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#### 4.3.2 Planner Flags

All of the planner routines in FFTW accept an integer `flags` argument, which is a bitwise OR (``|`') of zero or more of the flag constants defined below. These flags control the rigor (and time) of the planning process, and can also impose (or lift) restrictions on the type of transform algorithm that is employed.

Important: the planner overwrites the input array during planning, so you should initialize your input data after creating the plan. The only exception to this is the `FFTW_ESTIMATE` flag, as mentioned below.

##### Planning-rigor flags

- `FFTW_ESTIMATE` specifies that, instead of actual measurements of different algorithms, a simple heuristic is used to pick a (probably sub-optimal) plan quickly. With this flag, the input/output arrays are not overwritten during planning.
- `FFTW_MEASURE` tells FFTW to find an optimized plan by actually computing several FFTs and measuring their execution time. Depending on your machine, this can take some time (often a few seconds). `FFTW_MEASURE` is the default planning option.
- `FFTW_PATIENT` is like `FFTW_MEASURE`, but considers a wider range of algorithms and often produces a “more optimal” plan (especially for large transforms), but at the expense of several times longer planning time (especially for large transforms).
- `FFTW_EXHAUSTIVE` is like `FFTW_PATIENT`, but considers an even wider range of algorithms, including many that we think are unlikely to be fast, to produce the most optimal plan but with a substantially increased planning time.

##### Algorithm-restriction flags

- `FFTW_DESTROY_INPUT` specifies that an out-of-place transform is allowed to overwrite its input array with arbitrary data; this can sometimes allow more efficient algorithms to be employed.
- `FFTW_PRESERVE_INPUT` specifies that an out-of-place transform must not change its input array. This is ordinarily the default, except for c2r and hc2r (i.e. complex-to-real) transforms for which `FFTW_DESTROY_INPUT` is the default. In the latter cases, passing `FFTW_PRESERVE_INPUT` will attempt to use algorithms that do not destroy the input, at the expense of worse performance; for multi-dimensional c2r transforms, however, no input-preserving algorithms are implemented and the planner will return `NULL` if one is requested.
- `FFTW_UNALIGNED` specifies that the algorithm may not impose any unusual alignment requirements on the input/output arrays (i.e. no SIMD may be used). This flag is normally not necessary, since the planner automatically detects misaligned arrays. The only use for this flag is if you want to use the guru interface to execute a given plan on a different array that may not be aligned like the original. (Using `fftw_malloc` makes this flag unnecessary even then.)

##### Limiting planning time

         extern void fftw_set_timelimit(double seconds);

This function instructs FFTW to spend at most `seconds` seconds (approximately) in the planner. If `seconds == FFTW_NO_TIMELIMIT` (the default value, which is negative), then planning time is unbounded. Otherwise, FFTW plans with a progressively wider range of algorithms until the the given time limit is reached or the given range of algorithms is explored, returning the best available plan. For example, specifying `FFTW_PATIENT` first plans in `FFTW_ESTIMATE` mode, then in `FFTW_MEASURE` mode, then finally (time permitting) in `FFTW_PATIENT`. If `FFTW_EXHAUSTIVE` is specified instead, the planner will further progress to `FFTW_EXHAUSTIVE` mode.

Note that the `seconds` argument specifies only a rough limit; in practice, the planner may use somewhat more time if the time limit is reached when the planner is in the middle of an operation that cannot be interrupted. At the very least, the planner will complete planning in `FFTW_ESTIMATE` mode (which is thus equivalent to a time limit of 0).