How to check if a pointer variable is junk during runtime?

Question

I use valgrind to validate my code and it reports "Conditional jump or move depends on uninitialised value(s)" in one of my functions, which takes an array of pointers as argument.

Now, how do I check if an array contains junk values (might be using conditional break point) during run-time? Say, I don't access the pointer and hence the program doesn't break.

What is the condition to be checked for to identify a junk pointer?

There is no way to tell if a pointer is "junk" or not, except when the pointer is `NULL`. — Sergey Kalinichenko, Sep 12 '12 at 10:06
@dasblinkenlight Addition: No legal way and often no simple practical way. — Alexey Frunze, Sep 12 '12 at 10:47

score 3 · Answer 1 · answered Sep 14 '12 at 09:29

While the other answers are correct, you can also get valgrind to help you identify which entry or entries in the array exactly are causing the problem.

What you need to do is to add code to your program which loops over the array (you may already have such a loop of course) and then include valgrind/memcheck.h and add something like this to the loop:

if (VALGRIND_CHECK_VALUE_IS_DEFINED(entry)) {
  printf("index %d undefined\n", index);
}

where entry is the actual value from the array and index is the index of that value in the arry.

score 2 · Answer 2 · answered Sep 12 '12 at 10:11

2

You can't differentiate a valid pointer and junk(uninitialized) pointer, they are all just numbers.

The fact that you are dealing with a "junk" pointer at some point in your code indicates, there's a problem before reaching that point.

answered Sep 12 '12 at 10:11

P.P

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score 1 · Answer 3 · answered Sep 12 '12 at 10:19

1

You need to systematically initialize all your pointers to NULL. When you deallocate memory reset your pointer to NULL as well. This can be done using "constructor/destructor" functions wrapping malloc/free for instance. Only then you can test for NULL valued pointer to see if something went wrong.

answered Sep 12 '12 at 10:19

Nicolas Barbey

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Not even then. For example `char *a = calloc(1,1); char *b = a; free(a); a = 0; if (b) printf("%d\n", *b);`. I set `a` to a null pointer after freeing, just like the rules say, and I checked `b` for null before using it, just like the rules say. But it did me no good, I still got undefined behavior. There is no substitute in C for designing, documenting, and respecting object lifetime. – Steve Jessop Sep 12 '12 at 10:25
Good point.... This is why I like SO, good place to learn some stuff and put your knwoledge on test :) – Nicolas Barbey Sep 12 '12 at 10:30
+1 for solution to [avoid dangling pointers](http://en.wikipedia.org/wiki/Dangling_pointer#Avoiding_dangling_pointer_errors). @Steve said correctly that this will not check against pointer copies, but still I use this concept because I think some protection is better than zero protection. – Agnius Vasiliauskas Sep 12 '12 at 10:34

Steve Jessop · Accepted Answer · 2012-09-12T10:28:47.307

You don't test for junk, you put non-junk values in the array at some point between the time you create the array, and the first time you consider using the values. Usually you do it when the array is created:

const char* strings[] = {0, "junk", "here"};
int some_values[10] = { 0 };

Valgrind uses its own tricks to identify what it thinks is junk, but those tricks are outside the scope of the standard, and regular C code can't use them (or anyway shouldn't try). Even if you could somehow hook into what valgrind does, you'd end up with code that doesn't work on all implementations, or that only works when run under valgrind.

How to check if a pointer variable is junk during runtime?

4 Answers4