void mongoc_client_set_write_concern (mongoc_client_t *client, const mongoc_write_concern_t *write_concern);
Sets the write concern for the client. This only affects future operations, collections, and databases inheriting from client.
The default write concern is MONGOC_WRITE_CONCERN_W_DEFAULT: the driver blocks awaiting basic acknowledgment of write operations from MongoDB. This is the correct write concern for the great majority of applications.
It is a programming error to call this function on a client from a mongoc_client_pool_t. For pooled clients, set the write concern with the instead.