怎么查看动态分配内存空间的大小(GNU Linux下C语言版)

/********************************************************************

* Function: Test malloc and realloc,get malloc size
* Author  : Samson
* Date    : 11/23/2011
* Test platform:
*               GNU Linux version 2.6.29.4
*               gcc version 4.4.0 20090506 (Red Hat 4.4.0-4) (GCC)

* Online citations: http://fuse4bsd.creo.hu/localcgi/man-cgi.cgi?malloc+3 
* *****************************************************************/

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

int
main()
{

char *strP, *tem2P;
int ntem;

strP = malloc(20);

ntem = malloc_usable_size(strP);

printf("first malloc_usable_size is %d\n", ntem);
tem2P = strP;

if(NULL != realloc(strP, 100))
{
ntem = malloc_usable_size(strP);
printf("realloc malloc_usable_size is %d\n", ntem);
}

free(tem2P);
exit(0);

}

[root@UFO]# ./a.out
first malloc_usable_size is 20
realloc malloc_usable_size is 100