Adds notes on program termination and signal handlers.

I revised the text after dd's kind review. So, if you find
any error, it is probably introduced by my last minutes'
update and is entirely my fault, not dd's.

Reviewed by: dd

lib/libvgl/vgl.3

@@ -408,6 +408,54 @@ blank the display if the argment
 .Va blank
 != 0. This can be done to shut off the screen during display updates that
 the user should first see when it's done.
+.Ss Program termination and signal processing
+It is important to call
+.Fn VGLEnd
+before terminating the program.
+Care must be taken if you install signal handlers and try to call
+.Fn VGLEnd
+and
+.Xr exit 3
+to end the program.
+If a signal is caught while the program is inside
+.Nm libvgl
+functions,
+.Fn VGLEnd
+may not be able to properly restore the graphics hardware.
+.Pp
+The recommended way to handle signals and program termination is to
+have a flag to indicate singnal's delivery.
+Your singnal handlers set this flag but do not terminate
+the program immediately.
+The main part of the program checks the flag to see if it is
+supporsed to terminate, and call
+.Fn VGLEnd
+and
+.Xr exit 3
+if the flag is set.
+.Pp
+Note that
+.Fn VGLInit
+installs its internal signal handlers for
+.Dv SIGINT , SIGTERM , SIGSEGV ,
+and
+.Dv SIGBUS ,
+and terminates the program at appropriate time,
+after one of these signals is caught.
+If you want to have your own signal handlers for these signals,
+install handers
+.Em after
+.Fn VGLInit .
+.Pp
+.Dv SIGUSR1
+and
+.Dv SIGUSR2
+are internally used by
+.Nm libvgl
+to control screen switching and the mouse pointer,
+and are not available to
+.Nm libvgl
+client programs.
 .Sh AUTHORS
 .An S\(/oren Schmidt Aq sos@FreeBSD.org
 .Sh HISTORY