Error when using realloc for an array that store read int from file

Question

I would like to resize the array when it reaches its max capacity. But error came up after i do ./a.out Please help me...

Error: a.out: malloc.c:3574: mremap_chunk: Assertion `((size + offset) & (mp_.pagesize-1)) == 0' failed.

code:

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


int main(void)
{
  int cap=5;
  int *arr = malloc(cap*sizeof(int));
  FILE *f;

  if((f=fopen("/home/file.txt","r"))==NULL)
    printf("You cannot open");  

  while(fscanf(f, "%d", arr++)!=EOF)
  {
    index++;
    if(index==cap-1)
      arr = realloc(arr, (cap +=1) * sizeof(int));
  } 

  return 0;
}

Carl Norum · Accepted Answer · 2012-04-07 18:13:01Z

5

You have arr++ in your loop condition. That means arr doesn't point to the start of the allocated memory anymore when you call realloc(). That's going to end up with the error you're seeing.

Also:

Programming safety note:

Don't call realloc() in the form:
```
foo = realloc(foo, bar);
```
If an error occurs, foo will be set to NULL and you'll leak the original allocation.
Nonidiomatic code note:
```
(cap +=1) * sizeof(int)
```
is a bit weird. Why not ++cap * sizeof(int)? Or better yet, do it on two lines rather than cramming it all into one.

answered Apr 7, 2012 at 18:13

Carl Norum

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7 Comments

qwr qwr Over a year ago

So the solution will be using a pointer to point to the arr and then read from the pointer right?

Carl Norum Over a year ago

Just don't lose the pointer to the beginning of your allocation. Make a copy and use that when calling realloc(), for example.

qwr qwr Over a year ago

I did the following: int *y = arr; while(fscanf(f, "%d", y++)!=EOF) so now i have a new pointer do the work, and arr is not change it original location. But instead i got a segmentation error :(

Carl Norum Over a year ago

When you call realloc(), the base pointer to the array (arr) might change. You'll need to adjust y to match if that happens.

qwr qwr Over a year ago

But how do i ensure that y is not reading from the beginning again after i adjust the y?

|

MByD · Accepted Answer · 2012-04-07 18:13:31Z

0

You need to perform the realloc on the same address received fromm malloc, but you increase arr in while(fscanf(f, "%d", arr++)!=EOF)

answered Apr 7, 2012 at 18:13

MByD

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