Used a library-reference method to calibrate lightpen drivers.

The mouse reference is a pointer.  If it's NULL, the driver uses a default.  If it's non-NULL, then it points to a function that the driver can call.  That function will adjust the driver's calibration value.  It could ask the user to adjust the pen; or, it could read a file that holds a value from a previous calibration.

Application writers can choose how it's done: a function that's provided by the library, a custom function, or nothing.

libsrc/cbm/penadjust.c

@@ -0,0 +1,53 @@
+/*
+** Main lightpen driver calibration functions.
+**
+** 2013-06-23, Greg King
+*/
+
+
+#include <fcntl.h>
+#include <unistd.h>
+#include <mouse.h>
+
+
+static const char *name;
+
+
+/* Get a lightpen calibration value from a file if it exists.  Otherwise, call
+** pen_calibrate() to create a value; then, write it into a file, so that it
+** will be available at the next time that the lightpen is used.
+** Might change the screen.
+*/
+static void __fastcall__ adjuster (unsigned char *XOffset)
+{
+    int fd = open (name, O_RDONLY);
+
+    if (fd < 0) {
+        pen_calibrate (XOffset);
+        fd = open (name, O_WRONLY | O_CREAT | O_EXCL);
+        if (fd >= 0) {
+            (void) write (fd, XOffset, 1);
+            close (fd);
+        }
+    } else {
+        (void) read (fd, XOffset, 1);
+        close (fd);
+    }
+}
+
+
+/* pen_adjust() is optional; if you want to use its feature,
+** then it must be called before a driver is installed.
+** Note:  This function merely saves the file-name pointer, and sets
+** the mouse_adjuster pointer.  The file will be read only when a driver
+** is installed, and only if that driver wants to be calibrated.
+*/
+void __fastcall__ pen_adjust (const char *filename)
+{
+    if (filename != NULL && filename[0] != '\0') {
+        name = filename;
+        mouse_adjuster = adjuster;
+    } else {
+        mouse_adjuster = pen_calibrate;
+    }
+}