Using realloc to extend memory space taken with malloc?

Question

As a school assigment I need to create memory block with malloc and when its full it should extend itself with realloc.This will continue in a infnite loop.But I am getting segmantation fault error and I think the mistake is in realloc so can anyone tell me what I am doing wrong ?

#include "stdio.h"
#include "stdlib.h"
int main()
{
  int boyut=5;
  int* dizi;
  dizi=(int *)malloc((boyut*sizeof(int)));
  int b=0;
while(b<1)  
    {   
if(*dizi<boyut){                
 scanf("%d",dizi);
dizi++;                
 }
else if(boyut=*dizi)         
 {
    printf("boyut yetersiz");
    dizi = (int*) realloc (dizi,2*sizeof(int));
    boyut=*dizi*2;                  
 }  
}
  return 0;
}

Edited according to answers its extending the malloc area by *2.

int main()
{
  int boyut=2;
  int* dizi;
  int* dizi1;
  int n;
  dizi=(int *)malloc((boyut*sizeof(int)));
  int b=0;
  int i;
while(b<1)  
    { 

if(i<boyut){           

  for(i=0;i<boyut;i++){ 
    printf("\nDeğer Girin:");
 scanf("%d",dizi);
 }
 }
else if(boyut=i)         
 {

    boyut=boyut*2; 
    printf("\nBoyut yetersiz yeni boyutu %d yapıyoruz",boyut);
    dizi1 = (int*) realloc (dizi,boyut*sizeof(int));
    dizi=dizi1;


 }

}
  return 0;
}

Correct answer thanks for those who have same questions can use it.

You do have a debugger? Use it, it will tell you *where* the crash is, let you examine and walk up the function call stack, and examine values of variables. — Some programmer dude, Jan 04 '14 at 14:26
`*dizi` not initialize until scanf. and after `dizi++;` cannot realloc. — BLUEPIXY, Jan 04 '14 at 14:28
But I can take 5 integers at the beginnging the segmantation fault comes when relloc happens. — Bugra Sezer, Jan 04 '14 at 14:30

score 2 · Accepted Answer · edited May 23 '17 at 12:03

2

You actually reallocate to less than before the reallocation. First you allocate space for 5 (boyut) integers, then you reallocate space for only two integers. You need to increase boyut and reallocate with that as a multiplier.

You also need to remember that realloc can fail, and in that case will set dizi to NULL, making you both dereference a NULL pointer and loose the original pointer so you can't free it.

Also, don't cast the result of malloc (or realloc for that matter).

The real problem, as pointed out by BLUEPIXY, is because of undefined behavior. For realloc to work it needs to be passed the original pointer. You no longer have it after you do dizi++.

Also, when you allocate memory, the contents is uninitialized, meaning it will contain seemingly random data. So doing *dizi before you initialize that data will also result in undefined behavior. You might want to use e.g. calloc instead.

edited May 23 '17 at 12:03

Community

1
1

answered Jan 04 '14 at 14:15

Some programmer dude

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I did what you say but still same error.Code modified I added the new code. – Bugra Sezer Jan 04 '14 at 14:24
@BugraSezer Most likely it's because of `dizi1=dizi;`. I think you mean the opposite. – Some programmer dude Jan 04 '14 at 14:27
nope nothing changed I modified the code again please have a look. – Bugra Sezer Jan 04 '14 at 14:29
@BugraSezer Read the comment by BLUEPIXY. – Some programmer dude Jan 04 '14 at 14:31
I read what BLUEPIXY said but cannot understand what he meant ? – Bugra Sezer Jan 04 '14 at 14:32
So I removed my dizi++ but with if I do this I think it will keep writing on the same memory block over and over again so how can I make it go on a for statement ? – Bugra Sezer Jan 04 '14 at 14:37
Oke the working code is above its extending the memory by *2 thanks everyone. – Bugra Sezer Jan 04 '14 at 14:53

score 2 · Answer 2 · answered Jan 04 '14 at 14:15

2

The second argument to realloc is the total size of the allocation requested, not the amount by which it should grow. You should keep track of the size of the allocation:

size_t size = some_size;
void *ptr = malloc(some_size);
// check for errors

while (some_condition) {
    size += some_increment;
    void *newptr = realloc(size);
    // check for errors
    ptr = newptr;
}

answered Jan 04 '14 at 14:15

Fred Foo

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You said the same thing the answer above but still same error code modified please look.. – Bugra Sezer Jan 04 '14 at 14:25

Using realloc to extend memory space taken with malloc?

2 Answers2