Use vga_modeinfo(3) to check for availability of the function in a given mode.
Furthermore, some cards (Cirrus) just enable this buffer at a fixed hardware address. For Cirrus it is mapped at 14MB so you should never used it if you have more than 14MB of memory (But how does an application know?). The Mach32 support for this is smarter. It makes this feature only available when it is safe to be used.
To avoid all this problems you can use
in the /etc/vga/libvga.config file to disable the linear frame buffer if it cannot be used.
Returns the size of the mapped framebuffer if successful (can be less than total video memory), -1 if not.
The testlinear(6) demo shows the use of this feature (and if it works for you).
svgalib(7), vgagl(7), libvga.config(5), testlinear(6), vga_modeinfo(3), vga_getgraphmem(3), vga_setpage(3), vga_setreadpage(3), vga_setwritepage(3), vga_setlinearaddressing(3)
This manual page was edited by Michael Weller <firstname.lastname@example.org>. The exact source of the referenced function as well as of the original documentation is unknown.
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.