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cannam@95: <h3 class="section">5.4 Thread safety</h3>
cannam@95:
cannam@95: <p><a name="index-threads-341"></a><a name="index-OpenMP-342"></a><a name="index-thread-safety-343"></a>Users writing multi-threaded programs (including OpenMP) must concern
cannam@95: themselves with the <dfn>thread safety</dfn> of the libraries they
cannam@95: use—that is, whether it is safe to call routines in parallel from
cannam@95: multiple threads. FFTW can be used in such an environment, but some
cannam@95: care must be taken because the planner routines share data
cannam@95: (e.g. wisdom and trigonometric tables) between calls and plans.
cannam@95:
cannam@95: <p>The upshot is that the only thread-safe (re-entrant) routine in FFTW is
cannam@95: <code>fftw_execute</code> (and the new-array variants thereof). All other routines
cannam@95: (e.g. the planner) should only be called from one thread at a time. So,
cannam@95: for example, you can wrap a semaphore lock around any calls to the
cannam@95: planner; even more simply, you can just create all of your plans from
cannam@95: one thread. We do not think this should be an important restriction
cannam@95: (FFTW is designed for the situation where the only performance-sensitive
cannam@95: code is the actual execution of the transform), and the benefits of
cannam@95: shared data between plans are great.
cannam@95:
cannam@95: <p>Note also that, since the plan is not modified by <code>fftw_execute</code>,
cannam@95: it is safe to execute the <em>same plan</em> in parallel by multiple
cannam@95: threads. However, since a given plan operates by default on a fixed
cannam@95: array, you need to use one of the new-array execute functions (see <a href="New_002darray-Execute-Functions.html#New_002darray-Execute-Functions">New-array Execute Functions</a>) so that different threads compute the transform of different data.
cannam@95:
cannam@95: <p>(Users should note that these comments only apply to programs using
cannam@95: shared-memory threads or OpenMP. Parallelism using MPI or forked processes
cannam@95: involves a separate address-space and global variables for each process,
cannam@95: and is not susceptible to problems of this sort.)
cannam@95:
cannam@95: <p>If you are configured FFTW with the <code>--enable-debug</code> or
cannam@95: <code>--enable-debug-malloc</code> flags (see <a href="Installation-on-Unix.html#Installation-on-Unix">Installation on Unix</a>),
cannam@95: then <code>fftw_execute</code> is not thread-safe. These flags are not
cannam@95: documented because they are intended only for developing
cannam@95: and debugging FFTW, but if you must use <code>--enable-debug</code> then you
cannam@95: should also specifically pass <code>--disable-debug-malloc</code> for
cannam@95: <code>fftw_execute</code> to be thread-safe.
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