Path: utzoo!mnetor!uunet!husc6!uwvax!oddjob!gargoyle!ihnp4!inuxc!iuvax!pur-ee!uiucdcs!bradley!bucc2!brian
From: brian@bucc2.UUCP
Newsgroups: comp.lang.c
Subject: nasty evil pointers
Message-ID: <13100003@bucc2>
Date: 7 Mar 88 11:10:00 GMT
Lines: 95
Nf-ID: #N:bucc2:13100003:000:2598
Nf-From: bucc2.UUCP!brian    Mar  7 05:10:00 1988


  We lose a lot of a time to pointers running off all over the place.
We are using Microsoft C 5.0 under MESS-DOS, and that means no control
of memory. We've tried using things like _nullcheck() and _heapwalk(), but
the don't help much. It would be nice if we could check every pointer as
it was used... something like

void pointer_validate(p, n, f)
  void *p;
  int n;
  char *f;

  That would be called with a pointer, __LINE__, and __FILE__. If pointer
pointed, say, into the operating system or the text space, the function
would print a message and exit(). Otherwise it would return. Writing
such a function is not too difficult. However, getting it called is. It
would be nice if we had a utility program like ctrace we could run our
code through that would put in calls to this function. Unfortunatley, this
would have to have considerable knowledge of the C language to work...
In simple cases like

void foo(ip)
  int *ip;
  {
  printf("%d", *ip);
  }

would become:

void foo(ip)
  int *ip;
  {
  pointer_validate((void *) ip, __LINE__, __FILE__)
  printf("%d", *ip);
  }

  however, something like:

char *bar(bp, ip, sp)
  int *bp, *ip;
  char *sp;
  {
  register int i;
  char *s;

  if (sp)
    s = sp;

  for (i = *bp; i < 50; i += *ip)
    s[i] = 'X';

  return(s);
  }


  is another story entirley. The program would have to put in braces
in the if statement, and do something to the for like:

char *bar(bp, ip, sp)
  int *bp, *ip;
  char *sp;
  {
  register int i;
  char *s;

  pointer_validate((void *) sp, __LINE__, __FILE__)
  if (sp)
    {
    pointer_validate((void *) sp, __LINE__, __FILE__)
    s = sp;
    }
  else if (!(s = malloc(50)))
    return(NULL);

  pointer_validate((void *) bp, __LINE__, __FILE__)
  for (i=*bp; i<50; pointer_validate((void *) ip, __LINE__, __FILE__), i += *ip)
    s[i] = 'X';

  return(s);
  }

  I know these aren't good examples, it 5:00 AM... anyway, you get the idea
of the difficulties involved here. Without source to the compiler it could
be a major project. Does anybody know a way this could be done, perhaps with
something like lex? We don't have lex on the PC, but if we could come up
with source from lex on the unix system and move it to the PC, wouldn't it
work? (I don't know anything about lex)

...............................................................................

  When the going gets weird, the weird turn pro.

  Brian Michael Wendt       UUCP: {cepu,ihnp4,uiucdcs,noao}!bradley!brian
  Bradley University        ARPA: cepu!bradley!brian@seas.ucla.edu
  (309) 691-5175            ICBM: 40 40' N  89 34' W