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4.2 Using Plans

Plans for all transform types in FFTW are stored as type fftw_plan (an opaque pointer type), and are created by one of the various planning routines described in the following sections. An fftw_plan contains all information necessary to compute the transform, including the pointers to the input and output arrays.

void fftw_execute(const fftw_plan plan);

This executes the plan, to compute the corresponding transform on the arrays for which it was planned (which must still exist). The plan is not modified, and fftw_execute can be called as many times as desired.

To apply a given plan to a different array, you can use the new-array execute interface. See section New-array Execute Functions.

fftw_execute (and equivalents) is the only function in FFTW guaranteed to be thread-safe; see Thread safety.

This function:

void fftw_destroy_plan(fftw_plan plan);

deallocates the plan and all its associated data.

FFTW’s planner saves some other persistent data, such as the accumulated wisdom and a list of algorithms available in the current configuration. If you want to deallocate all of that and reset FFTW to the pristine state it was in when you started your program, you can call:

void fftw_cleanup(void);

After calling fftw_cleanup, all existing plans become undefined, and you should not attempt to execute them nor to destroy them. You can however create and execute/destroy new plans, in which case FFTW starts accumulating wisdom information again.

fftw_cleanup does not deallocate your plans, however. To prevent memory leaks, you must still call fftw_destroy_plan before executing fftw_cleanup.

Occasionally, it may useful to know FFTW’s internal “cost” metric that it uses to compare plans to one another; this cost is proportional to an execution time of the plan, in undocumented units, if the plan was created with the FFTW_MEASURE or other timing-based options, or alternatively is a heuristic cost function for FFTW_ESTIMATE plans. (The cost values of measured and estimated plans are not comparable, being in different units. Also, costs from different FFTW versions or the same version compiled differently may not be in the same units. Plans created from wisdom have a cost of 0 since no timing measurement is performed for them. Finally, certain problems for which only one top-level algorithm was possible may have required no measurements of the cost of the whole plan, in which case fftw_cost will also return 0.) The cost metric for a given plan is returned by:

double fftw_cost(const fftw_plan plan);

The following two routines are provided purely for academic purposes (that is, for entertainment).

void fftw_flops(const fftw_plan plan, 
                double *add, double *mul, double *fma);

Given a plan, set add, mul, and fma to an exact count of the number of floating-point additions, multiplications, and fused multiply-add operations involved in the plan’s execution. The total number of floating-point operations (flops) is add + mul + 2*fma, or add + mul + fma if the hardware supports fused multiply-add instructions (although the number of FMA operations is only approximate because of compiler voodoo). (The number of operations should be an integer, but we use double to avoid overflowing int for large transforms; the arguments are of type double even for single and long-double precision versions of FFTW.)

void fftw_fprint_plan(const fftw_plan plan, FILE *output_file);
void fftw_print_plan(const fftw_plan plan);

This outputs a “nerd-readable” representation of the plan to the given file or to stdout, respectively.


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