Their internal states in Rust are also namespaced, so two incompatible crates in the same process won't observe each others symbols. If they access external resources that are not namespaced though, that could be a problem.
The free() function frees the memory space pointed to by ptr, which must have been returned by a previous call to malloc(), calloc() or realloc(). Otherwise, or if free(ptr) has already been called before, undefined behavior occurs. If ptr is NULL, no operation is performed.I haven't ever observed a problem of concurrent access to stdout/err though, I expect because the methods for accessing stdout/err lock them for the duration of their printing. If you Google for "Rust print console slow", you'll probably find advice to explicitly lock it, to avoid individual printlns from each acquiring the lock.