allow to print a null-terminated string when binary tracing

[Gui,Jian]

Currently, printf("%s",str) will not append a '\0' at the end
of str for the requirement of ascii tracing. It will make
binary trace data difficult to decode, for we cannot locate
the end of current string.

e.g.
 stap -o probe.out -e 'probe kernel.function("sys_open"){ name="open";
   printf("%s%s%4b",name,user_string($filename),$flags);exit()}'

 will generate:
0000000: 6f70 656e 2f65 7463 2f6c 642e 736f 2e63  open/etc/ld.so.c
0000010: 6163 6865 0000 0000 0a                   ache.....

How about allowing to append a '\0' at the end of each string?
As Martin has suggested in
 http://sources.redhat.com/ml/systemtap/2006-q1/msg00284.html,
we can  use "%0s" to print a null-teminated string.

e.g.
 stap -o probe.out -e 'probe kernel.function("sys_open"){ name="open";
   printf("%0s%0s%4b",name,user_string($filename),$flags);exit()}'

 will generate:
0000000: 6f70 656e 002f 6574 632f 6c64 2e73 6f2e  open./etc/ld.so.
0000010: 6361 6368 6500 0000 0000 0a              cache......

Here is my patch to do this.  Thanks for your comments.

diff -Nur src-20060506/runtime/vsprintf.c src.new/runtime/vsprintf.c
--- src-20060506/runtime/vsprintf.c     2006-04-28 
16:36:28.000000000 -0400
+++ src.new/runtime/vsprintf.c  2006-05-08 07:38:58.000000000 -0400
@@ -275,6 +275,12 @@
                                        *str = ' ';
                                ++str;
                        }
+                       if(flags & STP_ZEROPAD) {
+                               if (str <= end)
+                                       *str = '\0';
+                               ++str;
+                       }
+
                        continue;

                case 'X':