How to update and array of pointers to elements of an array after realloc moves the memory location of that array

Question

I am a beginner and am having a really hard time with dynamic memory allocation. If anyone can help me with this problem, I would be really grateful.

I allocate some memory using malloc to an array Node using:

struct nodeT {
int id;
nodeT *parent, *T1, *T2;
};

struct nodeT* T;
T =  (struct nodeT*) malloc( 256*sizeof(struct nodeT) );

Then in order to reference T, I use a an array of pointers to T, call it Tptr:

struct nodeT** Tptr;
Tptr = (struct nodeT**) malloc( 256*sizeof(struct nodeT*) );

In my code, I fill data in T and then I set *Tptr = T iteratively as follows:

nodeT* s = &T[ 1*21 ];
s->id=23;
s->T1=...
s->T2=...
s->parent=...

and then

sptr = &Tptr[ 1*21 ];
*sptr=s;
and it works fine.

But sometimes, when I call realloc to increase the size of T, and then I check *sptr, it is no longer valid, I get a segmentation fault at runtime. I think the realloc may sometimes move the entire memory block to a new location, while the *sptr keeps pointing to the old location.

Any idea how can I update all Tptr after every realloc.

If I use a big size of T from the beginning and disable realloc for T, everything works fine. But I want it to work with dynamically increasing array sizes.

Best Regards Wajahat

score 0 · Answer 1 · answered Oct 28 '12 at 18:46

Your call to malloc sets T. When you call re-alloc you can not assume that T will be in the same place Since Tptr is set Tptr = T after calling

T =  (struct nodeT*) malloc( 256*sizeof(struct nodeT) );

your array of pointers TPtr needs to be re-populated after every realloc on T. If you don't want to do this yourself, use vectors rather than arrays and realloc

How to update and array of pointers to elements of an array after realloc moves the memory location of that array

1 Answers1