CODEWITHSUNDEEP
crealloc
Aleksei Udalov
Aleksei Udalov

Reputation: 3

How to use realloc (strange behavior)

So I have a program and its work fine.

#include <stdio.h>
#include <stdlib.h>
#define STACKDEFSIZE 1
typedef struct
{
  unsigned long int maxsize;
  unsigned long int cursize;
  unsigned long int* arr;
} stack;

stack* create_stack()
{
  stack* res = (stack*)malloc(sizeof(stack));
  res->arr = malloc(sizeof(long) * STACKDEFSIZE);
  res->maxsize = STACKDEFSIZE;
  res->cursize = 0;
  return res;
}

void push(stack* st, int val)
{
  if (st->cursize  == st->maxsize)
  {
    unsigned long int* old = st->arr;
    st->maxsize *= 2;
    st->arr = malloc(sizeof(unsigned long int) * st->maxsize);
    int i;
    for(i = 0; i < st->cursize; i++)
      st->arr[i] = old[i];
    free(old);
  }
  st->arr[st->cursize] = val;
  st->cursize += 1;
}

int main() {
  stack* s = create_stack();
  int i;
  for(i = 0; i < 10000; i++)
  {
    push(s, i);
  }
  return 0;
}

But if I change function 'push' to use realloc instead of malloc and free, program crash with message " Error in `./t': realloc(): invalid next size: 0x0000000001031030 Aborted"

void push(stack* st, int val)    
{    
  if (st->cursize  == st->maxsize)    
  {    
    st->maxsize *= 2;    
    st->arr = realloc(st->arr, st->maxsize);    
  }    
  st->arr[st->cursize] = val;    
  st->cursize += 1;    
}

Also valgrind print message 'Invalid write of size 8' when I trying to use realloc. What I doing wrong? I use gcc and Debian Jessie x86_64.

Upvotes: 0

Views: 66

Answers (1)

R Sahu
R Sahu

Reputation: 206717

You are passing the wrong size to realloc. As a consequence, your program runs into undefined behavior in no time.

Use:

st->arr = realloc(st->arr, sizeof(*st->arr)*st->maxsize);

Upvotes: 1

Related Questions