void gl_line(int x1, int y1, int x2, int y2, int c);
You should not assume that the same drawing trajectory is used when you exchange start and end points.
To use this program one first sets up a mode with a regular vga_setmode call and vga_setpage(0), with possibly a vga_setlinearaddressing call. Then a call to gl_setcontextvga(mode) is made. This makes the information about the mode available to gl_line.
The pixels are placed directly into video memory using inline coded commands.
This manual page was edited by Michael Weller <email@example.com>. The exact source of the referenced demo as well as of the original documentation is unknown. This page was modified by Don Secrest <firstname.lastname@example.org>.
It is very likely that both are at least to some extent are due to Harm Hanemaayer <H.Hanemaayer@inter.nl.net>.
Occasionally this might be wrong. I hereby asked to be excused by the original author and will happily accept any additions or corrections to this first version of the svgalib manual.