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Concurrency is the word used to describe causal independence between a number of actions, such as the execution of a number of instructions ``at the same time''. This is also the definition which we give of the term ``parallel'' in the introduction of this fourth part. The processes of the Unix library presented in the preceding chapter could be considered as concurrent to the extent that the Unix system provides the appearance of their simultaneous execution on a uniprocessor machine. But the notion of process and concurrency does not apply only to those obtained by the fork system call.

The Objective CAML language possesses a library for lightweight processes (threads.) The principle difference between a thread and a process is in the sharing or non-sharing of memory between the different child processes of the same program. Only the context of execution differs between two threads: the code and memory sections of the data are shared. Threads do not improve the execution time of an application. Their principal attraction is to make it possible to express the programming of concurrent algorithms within a language.

The nature of the chosen language, imperative or functional, affects the model of concurrency. For an imperative program, as every thread can modify the communal/shared memory, we are in a shared memory model. Communication between processes can be achieved by values written and read in this memory. For a purely functional program, that is to say, without side effects, even though the memory is shared, the calculations which each process executes do not act on this shared memory. In this case, the model used is that of separate memory and interaction between processes must be achieved by communication of values though channels.

The Objective CAML language implements both models in its thread library. The Thread module makes it possible to start new processes corresponding to a function call with its argument. Modules Mutex and Condition provide the synchronization tools for mutual exclusion and waiting on a condition. The Event model implements a means of communication of language values by events. These values can themselves be functional, thus making it possible to exchange calculations to be carried out between threads. As always in Objective CAML it is possible to mix the two models.

This library is portable to the different systems where OCAML runs. Unlike the Unix module, the Thread library facilitates the use of processes on machines that are not running Unix.

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