size_t _msize_dbg(  
   void *userData,  
   int blockType   
);  

// crt_msize_dbg.c  
// compile with: /MTd  
/*  
 * This program allocates a block of memory using _malloc_dbg  
 * and then calls _msize_dbg to display the size of that block.  
 * Next, it uses _realloc_dbg to expand the amount of  
 * memory used by the buffer and then calls _msize_dbg again to  
 * display the new amount of memory allocated to the buffer.  
 */  
  
#include <stdio.h>  
#include <malloc.h>  
#include <stdlib.h>  
#include <crtdbg.h>  
  
int main( void )  
{  
        long *buffer, *newbuffer;  
        size_t size;  
  
        /*   
         * Call _malloc_dbg to include the filename and line number  
         * of our allocation request in the header  
         */  
        buffer = (long *)_malloc_dbg( 40 * sizeof(long), _NORMAL_BLOCK, __FILE__, __LINE__ );  
        if( buffer == NULL )  
               exit( 1 );  
  
        /*   
         * Get the size of the buffer by calling _msize_dbg  
         */  
        size = _msize_dbg( buffer, _NORMAL_BLOCK );  
        printf( "Size of block after _malloc_dbg of 40 longs: %u\n", size );  
  
        /*   
         * Reallocate the buffer using _realloc_dbg and show the new size  
         */  
        newbuffer = _realloc_dbg( buffer, size + (40 * sizeof(long)), _NORMAL_BLOCK, __FILE__, __LINE__ );  
        if( newbuffer == NULL )  
               exit( 1 );  
        buffer = newbuffer;  
        size = _msize_dbg( buffer, _NORMAL_BLOCK );  
        printf( "Size of block after _realloc_dbg of 40 more longs: %u\n", size );  
  
        free( buffer );  
        exit( 0 );  
}  

Size of block after _malloc_dbg of 40 longs: 160  
Size of block after _realloc_dbg of 40 more longs: 320