Compile and execute CoBOL with GraalVM and GnuCOBOL

GraalVM the polyglot Virtual Machine by Oracle

The GraalVM supports various languages, Java, JavaScript, Ruby, Python, R, WebAssembly, C/C++1. In another document2 Oracle explains the VM can be an interpreter from native codes using compiler called a Low-Level Virtual Machine (LLVM). Those native code being C, C++, FORTRAN, Rust, COBOL, and Go. It is now getting even more exiting! As a former mainframe/CoBOL developer, I would like to see those billion lines of CoBOL modernized in a more open environments.

CoBOL modernization

This article's goal is to explain how the GraalVM could be running CoBOL on pretty much any platform. IT departments, mainly in financial institutions and governments are desperately seeking CoBOL and mainframe experts but the lack of training course and the repelling Z/OS TSO environment are not encouraging vocations. Another major issue with those technologies resides in the costs of mainframe licenses. I think, hope, both could be solved developing CoBOL in modern environments. In case you don't know, rewriting the code and just shutting down the mainframes is not an option, see this great article.


Using Flex and Bison for lexical parsing, GnuCOBOL can transpile CoBOL to C. It can directly compile CoBOL using your platform toolchain but it is not our goal here, as we want to execute it with GraalVM. There are many CoBOL compilers out there. This one implement major part of CoBOL 1985, 2002 and several extensions of other compilers. This compiler and it's dependency libcob can easily be installed using synaptic package manager command apt-get install open-cobol. Open CoBOL is the former name of GnuCOBOL, it can be checked that way:

chrichri@chrichri-debian:~$ cobc -version
cobc (GnuCOBOL) 2.2.0
Copyright (C) 2017 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <>
This is free software; see the source for copying conditions.  There is NO
Written by Keisuke Nishida, Roger While, Ron Norman, Simon Sobisch, Edward Hart
Built     Jul 17 2018 20:29:40
Packaged  Sep 06 2017 18:48:43 UTC
C version "7.3.0"

Native code in GraalVM

If not already installed, GraalVM installation is described on their website. Executing native code require a GraalVM package call llvm-toolchain. On my system I already have clang and lli so I created symlinks clang-gu and lli-gu. I prefer to create a symlink with a different name for clang and lli executables rather that extending the path which will required the use of update-alternatives. A great medium post from an Oracle collaborator detail the GraalVM llvm-toolchain.

Compiling CoBOL C intermediate and execution

Let use the Mandelbrot set implemented in CoBOL in our example, see mandelbrot.cob:

identification division.
program-id. MandelbrotSet.

data division.
working-storage section.
01 ResolutionX             constant 240.
01 ResolutionY             constant 100.
01 RealPlaneMin            constant -2.5.
01 RealPlaneMax            constant 0.8.
01 ImaginaryPlaneMin    constant -1.25.
01 ImaginaryPlaneMax    constant 1.25.
01 ProportionalX           pic S99V9(16) usage comp-5 value zeros.
01 ProportionalY           pic S99V9(16) usage comp-5 value zeros.
01 IterationsMax           constant 60.
01 Threshold               constant 10000.

01 ScreenX                    pic 999 usage comp-5.
01 ScreenY                    pic 999 usage comp-5.
01 MathPlaneX              pic S99V9(16) usage comp-5.
01 MathPlaneY              pic S99V9(16) usage comp-5.

01 PointX                     pic S9(7)V9(8) usage comp-5.
01 PointY                     pic S9(7)V9(8) usage comp-5.
01 XSquared                pic S9(10)V9(8) usage comp-5.
01 YSquared                pic S9(10)V9(8) usage comp-5.
01 Iteration               pic 999 value zero.
01 TempVar                    pic S9(5)V9(8) usage comp-5.

procedure division.

compute ProportionalX = (RealPlaneMax - RealPlaneMin) /
    (ResolutionX - 1)
compute ProportionalY = (ImaginaryPlaneMax - ImaginaryPlaneMin) /
    (ResolutionY - 1)

perform varying ScreenY from 0 by 1 until ScreenY is equal to

    compute MathPlaneY = ImaginaryPlaneMin +
           (ProportionalY * ScreenY)

    perform varying ScreenX from 0 by 1 until ScreenX is equal to

           compute MathPlaneX = RealPlaneMin +
               (ProportionalX * ScreenX)

           move zero to PointX
           move zero to PointY
           multiply PointX by PointX giving XSquared
           multiply PointY by PointY giving YSquared

           perform with test after varying Iteration from 0 by 1
               until Iteration >= IterationsMax or
                        XSquared + YSquared >= Threshold
                    compute TempVar = XSquared - YSquared + MathPlaneX
                    compute PointY = 2 * PointX * PointY + MathPlaneY
                    move TempVar to PointX
                    compute XSquared = PointX * PointX
                    compute YSquared = PointY * PointY

           if Iteration is equal to IterationsMax
               display "*" with no advancing
               display " " with no advancing

    display " "

stop run.
end program MandelbrotSet.

Producing the C intermediate

Using GnuCOBOL, the C intermediate can be produced with the following command:

cobc -C -x mandelbrot.cob

The project should look like:

├── mandelbrot.c
├── mandelbrot.c.h
├── mandelbrot.c.l.h
└── mandelbrot.cob

Compiling C to LLVM bytcode

One point not completely clear from their documentation, is the difference with the regular Clang.


On contrary to regular Clang, Graal one compile directly to LLVM bytecode without -emit-llvm. Do not mix regular LLVM toolchain with GraalVM toolchain in general.

Don't forget to include the libcob dependency with -lcob, our compiling command is:

clang-gu mandelbrot.c -o bin/mandelbrot-graal.bc -lcob

The project should look like:

├── bin
│   └── mandelbrot-graal.bc
├── mandelbrot.c
├── mandelbrot.c.h
├── mandelbrot.c.l.h
└── mandelbrot.cob

Execution in the LLVM interpreter

The LLVM interpreter lli command:

lli-gu ./bin/mandelbrot-graal.bc

Christophe Brun,