realloc fails with pointer array

Question

I want to create and resize an array of pointers to an arbitrary type. But for some reason, this crashes in the fifth iteration:

#include <stdlib.h>
#include <stdio.h>

int main() {
    int **data = NULL;
    size_t size = 0;

    for (size_t i = 0; i < 10; ++i) {
        printf("begin iteration #%lu...\n", i + 1);
        int *a = malloc(sizeof *a);
        data = realloc(data, ++size);
        data[size - 1] = a;
    }
}

The output is:

begin iteration #1...
begin iteration #2...
begin iteration #3...
begin iteration #4...
begin iteration #5...
realloc(): invalid next size
Aborted (core dumped)

I know there are already houndreds of questions regarding realloc(): invalid next size and I looked through many of them but I still wasn't able to figure it out. I am pretty sure I am missing something fairly basic here.

"an array of pointers to an arbitrary type" hmmm... then why are you using int* — 4386427
– 4386427, Commented Oct 20, 2021 at 13:52
regarding: int *a = malloc(sizeof *a); This results in an unrecoverable memory leak — user3629249
– user3629249, Commented Oct 21, 2021 at 20:45

dbush · Accepted Answer · 2021-10-20 13:41:33Z

5

You're not allocating enough space:

data = realloc(data, ++size);

This is telling realloc to allocate ++size bytes, so 1 byte on the first interation, 2 on the second, etc. This is not enough to hold what you want to store, so you're writing past the end of allocated memory, triggering undefined behavior.

You need to multiply this value by the element size.

data = realloc(data, ++size * sizeof *data);

Also, you're using the wrong format specifier here:

printf("begin iteration #%lu...\n", i + 1);

For a size_t, the size modifier is z:

printf("begin iteration #%zu...\n", i + 1);

answered Oct 20, 2021 at 13:41

dbush

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Comments

klutt · Accepted Answer · 2021-10-20 13:43:53Z

2

Well, at least one problem is that you're allocating size bytes instead of elements.

data = realloc(data, ++size * sizeof *data);

answered Oct 20, 2021 at 13:43

klutt

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