Re: using realloc()

Jens.Toerring_at_physik.fu-berlin.de
Date: 07/30/04 

Date: 29 Jul 2004 23:31:51 GMT

John Hanley <jdhanley@telusplanet.net> wrote:
>> I start out with int * elements = (int *) malloc(sizeof(int)),
>>
>> then in my next function, I pass it 'elements' which I reallocate memory
> for
>> in order to add another element dynamically:
>>
>> elements=(int *)realloc(elements, sizeof(int)*(j+1));
>>
>> When debugging this in gdb, I notice that when j=2, and I realloc, my
>> address of 'elements' changes. But when j=1, it was fine.

> When thinking about this further, I am guessing that it has something to do
> with alignment in memory. Does that sound right?

No, not at all, see my previous post.

> If so, is there a way around this to make sure it's always going to be
> pointing to the same memory address? This is only because I pass this
> pointer around to a couple of functions. If the second function changes the
> address, the first function will still be pointing to the original address.

If there is the need to increase the memory a pointer is pointing to
in a function you don't pass just the pointer itself to a function,
but a pointer to that pointer. That way you can change what it's
pointing to with the change becoming visible to the calling functing.
Here's a rather contrived example:

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

int mail( void )
{
    int *p = malloc( 100 );

    if ( p == NULL )
        printf( "Couldn't make p point to even 100 int's\n" );
        exit( EXIT_FAILURE );
    }

    if ( increase_p( &p ) == 0 )
        printf( "Wow, p now points to 500 int's!\n" );
    else
        printf( "Sorry, couldn't get more, p still points to 100 int's\n" );
    ...
}

int increase_p( int **p )
{
    int *new_p = realloc( *p, 500 * sizeof **p );

    if ( new_p != NULL )
    {
        *p = new_p;
        return 0;
    }
    return -1;
}

Admittedly, pointers to pointers can be a bit scary at first, but after
a rather short time you will get the hang of it - and they are the only
way to go in these cases. Just wait 'til you have to deal with triple
pointers;-)
                                    Regards, Jens 

-- 
  \   Jens Thoms Toerring  ___  Jens.Toerring@physik.fu-berlin.de
   \__________________________  http://www.toerring.de

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