# Chapter ‍33 The threads library

The threads library allows concurrent programming in OCaml. It provides multiple threads of control (also called lightweight processes) that execute concurrently in the same memory space. Threads communicate by in-place modification of shared data structures, or by sending and receiving data on communication channels.

The threads library is implemented on top of the threading facilities provided by the operating system: POSIX 1003.1c threads for Linux, MacOS, and other Unix-like systems; Win32 threads for Windows. Only one thread at a time is allowed to run OCaml code on a particular domain ‍9.5.1. Hence, opportunities for parallelism are limited to the parts of the program that run system or C library code. However, threads provide concurrency and can be used to structure programs as several communicating processes. Threads also efficiently support concurrent, overlapping I/O operations.

Programs that use threads must be linked as follows:

            ocamlc -I +unix -I +threads other options unix.cma threads.cma other files
            ocamlopt -I +unix -I +threads other options unix.cmxa threads.cmxa other files

Compilation units that use the threads library must also be compiled with the -I +threads option (see chapter ‍13).

- Module Thread: lightweight threads
- Module Event: first-class synchronous communication