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Thread safety

This library does not contain any locking code to protect objects against simultaneous modification in multi-threaded programs. Therefore it is up to you, the user of the library, to ensure that your threaded client programs perform no conflicting operations concurrently.

Most of the time this isn't hard. Result sets are immutable, so you can share them between threads without problem. The main rule is:

- Treat a connection, together with any and all objects related to it, as a "world" of its own. You should generally make sure that the same "world" is never accessed by another thread while you're doing anything non-const in there.

That means: don't issue a query on a transaction while you're also opening a subtransaction, don't access a cursor while you may also be committing, and so on.

In particular, cursors are tricky. It's easy to perform a non-const operation without noticing. So, if you're going to share cursors or cursor-related objects between threads, lock very conservatively!

Use `pqxx::describe_thread_safety` to find out at runtime what level of thread safety is implemented in your build and version of libpqxx. It returns a `pqxx::thread_safety_model` describing what you can and cannot rely on. A command-line utility `tools/pqxxthreadsafety` prints out the same information.