# How to Bessel Functions in C

### From Difdopwiki

### The problem

- There is a basic implementation of the bessel functions in C/C++ in the gsl library. However this implementation does not work for complex arguments, and therefore is not very useful for a general complex problem.

- Slatec, which is a fortran library has this implemented for complex arguments for double precision, which is the case we are interested in.

- However Slatec is a huge, ancient beast, which uses some weird methods to figure out machine specifications. Or maybe it is just that it is written in fortran.

### The Solution

Here I will describe the steps I followed to rip the function zbesj.f from the slatec library and get it working as a standalone fortran library which contains just this file and its dependencies. Here we go.

**Step1:** The Netlib website has the nice feature that it allows you to download a file and its dependencies. Go to this link and click next to zbesj.f which says "zbesj.f plus dependencies". It will ask you if you want a zip, tgz or plain text file. Choose tgz (don't choose plain text, we will replace some of the files later on).
**Step2:** Here is how I extract and organize the fortran files. You decide your own organization:

mkdir amos cp ~/down/netlibfiles.tgz ./amos/ cd amos tar xvf netlibfiles.tgz mv slatec/src *.f ./ rm -rf slatec

**Step3:** The machine files i1mach.f and d1mach.f in slatec suck worse than slatec itself ( see this article for instance for a discussion). Apparently the ones in blas are newer. So download these files from netlib and overwrite the existing ones:

wget -N http://www.netlib.org/blas/d1mach.f wget -N http://www.netlib.org/blas/i1mach.f

**Step4:** In my case I was interested in making a static lib and a wrapper header which I would eventually include in my c project. So I actually used a automake file as part of the autoconf/automake scripts in the project. You can find a single Makefile I produced for testing here (Obviously this is not very portable in comparison to autoconf/automake). This will create a "libamos.a" when you type "make".

**Step5:** We need to first test that the thing actually works in fortran, before attempting to call it from c. So here is my attempt at a fortran program that prints out the value of J0(0.1+i*0.2):

program bestest integer nz,ierr double precision zr,zi,cyr,cyi external zbesj dimension cyr(4),cyi(4) zr=0.1d0 zi=0.2d0 call zbesj(zr,zi,0.0d0,1,1,cyr,cyi,nz,ierr) print *, cyr(1),cyi(1) end program

save this as bestest.f in that directory, so that you can compile it using

make test

with the above makefile. When I run it, I get this:

./bestest 1.0074890116830091 -1.00375190744146272E-002

to compare with the octave result:

octave:1> output_precision(17) octave:2> besselj(0,0.1+i*0.2) ans = 1.0074890116830091e+00 - 1.0037519074414627e-02i

**Step6:** Ok, so it works. We can add the function declaration to our header file, and link it with -lamos as well as -lgfortran. This obviuosly depend on your project organization. Here is a minimal c file for testing purposes:

#include <stdio.h> #include <complex.h> extern void zbesj_(double*, double*, double*, int*, int*, double*, double*, int*, int*); complex besselj(double nu, complex z){ int kode=1; int n=1; double zr=creal(z); double zi=cimag(z); int nz,ierr; double cyr[1],cyi[1]; complex res; zbesj_(&zr,&zi,&nu,&kode,&n,cyr,cyi,&nz,&ierr); if(ierr!=0){ printf("error!\n"); } res=cyr[0]+I*cyi[0]; return res; } int main(void){ complex J0=besselj(0.0,0.1+0.2*I); printf("\nJ0(0.1+0.2i)= %.17f %+.17f I\n",creal(J0),cimag(J0)); }

you can save this as "cbestest.c". This way, you can compile it using the above Makefile by typing "make ctest". Note that the above besselj function is a reasonable first approximation to a wrapper function. Here is the result:

./cbestest J0(0.1+0.2i)= 1.00748901168300908 -0.01003751907441463 I

note also that the little differences in the last digits are in fact due to output roundup.

if you want the result of all this as a tar.gz archive, here it is.

**Corollary:** If you want to use other similar functions (e.g. the modified bessel function), all you need to download is the netlib file corresponding to that function (e.g. zbesi.f), and add it similarly to the make files etc. (see above) most special functions are dependent on the same basic functions.