# FFTW FAQ - Section 5
Known bugs

- Q5.1. FFTW 1.1 crashes in rfftwnd on Linux.
- Q5.2. The MPI transforms in FFTW 1.2 give incorrect results/leak memory.
- Q5.3. The test programs in FFTW 1.2.1 fail when I change FFTW to use single precision.
- Q5.4. The test program in FFTW 1.2.1 fails for n > 46340.
- Q5.5. The threaded code fails on Linux Redhat 5.0
- Q5.6. FFTW 2.0's rfftwnd fails for rank > 1 transforms with a final dimension >= 65536.
- Q5.7. FFTW 2.0's complex transforms give the wrong results with prime factors 17 to 97.
- Q5.8. FFTW 2.1.1's MPI test programs crash with MPICH.
- Q5.9. FFTW 2.1.2's multi-threaded transforms don't work on AIX.
- Q5.10. FFTW 2.1.2's complex transforms give incorrect results for large prime sizes.
- Q5.11. FFTW 2.1.3 crashes on AIX

## Question 5.1. FFTW 1.1 crashes in rfftwnd on Linux.

This bug was fixed in FFTW 1.2. There was a bug in `rfftwnd` causing an incorrect amount of memory to be allocated. The bug showed up in Linux with libc-5.3.12 (and nowhere else that we know of).

## Question 5.2. The MPI transforms in FFTW 1.2 give incorrect results/leak memory.

These bugs were corrected in FFTW 1.2.1. The MPI transforms (really, just the transpose routines) in FFTW 1.2 had bugs that could cause errors in some situations.

## Question 5.3. The test programs in FFTW 1.2.1 fail when I change FFTW to use single precision.

This bug was fixed in FFTW 1.3. (Older versions of FFTW did work in single precision, but the test programs didn't--the error tolerances in the tests were set for double precision.)

## Question 5.4. The test program in FFTW 1.2.1 fails for n > 46340.

This bug was fixed in FFTW 1.3. FFTW 1.2.1 produced the right answer, but the test program was wrong. For large n, n*n in the naive transform that we used for comparison overflows 32 bit integer precision, breaking the test.

## Question 5.5. The threaded code fails on Linux Redhat 5.0

We had problems with glibc-2.0.5. The code should work with glibc-2.0.7.

## Question 5.6. FFTW 2.0's rfftwnd fails for rank > 1 transforms with a final dimension >= 65536.

This bug was fixed in FFTW 2.0.1. (There was a 32-bit integer overflow due to a poorly-parenthesized expression.)

## Question 5.7. FFTW 2.0's complex transforms give the wrong results with prime factors 17 to 97.

There was a bug in the complex transforms that could cause incorrect results under (hopefully rare) circumstances for lengths with intermediate-size prime factors (17-97). This bug was fixed in FFTW 2.1.1.

## Question 5.8. FFTW 2.1.1's MPI test programs crash with MPICH.

This bug was fixed in FFTW 2.1.2. The 2.1/2.1.1 MPI test programs crashed when using the MPICH implementation of MPI with the `ch_p4` device (TCP/IP); the transforms themselves worked fine.

## Question 5.9. FFTW 2.1.2's multi-threaded transforms don't work on AIX.

This bug was fixed in FFTW 2.1.3. The multi-threaded transforms in previous versions didn't work with AIX's `pthreads` implementation, which idiosyncratically creates threads in detached (non-joinable) mode by default.

## Question 5.10. FFTW 2.1.2's complex transforms give incorrect results for large prime sizes.

This bug was fixed in FFTW 2.1.3. FFTW's complex-transform algorithm for prime sizes (in versions 2.0 to 2.1.2) had an integer overflow problem that caused incorrect results for many primes greater than 32768 (on 32-bit machines). (Sizes without large prime factors are not affected.)

## Question 5.11. FFTW 2.1.3 crashes on AIX

The FFTW 2.1.3 `configure` script picked incorrect compiler flags for the `xlc` compiler on newer IBM processors. This is fixed in FFTW 2.1.4.

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Extracted from FFTW Frequently Asked Questions with Answers, Copyright © 2026 Massachusetts Institute of Technology.