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
fe_setauthsvc are not thread-safe and should not be
used in multithread programs.
PQoidStatus can be
PQoidValue. There is no good reason to
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.