height);
   cairo_set_source_surface(csurface, image, 0, 0);
   //How do I apply the mask?
   //I tried cairo_set_operator(csurface, CAIRO_OPERATOR_SOURCE);
   cairo_paint(csurface);
}

But the icons do not have any transparencies. I've found no example to solve this with cairo on the internet. There is only a complicated way but it's so badly documented that it doesn't help me at all. Does someone have a link or an example for how to restore the original icon with its transparencies? Thank you in advance.

Solution

Here is an example from awesome where it "turns" the icon into a cairo surface: https://github.com/awesomeWM/awesome/blob/430f4fab15bb101b4af9fadbebb9a9bfa47ba9de/objects/client.c#L1501

This uses xcb instead of Xlib, but you should manage to still understand this. The part that handles a mask begins in line 1538. Basically, a new cairo surface is created and a cairo context is set up for it. The source surface is the icon and the mask is applied via cairo_mask_surface ("use the alpha channel of some cairo surface as the mask of a drawing operation"). You can just copy this part to your code so that you don't have to draw the icons to a temporary surface.

TL;DR: The answer to your //How do I apply the mask? is: Use cairo_mask_surface() instead of cairo_paint().