32.21. Behavior in Threaded Programs
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## 32.21. Behavior in Threaded Programs #

As of version 17, libpq is always reentrant and thread-safe. However, one restriction is that no two threads attempt to manipulate the same `PGconn` object at the same time. In particular, you cannot issue concurrent commands from different threads through the same connection object. (If you need to run concurrent commands, use multiple connections.)

`PGresult` objects are normally read-only after creation, and so can be passed around freely between threads. However, if you use any of the `PGresult`-modifying functions described in Section 32.12 or Section 32.14, it's up to you to avoid concurrent operations on the same `PGresult`, too.

In earlier versions, libpq could be compiled with or without thread support, depending on compiler options. This function allows the querying of libpq's thread-safe status:

`PQisthreadsafe` #

  Returns the thread safety status of the libpq library.

      int PQisthreadsafe();

  Returns 1 if the libpq is thread-safe and 0 if it is not. Always returns 1 on version 17 and above.

The deprecated functions `PQrequestCancel` and `PQoidStatus` are not thread-safe and should not be used in multithread programs. `PQrequestCancel` can be replaced by `PQcancelBlocking`. `PQoidStatus` can be replaced by `PQoidValue`.

If you are using Kerberos inside your application (in addition to inside libpq), you will need to do locking around Kerberos calls because Kerberos functions are not thread-safe. See function `PQregisterThreadLock` in the libpq source code for a way to do cooperative locking between libpq and your application. (Note that it is only safe to call `PQregisterThreadLock` when there are no open connections.) Clients may retrieve the current locking callback with `PQgetThreadLock`; if no custom callback has been registered, a default is used.

Similarly, if you are using Curl inside your application, and you do not already initialize libcurl globally before starting new threads, you will need to cooperatively lock (again via `PQregisterThreadLock`) around any code that may initialize libcurl. This restriction is lifted for more recent versions of Curl that are built to support thread-safe initialization; those builds can be identified by the advertisement of a `threadsafe` feature in their version metadata.

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