了解堆检查，堆损坏

Question

如果我跑：

char *buffer;
buffer = (char *)malloc( 8 );
strcpy(buffer,"abcdefghijklm");
int  heapstatus = _heapchk();
switch( heapstatus )
{
case _HEAPOK:
  printf(" OK - heap is fine\n" );
  break;
 case _HEAPEMPTY:
  printf(" OK - heap is empty\n" );
  break;
  case _HEAPBADBEGIN:
  printf( "ERROR - bad start of heap\n" );
  break;
  case _HEAPBADNODE:
  printf( "ERROR - bad node in heap\n" );
  break;
 }

它显示：

 ERROR - bad node in heap

好。 但如果我跑：

char *a;
for (int i=0;i<1000000;i++) a = (char*)malloc(1); // Line added
char *buffer;
buffer = (char *)malloc( 8 );
strcpy(buffer,"abcdefghijklm");
int  heapstatus = _heapchk();
switch( heapstatus )
{
case _HEAPOK:
  printf(" OK - heap is fine\n" );
  break;
 case _HEAPEMPTY:
  printf(" OK - heap is empty\n" );
  break;
  case _HEAPBADBEGIN:
  printf( "ERROR - bad start of heap\n" );
  break;
  case _HEAPBADNODE:
  printf( "ERROR - bad node in heap\n" );
  break;
 }

它显示：

OK - heap is fine

问题是为什么？堆腐败对吗？