# Chapter 25 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 by time-sharing on a single processor. It will not take advantage of multi-processor machines. Using this library will therefore never make programs run faster. However, many programs are easier to write when structured as several communicating processes.

Two implementations of the `threads` library are available, depending on the capabilities of the operating system:

- System threads. This implementation builds on the OS-provided threads facilities: POSIX 1003.1c threads for Unix, and Win32 threads for Windows. When available, system threads support both bytecode and native-code programs.
- VM-level threads. This implementation performs time-sharing and context switching at the level of the OCaml virtual machine (bytecode interpreter). It is available on Unix systems, and supports only bytecode programs. It cannot be used with native-code programs.

Programs that use system threads must be linked as follows:

            ocamlc -thread other options unix.cma threads.cma other files
            ocamlopt -thread other options unix.cmxa threads.cmxa other files

Compilation units that use the `threads` library must also be compiled with the `-thread` option (see chapter 8).

Programs that use VM-level threads must be compiled with the `-vmthread` option to `ocamlc` (see chapter 8), and be linked as follows:

            ocamlc -vmthread other options threads.cma other files

Compilation units that use `threads` library must also be compiled with the `-vmthread` option (see chapter 8).

- Module `Thread`: lightweight threads
- Module `Mutex`: locks for mutual exclusion
- Module `Condition`: condition variables to synchronize between threads
- Module `Event`: first-class synchronous communication
- Module `ThreadUnix`: thread-compatible system calls