By default, if shared memory is supported, libmongoc gathers statistics about client/cursor/socket usage. See the Basic Troubleshooting page for more info.

They are currently enabled by default on Linux if shared memory is supported. They may be opted-out at configuration time with -DENABLE_SHM_COUNTERS=OFF or at runtime with the MONGOC_DISABLE_SHM.

The shared memory segment is unlinked in mongoc_cleanup, but if a user does not call that function, or a crash occurs before that function is called, then this may result in shared memory segments being left around.

Let's consider whether this should be an opt-in feature rather than opt-out. This would be a bit of a behavior change, but it's not clear to me how disruptive changing this default would be.