realloc
will change the amount of memory that is allocated, if needed. It is also free to move the data to another place in memory if that's more efficient (avoiding memory fragmentation).
The function, then, returns a new pointer to the new location in memory where your data is hiding. You're calling realloc
, and allocating (probably) four times as much memory as before, so it's very likely that that allocated memory is situated elsewhere in memory.
In your comment, you said realloc
works like free
+ malloc
. Well, in some cases it can behave similarly, however: realloc
and free
are different functions, that do different tasks. Both are functions that manage the dynamic memory, so yes, obviously there are similarities, and in the case of realloc
, sometimes they can seem to be doing the same thing, however: As I explained here, realloc
and free
are fundamentally different functions
However, by not assigning the return value of realloc
to q.elems
, you're left with a pointer to a memory address that is no longer valid. The rest of your program can, and probably does, exhibit signs of undefined behaviour, then.
Unless you show some more code, I suspect this will take care of the problem:
//change:
Pqueue_elem* newElems = realloc(q.elems, 4 * q.capacity * sizeof *newElems);
//to
q.elems = realloc(q.elems, 4 * q.capacity * sizeof *newElems);
Or better yet, check for NULL pointers:
Pqueue_elem* newElems = realloc(q.elems, 4 * q.capacity * sizeof *newElems);
if (newElems == NULL)
exit( EXIT_FAILURE );// + fprintf(stderr, "Fatal error...");
q.elems = newElems;//<-- assign new pointer!
Looking at your pqueue_new
function, I would suggest a different approach. Have it return the pointer to Pqueue
. You're working with a piece of dynamic memory, treat it accordingly, and have your code reflect that all the way through:
Pqueue * pqueue_new(size_t size)
{//size_t makes more sense
if (size < 4)
size = 4;
Pqueue* q = malloc(sizeof *q);
check(q, "a new queue.");
q->capacity = size;
q->elems = malloc(q->capacity * sizeof *(q->elems));
check(q->elems, "queue's elements");
return q;
}
Alternatively, pass the function a pointer to a stack variable:
void pqueue_new(Pqueue *q, size_t size)
{
if (q == NULL)
{
fprintf(stderr, "pqueue_new does not do NULL pointers, I'm not Chuck Norris");
return;//or exit
}
if (size < 4)
size = 4;
check(q, "a new queue.");
q->capacity = size;
q->elems = malloc(q->capacity * sizeof *(q->elems));
check(q->elems, "queue's elements");
}
//call like so:
int main ( void )
{
Pqueue q;
pqueue_new(&q, 3);
}
Those would be the more common approaches.