Only the storage referenced by ptr is modified. No other storage locations are accessed by the call.

free - C++ Reference http://www.cplusplus.com/reference/cstdlib/free/

Data races

Only the storage referenced by ptr is modified. No other storage locations are accessed by the call.
If the function releases a unit of storage that is reused by a call to allocation functions (such as calloc or malloc), the functions are synchronized in such a way that the deallocation happens entirely before the next allocation.