[R-pkg-devel] SystemRequirements & configure check for FFTW with single precision support

Lukas,

I have not seen the communication so I'm not commenting on that
specifically, I only looked at the GitHub link.

Although your configure could be improved (more below), it works well
enough to detect fftw3f.

Unfortunately, SystemRequirements don't have a well-defined structure, but
there are two commonly used notations:

a) library name and version so in your case that would be something like
libfftw3f (>=3.3.0)

b) deb/rpm package names
libfftw3-dev (deb), fftw3-devel (rpm)

fftw is a bit of a mess, because Debian ships the single-precision static
library in libfftw3-dev, but also has libfftw3-single3 which is the dynamic
version of the same, while RH does not distinguish. macOS recipe names are
usually included only if they are non-obvious, but that would be the case
here since installing fftw recipe does not work for you, so mentioning
fftw-s is probably a good idea (there are community scripts that try to
extract that information from the packages so that the dependencies can be
installed).

As for your configure, my main objection would be that it ignores CPPFLAGS
(they are not substituted at all even though they *are* used in the tests)
and doesn't use pkg-config to get the correct flags. (Also the brew part
should go - it's makes unwarranted assumptions [see below] and is entirely
superfluous if you use pkg-config instead.) To illustrate what I mean, you
get

$ pkg-config --cflags fftw3f
-I/opt/R/x86_64/include
$ pkg-config --libs fftw3f
-L/opt/R/x86_64/lib -lfftw3f -lm

while fCWTr on GitHub only uses

  Configuration for fCWTr 0.2.9000

    cppflags:
    cxxflags:
    ldflags:
    libs:     -lfftw3f

It actually works despite that, because R will inject the other flags for
you, but that will only work is fftw is installed in the same location as
the other libraries used by R (which is common, but not guaranteed). I
wouldn't say it is dealbreaker, but it would recommend it for robustness.

FWIW with homebrew the correct flags are obtainable from pkg-config:

$ pkg-config --libs fftw3f
-L/opt/brew/Cellar/fftw/3.3.8_1/lib -lfftw3f
$ pkg-config --cflags fftw3f
-I/opt/brew/Cellar/fftw/3.3.8_1/include

Cheers,
Simon

On Jan 11, 2025, at 10:37 PM, Lukas Schneiderbauer <

lukas.schneiderbauer at gmail.com> wrote:

Hi list,

I am working on getting a package <

https://github.com/lschneiderbauer/fCWTr> to

CRAN. It depends on the FFTW library <https://www.fftw.org/> that is

built

with single precision support. I am stuck in the submission process and I
require your help.

Before I come to my questions, some key facts about the package:

The package has an autoconf configure script that uses (among other

things,

like OpenMP checks, etc..) AC_SEARCH_LIBS to check whether required
functions of the library 'fftwf' exist, if yes, it adds the corresponding
compiler/linker flags; if no, it errs with a descriptive error message.

The CRAN Windows as well as Linux build service included fftwf in their
fftw build out of the box, and so building there was no problem, R CMD
Check passes there. In the past, building for MacOS was more trouble,

since

its fftw package does not include a single-precision build. I reached out
to Simon Urbanek, and he was so kind as to add an appropriate new recipe
"fftw-s" that provided an fftw version with single precision support. As

of

now, R CMD check also passes cleanly on the MacOS build service, thanks

to

Simon Urbanek's efforts.

Now, I am stuck at submission for two reasons:
1. The SystemRequirements specification in the DESCRIPTION file is
incorrect.
2. It is said that "the package needs a configure check for fftwf".

Add 1.
Initially, I had no mention of the "single precision" version of fftw,
because I thought it is included everywhere by default. It was stated

that

I need to add that information. I naturally complied.
This is my current version:
"SystemRequirements: fftw3 (including single precision support fftw3f),
fftw3f_omp (optional), OpenMP (optional)"
In the second subscription run, I was told to add "fftw-s" since I

require

the fftw-s package on MacOS. This does not make much sense to me since
"fftw-s" is only the name of this package on Simon Urbanek's MacOS build
service. The library file itself is still called fftwf, like it is on any
other platform. If I added "fftw-s", I would also need to explain that

this

is only valid for MacOS which seems to make the SystemRequirements
unnecessary verbose.
* Can someone explain the reason behind this request to me?
* How exactly should I add "fftw-s" to pass the submission process?

Add 2.
I tried to explain now for the second time in the submission notes, that

a

check is already in place (see the AC_SEARCH_LIBS paragraph above). But

my

explanation gets ignored.
* What am I doing wrong?
* What additional configure checks do I need to add to the package?

Thanks a lot for your help!
Sincerely, Lukas Schneiderbauer

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