PostgreSQL 7.4.3 Documentation
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# 27.12. Behavior in Threaded Programs

libpq is reentrant and thread-safe if the `configure` command-line option `--enable-thread-safety` has been used when the PostgreSQL distribution was built. In addition, you might need to use additional compiler command-line options when you compile your application code. Refer to your system's documentation for information about how to build thread-enabled applications.

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, start up multiple connections.)

`PGresult` objects are read-only after creation, and so can be passed around freely between threads.

The deprecated functions `PQoidStatus` and `fe_setauthsvc` are not thread-safe and should not be used in multithread programs. `PQoidStatus` can be replaced by `PQoidValue`. There is no good reason to call `fe_setauthsvc` at all.

libpq applications that use the `crypt` authentication method rely on the `crypt()` operating system function, which is often not thread-safe. It is better to use the `md5` method, which is thread-safe on all platforms.

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