What it takes to make a parameter const for a function. You must edit (at a minimum) both the function prototype and function definition. Here, I am making the verbose argument const.

mbsystem-const-demo.diff

svn diff mb_define.h mb_defaults.c 
Index: mb_define.h
===================================================================
--- mb_define.h	(revision 2165)
+++ mb_define.h	(working copy)
@@ -315,7 +315,7 @@
 #define	MB_PROJECTION_PROJECTED		1
 
 /* MBIO core function prototypes */
-int mb_defaults(int verbose, int *format, int *pings,
+int mb_defaults(const int verbose, int *format, int *pings,
 		int *lonflip, double bounds[4],
 		int *btime_i, int *etime_i,
 		double *speedmin, double *timegap);

Index: mb_defaults.c
===================================================================
--- mb_defaults.c	(revision 2165)
+++ mb_defaults.c	(working copy)
@@ -120,7 +120,7 @@
 static char rcs_id[]="$Id$";
 
 /*--------------------------------------------------------------------*/
-int mb_defaults(int verbose, int *format, int *pings,
+int mb_defaults(const int verbose, int *format, int *pings,
 		int *lonflip, double bounds[4],
 		int *btime_i, int *etime_i,
 		double *speedmin, double *timegap)