[Bug translator/14431] New: NULL/invalid char * pretty printed as "<unknown>" string

["mjw at redhat dot com" <sourceware-bugzilla at sourceware dot org>]

http://sourceware.org/bugzilla/show_bug.cgi?id=14431

             Bug #: 14431
           Summary: NULL/invalid char * pretty printed as "<unknown>"
                    string
           Product: systemtap
           Version: unspecified
            Status: NEW
          Severity: normal
          Priority: P2
         Component: translator
        AssignedTo: systemtap@sourceware.org
        ReportedBy: mjw@redhat.com
    Classification: Unclassified


If you have a char or unsigned char pointer and pretty print it, but it doesn't
actually point to a valid string, then the result is not very useful. char *
and unsigned char * are often used as arbitrary pointers/markers in code, not
just for strings.

Contrived example:

#include <malloc.h>
#include <stddef.h>

struct foobar
{
  char *foo;
  size_t foo_size;
  unsigned char *bar;
  size_t bar_size;
};

static struct foobar *fb;

unsigned char
use ()
{
  if (fb->foo) *fb->foo = 'a';
  return fb->bar_size > 0 ? *fb->bar : 0;
}

int
main (int argc, char **argv)
{
  struct foobar stack;
  fb = &stack;
  fb->foo_size = 42;
  fb->bar_size = 47;
  fb->foo = (char *) calloc (fb->foo_size, 1);
  fb->bar = (unsigned char *) calloc (fb->bar_size, 1);
  fb->bar_size = use (&fb);
  fb->foo = NULL;
  fb->bar = fb->foo + fb->foo_size;
  fb->bar_size = 0;
  return use (&fb);
}

$ gcc -g -o foobar foobar.c 
$ stap -e 'probe process.function("use") { log($fb$) }' -c ./foobar
{.foo="", .foo_size=42, .bar="", .bar_size=47}
{.foo="<unknown>", .foo_size=42, .bar="<unknown>", .bar_size=0}

It would have been convenient if those last two foo/bar pointers were
represented by something different than just "<unknown>".

Maybe "<unknown@0x0>" and "<unknown@0x42>"?

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