# 9 Multithreaded applications

SWI-Prolog multithreading is based on standard C language multithreading support. It is not like ParLog or other parallel implementations of the Prolog language. Prolog threads have their own stacks and only share the Prolog heap: predicates, records, flags and other global non-backtrackable data. SWI-Prolog thread support is designed with the following goals in mind.

- Multithreaded server applications
  Today's computing services often focus on (internet) server applications. Such applications often have need for communication between services and/or fast non-blocking service to multiple concurrent clients. The shared heap provides fast communication, and thread creation is relatively cheap.154On an Intel i7-2600K, running Ubuntu Linux 12.04, SWI-Prolog 6.2 creates and joins 32,000 threads per second elapsed time.

- Interactive applications
  Interactive applications often need to perform extensive computation. If such computations are executed in a new thread, the main thread can process events and allow the user to cancel the ongoing computation. User interfaces can also use multiple threads, each thread dealing with input from a distinct group of windows. See also section 9.7.

- Natural integration with foreign code
  Each Prolog thread runs in a native thread of the operating system, automatically making them cooperate with MT-safe foreign code. In addition, any foreign thread can create its own Prolog engine for dealing with calling Prolog from C code.

SWI-Prolog multithreading is based on the POSIX thread standard Butenhof, 1997 used on most popular systems except for MS-Windows. On Windows it uses the http://sources.redhat.com/pthreads-win32/pthread-win32 emulation of POSIX threads mixed with the Windows native API for smoother and faster operation. The SWI-Prolog thread implementation has been discussed in the ISO WG17 working group and is largely addopted by YAP and XSB Prolog.155The latest version of the ISO draft can be found at http://logtalk.org/plstd/threads.pdf. It appears to have dropped from the ISO WG17 agenda.

## Section Index