One of the definitions of "against", taken from Merriam Webster: > 4 b: in contact with > "leaning against the wall" OP's code is "in contact with" the publi

One of the definitions of "against", taken from Merriam Webster: > 4 b: in contact with > "leaning against the wall" OP's code is "in contact with" the public APIs provided by GTK. That is, OP uses the GTK library.