When an attempt is made to resize a buffer through a call to the realloc() function, the pointer is checked for validity if it's a non-NULL value. If it's valid, the header of the heap buffer is checked for consistency. If the buffer is large enough to satisfy the request, the buffer header is modified, and the call returns. If a new buffer is required to satisfy the request, memory allocation is performed to obtain a new buffer large enough to satisfy the request with the same consistency checks being applied as in the case of memory allocation described above. The original buffer is then released.
If fill-area boundary checking is enabled (described in the "Manual Checking" section) the guard code checks are also performed on the allocated buffer before it's actually resized. If a new buffer is used, the guard code checks are done just before releasing the old buffer.