Memory of two variables are colliding when used malloc

Question

I'm trying to learn the memory allocation in C using malloc and increasing the size of allocated array using realloc inside a function. I came across this. when I use single variable the code is working well. But when I allocate memory for second variable, it gives me weird output.

Code is below:

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

# define N 3

void padd(int *a){
    int sizes = 10*sizeof(int);
    a = (void *)realloc(a,sizes);
    a[0] = 10;
    printf("inside func= %d \n",a[0]);
}

int main()
{
    int *d;
    int *pA;
    
    int size = N*sizeof(int);
    pA = (void *)malloc(size);
    //d = (int *)malloc(size);
    
    padd(pA);
    printf("outside func= %d",pA[0]);
    
}

it gives me output:

inside func= 10
outside func= 10

but if I uncomment the //d = (int *)malloc(size); line, It gives me

inside func= 10 
outside func= -1368048824

as output.

What might be wrong here?

Answer 1

If realloc can’t extend a buffer in place, it will allocate a new buffer, copy the contents of the old buffer to it, free the old buffer, and return the address of the new buffer (or NULL if it cannot satisfy the request); thus, the value of a can change in padd. However, that change is only applied to the formal parameter a - the actual parameter pA is not affected.

Based on the behavior, it looks like that if you allocate d, it’s allocated immediately after pA such that pA can’t be extended in place, so realloc is creating a new buffer and deallocating the old one, and the old buffer is overwritten before the printf statement in main.

You’ll want to write padd such that any changes to a are reflected in pA - either return the (potentially new) value of a or pass a pointer to pA:

void padd( int **a )
{
  size_t sizes = 10 * sizeof (int); // see note 1
  int *tmp = realloc( *a, sizes ); // see note 2
  
  if ( tmp )
  {
    *a = tmp;
    (*a)[0] = 10;
    printf( "Inside func, (*a)[0] = %d\n", (*a)[0] );
  }
  else
  {
    printf ( "Inside func, realloc failed!\n" );
  }
}

and you’d call it as

padd( &pA );

Note 1: sizeof has type size_t, not int.

Note 2: Since realloc can potentially return NULL, always assign the result to a temporary value and check it before assigning back to the original variable, otherwise you run the risk of losing access to memory you’ve already allocated. Also, the cast to void * is unnecessary and confusing since you’re assigning the result to an int * variable. Unless you’re compiling this code as C++ or under an ancient K&R C compiler, just leave the cast off completely - otherwise, your cast has to match the type of the thing you’re assigning to, which in this case is int *.