Hi, 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: Maarten