I have a program that reads a hexadecimal string representation from a configuration file, turns it into [a GdkRGBA struct from Gdk](https://docs.gtk.org/gdk4/method.RGBA.parse.html), uses `gtk_color_dialog_button_set_rgba ()` to set it as the default colour of a GtkColorDialogButton, and then listens to user changes to the aforementioned button.

On user change, I want to write back the chosen value to the configuration file so that it is persistent across program launches, and I need it to be a hexadecimal string representation for legacy compatibility reasons (the configuration file is `~/.Xresources`, so it needs to be compatible with many existing programs). However, while Gdk provides a [`gdk_rgba_parse ()`](https://docs.gtk.org/gdk4/method.RGBA.parse.html) function to parse a hexadecimal string representation of a colour into a GdkRGBA struct, I can't find anything to do the inverse, i.e., turn a GdkRGBA struct into a hexadecimal string representation. There is [`gdk_rgba_to_string ()`](https://docs.gtk.org/gdk4/method.RGBA.to_string.html), but it only returns strings of the form `rgb(r,g,b)` or `rgba(r,g,b,a)`, whereas I want `#rrggbb`.

How can I get the hexadecimal string representation of a GdkRGBA struct's value in C?