[LAD] malloc in fftw

Maarten de Boer mdb.list at resorama.com
Wed Apr 7 08:59:56 UTC 2010


I suppose that many of you are or might be using fftw for real-time audio processing, and I think the following information may be of your interest.

I just came to realize that fftw_execute calls malloc. This happens even though you create a 'plan' where you pass fftw_malloc preallocated input and output buffers.

It goes without saying that calling malloc is bad practice for audio callbacks, but as a reference I quote the jack_set_process_callback documentation:

 The code in the supplied function must be suitable for real-time execution.
 That means that it   cannot call functions that might block for a long time.
 This includes malloc, free, printf, pthread_mutex_lock, sleep, wait, poll,
 select, pthread_join, pthread_cond_wait, etc, etc.

I wrote the fftw developers and they confirmed the memory allocation takes place. They suggested the following:

> Try planning with the undocumented flag FFTW_NO_BUFFERING, which will
> prevent the use of malloc() in common cases.  This is not
> foolproof---some transforms are very hard to do without additional
> buffers, but if all you are trying to do is vanilla out-of-place 1D
> transforms without large prime factors it should work.


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