The biggest principle behind the design of the Cilk language is that the programmer should be responsible for exposing the parallelism, identifying elements that can safely be executed in parallel; it should then be left to the run-time environment, particularly the scheduler, to decide during execution how to actually divide the work between processors. It is because these responsibilities are separated that a Cilk program can run without rewriting on any number of processors, including one.

The Cilk language has been developed since 1994 at the MIT Laboratory for Computer Science. It is based on ANSI C, with the addition of just a handful of Cilk-specific keywords. When the Cilk keywords are removed from Cilk source code, the result is a valid C program, called the serial elision (or C elision) of the full Cilk program. Cilk is a faithful extension of C and the serial elision of any Cilk program is always a valid serial implementation in C of the semantics of the parallel Cilk program. Despite several similarities, Cilk is not directly related to AT&T Bell Labs’ Concurrent C.

A commercial version of Cilk, called Cilk++, that supports both C and C++ and is compatible with both GCC and Microsoft C++ compilers, was developed by Cilk Arts, Inc.. Academic and Open Source versions also exist, where the Open Source version is under an in-house license that falls somewhere between the updated BSD license and the LGPL. The original Cilk code is still available from MIT. In July 2009, Intel Corporation acquired Cilk Arts, the Cilk++ technology and the Cilk trademark. In 2010, Intel released a commercial implementation in its compilers combined with some data parallel constructs with the name Intel Cilk Plus. Intel has also released a specification to enable other compatible implementations, and has said the trademark will be usable by compliant implementations.

In the original MIT Cilk implementation, the first Cilk keyword is in fact cilk, which identifies a function which is written in Cilk. Since Cilk procedures can call C procedures directly, but C procedures cannot directly call or spawn Cilk procedures, this keyword is needed to distinguish Cilk code from C code.

The remaining keywords are:

They are described in further detail below.

The two remaining Cilk keywords are slightly more advanced, and concern the use of inlets. Ordinarily, when a Cilk procedure is spawned, it can return its results to the parent procedure only by putting those results in a variable in the parent’s frame, as we assigned the results of our spawned procedure calls in the example to x and y.

The alternative is to use an inlet. An inlet is a function internal to a Cilk procedure which handles the results of a spawned procedure call as they return. One major reason to use inlets is that all the inlets of a procedure are guaranteed to operate atomically with regards to each other and to the parent procedure, thus avoiding the bugs that could occur if the multiple returning procedures tried to update the same variables in the parent frame at the same time.

inlet — This keyword identifies a function defined within the procedure as an inlet.

abort — This keyword can only be used inside an inlet; it tells the scheduler that any other procedures that have been spawned off by the parent procedure can safely be aborted.