Checked Then Freed

Two threads in a connection server:

/* worker thread */
if (conn->hostname) {
    log_line("serving %s", conn->hostname);
}

/* reaper thread */
free(conn->hostname);
conn->hostname = NULL;

Under load the worker segfaults inside log_line on a pointer that tested non-NULL microseconds earlier. Which change fixes it?

Reorder the reaper to set conn->hostname = NULL first and call free afterwards.

Take one mutex around the check and the use in the worker, and around the free and the clear in the reaper.

Declare the field as _Atomic(char *) so the load and the store cannot tear.

Copy conn->hostname into a local, test the local, and pass the local to log_line.