creference realloc
Defined in header <stdlib.h>
void *realloc( void *ptr, size_t new_size );
Reallocates the given area of memory. It must be previously allocated by malloc(), calloc() or realloc() and not yet freed with a call to free or realloc. Otherwise, the results are undefined.
The reallocation is done by either:
a) expanding or contracting（缩小） the existing area pointed to by ptr, if possible. The contents of the area（由ptr所指向的区域） remain unchanged up to the lesser of the new and old sizes（the lesser of A and B的意思是A和B中的小者）. If the area is expanded, the contents of the new part of the array are undefined.
NOTE :
new_size可能大于ptr所指向的区域的长度，也可能小于ptr所指向区域的长度；即realloc即可用expand，也可以contract；那么新分配的空间的初始值是什么呢？最后两句话给出了解释。
b) allocating a new memory block of size new_size bytes, copying memory area with size equal the lesser of the new and the old sizes, and freeing the old block.
If there is not enough memory, the old memory block is not freed and null pointer is returned.
If ptr is NULL, the behavior is the same as calling malloc(new_size).
If new_size is zero, the behavior is implementation defined (null pointer may be returned (in which case the old memory block may or may not be freed), or some non-null pointer may be returned that may not be used to access storage).