use fftw3 with gfortran included in TDM-GCC-64 - fortran

Question

I am trying to use FFTW3 with gfortran compiler included in the TDM-GCC-64 suite in Windows 7 platform.
I have downloaded "fftw-3.3.4-dll64.zip" from the following page:
http://www.fftw.org/install/windows.html
I also created a fortran module described in the following tutorial:
http://www.fftw.org/doc/Defining-an-FFTW-module.html
Now, I tried to compile the following fortrans program
program test
use FFTW3
implicit none
integer N
parameter(N=4)
integer*8 plan
double complex in, out
dimension in(N),out(N)
integer i
write(*,*) 'Input array:'
do i = 1,N,1
in(i) = dcmplx(float(i),float(i+1))
write(*,*) ' in(',i,') = ',in(i)
enddo
call dfftw_plan_dft_1d ( plan, N, in, out, FFTW_FORWARD, FFTW_ESTIMATE )
call dfftw_execute ( plan )
write(*,*) 'Output array:'
do i = 1,N,1
write(*,*) ' out(',i,') = ',out(i)
enddo
call dfftw_destroy_plan ( plan )
call dfftw_plan_dft_1d ( plan, N, out, in, FFTW_FORWARD, FFTW_ESTIMATE )
call dfftw_execute ( plan )
write(*,*) 'Output array after inverse FFT:'
do i = 1,N,1
write(*,*) ' ',N,' * in(',i,') = ',in(i)
enddo
call dfftw_destroy_plan ( plan )
end
I tried to compile but the a couple of error messages popped up:
undefined reference to dfftw_plan_dft_1d_'
undefined reference todfftw_execute_'
...
The command used for compiling is:
gfortran test.f90 -ffree-form -o test_fftw.exe
I searched web up and down but have not found solutions to the problem. Could anyone help me out here? Thanks!!!

Answer 1

You need to learn the basics of compiling and linking programs with GCC
This tutorial refers to the GCC C compiler, gcc, and C++ compiler, g++, but the principles are the same for the Fortran compiler, gfortran.
Your program invokes functions, like dfftw_plan_dft_1d that are not defined
in your code but in the fftw3 library. You must therefore tell gfortran
to link that library with your program, after it is compiled, or no definitions will be found for
these functions, and no executable can be built. Instead, the linkage
fails with the undefined reference errors you are seeing.
The command you have used to build the program:
gfortran test.f90 -ffree-form -o test_fftw.exe
attempts to compile and link the program in a single command. You can do this,
if you clearly understand the underlying separate processes of compiling
and linking. If you don't, you should follow the better practice of compiling first,
and if that is successful, then linking. The command you have used fails in
its linkage step because it does not tell the linker that the fftw3 library
is needed (or where to find it) so no attempt is made to find it and link it.
You say you have downloaded fftw-3.3.4-dll64.zip. I presume you have
unzipped it somewhere on your system. Let's say you unzipped it to C:\fftw-3.3.4-dll64.
You also say you have made an FFTW3 module that apparently is being used successfully.
In that case, compile your program first:
gfortran -c -o test.o test.f90
generating an object file test.o. The option -c tells gfortran to compile only,
and not to link. (You do not need to specify --ffree-form: the file-extension .f90
implies it).
Next, link the object file test.o with the fttw3 library, to make an executable
program test_fftw.exe:
gfortran -o test_fftw.exe test.o -LC:\fftw-3.3.4-dll64 -lfftw3-3
The linker option:
-LC:\fftw-3.3.4-dll64
tells the linker to add C:\fftw-3.3.4-dll64 (where you unzipped the package)
to the list of directories that it will search to find libraries. And the linker option -lfftw3-3 tells it to link a library that matches the identifier fftw3-3. On Windows, the GCC linker
will match a library identfier name with any of the filenames:
libname.a (a static library)
libname.dll (a shared library)
name.dll (also a shared library)
name.lib (an import library for a shared library)
So, since C:\fftw-3.3.4-dll64 contains the shared library libfftw3-3.dll,
the linker will identify it in that directory as the one required by -lfftw3-3
Now, unless there are other problems, you program will link and you will have
an executable test_fftw.exe, which will run, provided that libfftw3-3.dll
can be found, at runtime, in one of the places where the Windows
loader will look for it.