## 27.14. 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, or look in `src/Makefile.global` for `PTHREAD_CFLAGS` and `PTHREAD_LIBS`.

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

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

libpq must ignore `SIGPIPE` signals generated internally by `send()` calls to backend processes. When PostgreSQL is configured without `--enable-thread-safety`, libpq sets `SIGPIPE` to `SIG_IGN` before each `send()` call and restores the original signal handler after completion. When `--enable-thread-safety` is used, libpq installs its own `SIGPIPE` handler before the first database connection. This handler uses thread-local storage to determine if a `SIGPIPE` signal has been generated by a libpq `send()`. If an application wants to install its own `SIGPIPE` signal handler, it should call `PQinSend()` to determine if it should ignore the `SIGPIPE` signal. This function is available in both thread-safe and non-thread-safe versions of libpq.

If you experience problems with threaded applications, run the program in `src/tools/thread` to see if your platform has thread-unsafe functions. This program is run by `configure`, but for binary distributions your library might not match the library used to build the binaries.