RE: [Valgrind-users] Problem with realloc() + valgrind 2.4.0 (incl. sample program)

[Mark S. <ms...@cl...>]

It looks like it's not a problem with realloc() directly, but a problem
with copying between large allocated chunks. This program:

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

void my_memcpy(char *dest, char *src, unsigned int len)
{
    for(int i=3D0; i<len; i++) {
        dest[i] =3D src[i];
    }
}

int main(int argc, char **argv)
{
    int len =3D 0;
    char *data =3D NULL;
    int do_copy =3D 1;

    for(int i=3D0; i<2000; i++) {
        len +=3D 4096;
        char *new_data =3D (char *) malloc(len*sizeof(char));
        if (data && do_copy) {
            my_memcpy(new_data, data, len-4096);
        }
        free(data);
        data =3D new_data;
    }

    free(data);

    return 0;
}

Exibits the same problem if do_copy is 1, but behaves properly if
do_copy is 0.

  --Mark


-----Original Message-----
From: val...@li...
[mailto:val...@li...] On Behalf Of
Nicholas Nethercote
Sent: Thursday, May 19, 2005 2:16 PM
To: Thomas Steffen
Cc: val...@li...
Subject: Re: [Valgrind-users] Problem with realloc() + valgrind 2.4.0
(incl. sample program)


On Thu, 19 May 2005, Thomas Steffen wrote:

> If you look at the realloc function in memcheck, you will find this is

> by design. Realloc always returns a fresh piece of address space, so=20
> that it can check for any old pointer that are not updated. Of course=20
> this uses a lot of address space, which means it uses a lot of memory.

Are you sure?  It looks to me like it reuses the same memory if the new=20
size is smaller or equal to the old size.  It allocates new memory if
the=20
new size is bigger.

But the allocator does avoid recycling freed blocks for a while, which=20
might explain the 700MB figure.

N


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