FFTW3 example.

FFTW3.c

#include <complex.h>
#include <fftw3.h>
#include <stdio.h>

int main(void)
{
    int N = 8;
    double *in;
    fftw_complex *d_out;
    double *i_out;
    fftw_plan fft_plan_direct;
    fftw_plan fft_plan_inverse;

    in =  fftw_alloc_real(N); // (fftw_complex*) fftw_malloc(sizeof(fftw_complex)*N);
    d_out = fftw_alloc_complex(N); // (fftw_complex*) fftw_malloc(sizeof(fftw_complex)*N);
    i_out = fftw_alloc_real(N); // (fftw_complex*) fftw_malloc(sizeof(fftw_complex)*N);
    
    fft_plan_direct = fftw_plan_dft_r2c_1d(N, in, d_out, FFTW_PATIENT);
    fft_plan_inverse = fftw_plan_dft_c2r_1d(N, d_out, i_out, FFTW_PATIENT);

    // printf("plan1 \n");
    // fftw_print_plan(fft_plan_direct);
    // printf("\n");
    // printf("plan2 \n");
    // fftw_print_plan(fft_plan_inverse);
    // printf("\n");
    // printf("\n");

    printf("input array: ");
    for(int i=0; i<N ; i++)
    {
        in[i] = i;
        d_out[i] = 9+9*_Complex_I;
        printf("%.1f, ", in[i]);
    }
    printf("\n");

    fftw_execute(fft_plan_direct);

    printf("input array: ");
    for(int i=0; i<N ; i++)
        printf("%.1f, ", in[i]);
    printf("\n");
    
    printf("direct output array: ");
    for(int i=0; i<N ; i++)
        printf("%.1f + i%.1f, ", creal(d_out[i]), cimag(d_out[i]));
    printf("\n");

    fftw_execute(fft_plan_inverse);

    printf("direct output array: ");
    for(int i=0; i<N ; i++)
        printf("%.1f + i%.1f, ", creal(d_out[i]), cimag(d_out[i]));
    printf("\n");

    printf("inverse output array: ");
    for(int i=0; i<N ; i++)
    {
        i_out[i] *= 1./N;
        printf("%.1f, ", i_out[i]);
    }
    printf("\n");

    fftw_destroy_plan(fft_plan_direct);
    fftw_destroy_plan(fft_plan_inverse);
    fftw_free(in);
    fftw_free(d_out);
    fftw_free(i_out);
    return 0;
}

Makefile

TARGET = ex1
LIBS = -lfftw3 -lm
CC = gcc
CFLAGS = -g -Wall -fPIC #-Ofast -ffast-math

.PHONY: clean all default

default: $(TARGET)
all: default

OBJECTS = $(patsubst %.c, %.o, $(wildcard *.c))
HEADERS = $(wildcard *.h)

PROTO_OBJECTS = $(wildcard *.proto)

%.o: %.c $(DEPS)
	$(CC) -c -o $@ $< $(CFLAGS)

.PRECIOUS: $(TARGET) $(OBJECTS)

$(TARGET): $(OBJECTS)
	$(CC) $(OBJECTS) -Wall $(LIBS) -o $@

clean:
	-rm -f *.o
	-rm -f $(TARGET)