This is the GPC library. GPC means "General Polygon Clipper". The author is Alan Murta. This is version 2.32, of 2004. GPC was distributed on the website of the university of Manchester but it is no longer available there, since August 2020. There is a version 2.33 of GPC, of 2014, but version 2.33 calls fscanf_s, which is in the extension, annex K, of C11, implemented by Microsoft only. So we stay with version 2.32.