Error :"pointer being realloc'd was not allocated" on macOS but works on Windows when trying to realloc twice

Question

I'm trying to implement a function that concatenate two strings, but I keep getting the same error. "pointer being realloc'd was not allocated" When I compiled the same code on a windows machine it worked, is it something that I'm missing? The code below is basically what I'm trying to do. main:

int main() {

int length = 4096;
char *string = malloc(length * sizeof(char));
createString(string, length);
realloc(string, 30);
return 0;
}

createString:

void createString(char * string, int length) {
  char *copyAdress = string;
  char *temp ="";
  int counter2 = 0;

  fflush(stdin);
  fgets(string, length,stdin);
  while(*string != EOF && *string != *temp ) {
    string++;
    counter++;
  }
  string = copyAdress;
  realloc(string, (counter)*sizeof(char));
}

Thanks! Edit: I want createString to change the size of string to the length of the string that I get with fgets, while having the same address as the string that I sent in, so I can allocate more memory to it later when I want to add another string to it.

`*string != *temp ` is just comparing with `'\0'` because `temp` is `""`. — Paul Ogilvie, Oct 04 '18 at 13:51

Jabberwocky · Answer 1 · 2018-10-04T13:42:08.427

3

There are several issues:

realloc(string, (counter)*sizeof(char)); is wrong, you need string = realloc(string, (counter)*sizeof(char)); because realloc may return a different address.
Calling createString(string, length); won't modify string

If you want a more accurate answer you need to tell us what exactly createString is supposed to do. In your code there is no attempt to concatenate two strings.

edited Oct 04 '18 at 13:42

answered Oct 04 '18 at 13:31

Jabberwocky

48,281
17
65
115

My goal is to change the size of the string in main with createString while having the same address as before. Then I want to add more space to the string, so I can concatenate another string to it. – Axel Pettersson Oct 04 '18 at 13:46
...and `realloc` may return 0 if it can;t realloc, so `tmpstring= realloc(...` – Paul Ogilvie Oct 04 '18 at 13:50

score 1 · Answer 2 · edited Oct 04 '18 at 13:42

1

If you realloc some memory, the pointer pointing to the original memory becomes invalid (unless realloc failed and returned NULL). So calling realloc twice on the same pointer should indeed not work (if it didn't return NULL the first time).

edited Oct 04 '18 at 13:42

Eric Postpischil

195,579
13
168
312

answered Oct 04 '18 at 13:34

Blaze

16,736
2
25
44

2

It doesn not _need_ to become invalid of realloc could expand the memory. It _may_, though. – Paul Ogilvie Oct 04 '18 at 13:53

Kami Kaze · Accepted Answer · 2018-10-04T14:18:36.190

Let's work through this in order of execution.

fflush(stdin); is undefined behaviour. If you really need to clear everything in the stdin you have to find another way (a loop for example). There are compilers/systems with a defined implementation but I would not count on it.

string++; is superflous as you overwrite string after the loop.

realloc(string, (counter)*sizeof(char));

should be

char *temp = realloc(string, (counter)*sizeof(char));
if (temp != NULL)
  string = temp;

This way you get the pointer where your new string is located, but I suggest you read the refecerence for realloc. In essence you do not know if it has been moved and the old address might be invalid from that point on. So dereferencing it is also undefined behaviour.

After this you would have to return the new address of string or pass the address of the pointer to your function.

The same problem repeats with the second realloc. You only got to know your first call was wrong, because the second call noticed that you do not have valid data in what you thought would be your string.

In regards to your comment: It is not possible to use realloc and to be sure that the reallocated memory is in the same place as before.

score 0 · Answer 4 · answered Oct 04 '18 at 14:00

See the answers from others about what you do wrong. However, the eror message means that on MacOS, the realloc in createString deallocated the orignal string and allocated a new one, and now your realloc in main tries to realloc a pointer that is no longer valid (allocated). On Windows, the memory was not deallocated in createString and so the second call of realloc (in main) is given a valid pointer.

Error :"pointer being realloc'd was not allocated" on macOS but works on Windows when trying to realloc twice

4 Answers4