Is there anyway to test if a pointer points to allocated memory or
not?
For example if I have a pointer such as char *p is there a standard
way to test whether an assignment such as the following has been
applied?
p = (char *) malloc(sizeof(char) * n);
Well, if you need to do this, there is one portable option. You could
"wrap" malloc, calloc, realloc, and free, and keep track of
the things allocated/freed. Of course, this only works for things
allocated under your control, which may not always be true of
all dynamically allocated memory you encounter (e.g. stuff from
libraries not owned by you). Anyway, the following might work
for you:
example program fragment, assumes you have some static hash
table accessed through the hash functions which store or
retrieve pointer values.
void *my_malloc(size_t size)
{
void *tmp = malloc(size);
if (tmp != NULL) {
hash_enter(tmp);
}
return tmp;
}
void my_free(void *ptr)
{
if (ptr != NULL) {
hash_remove(ptr);
free(ptr);
}
}
int ptr_was_allocated(void *ptr)
{
if (ptr != NULL && hash_contains(ptr)) {
return 1;
}
else {
return 0;
}
}
If the above doesn't suit you, you are out of luck in portable
C AFAIK. But if you are willing to use platform specific extensions,
you may have some options. If so, ask in a group dedicated to
your platform. Or you could leave such checking of
pointers out of your code, and use the tools available for your
platform while testing to detect invalid memory accesses -- for example
valgrind/electric fence/bounds checker/Purify/etc.
Good luck.
-David
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