Friday 18 January 2019

Gambas: using external C libraries

It may be useful sometimes to use C-language library functions in your Gambas programs.

There may be a performance advantage in writing your own C functions or you may simply wish to use existing C libraries.

Here are my notes which may help you get started.

Following on from my earlier post on Gambas Libraries, this post uses simple C-code examples as a means of describing the principles of using C libraries.

I want to build an example where Gambas uses functions from 2 dynamic (shared) libraries (#1 & #2), and where library #1 also uses functions from library #2.

This is rather more complicated that using a single C library, but I hope this example will provide a better understanding of the subject.

the C code

The first library source file (mathlib1.c) has two functions:-
  • timesTwo; this just multiplies a number by 2
  • timesTwoPlusNine; multiplies a number by 2 then adds 9

The second library source file (mathlib2.c) has 1 function:-
  • plusNine; simply adds 9 to a number

// mathlib1.c
//trivial maths library

#include "mathlib1.h"
#include "mathlib2.h"

extern int plusNine (int anInteger);

int timesTwo (int anInteger)
   int x;
   x = 2 * anInteger;
   return x;

int timesTwoPlusNine (int anInteger)
   int a, b;
   a = 2 * anInteger;
   b = plusNine (a);
   return b;

As you can see, the timesTwoPlusNine(int) function depends upon an EXTERNal function: plusNine(int).

//trivial maths function

#include "mathlib1.h"
#include "mathlib2.h"

int plusNine (int anInteger)
   int x;
   x = anInteger + 9;
   return x;

We also need header files, mathlib1.h:-

extern int timesTwo (int anInteger) ;

...and mathlib2.h:-

extern int plusNine (int anInteger) ;

Both files need to be compiled, so open a terminal at the current directory and type:-

  gcc -fPIC -c mathlib1.c
  gcc -fPIC -c mathlib2.c

...which produces object files; mathlib1.o & mathlib2.o

Create a shared library from mathlib2.o:-

 gcc -shared -o mathlib2.o

Create a shared library from mathlib1.o linking with mathlib2.o:-

 gcc -shared -o mathlib1.o mathlib2.o

We should end up with a shared library with no dependency to mathlib1
and a shared library linked to mathlib2

Gambas test code

I started by creating a simple Gambas gui app with one ValueBox, one Button and three TextBoxes. Here are the declarations:-

 Library "/home/steve/C_stuff/codeexamples/mathlib1"

 Public Extern timesTwo(iNum As Integer)As Integer
 Public Extern timesTwoPlusNine(iNum As Integer)As Integer

 Library "/home/steve/C_stuff/codeexamples/mathlib2"

 Public Extern plusNine(iNum As Integer)As Integer

Note: the Library path applies to all external functions that follow, until the next stated Library path.

The rest of the Gambas code:-

Public Sub Form_Open()
  vbNumber.Value = 5

Public Sub Button1_Click()
  TextBox1.Text = timesTwo(vbNumber.Value)
  TextBox2.Text = plusNine(vbNumber.Value)
  TextBox3.Text = timesTwoPlusNine(vbNumber.Value)

When the Gambas project is run in the IDE you should get the expected results.


This example should illustrate how to compile, link and create shared (dynamic) C libraries for use with Gambas. Creating multiple libraries which have inter-dependencies is clearly more complicated than just linking to a single library. But I hope this example conveys what is required for both simple and more complex configurations.

And of course you can use existing C libraries such as the C Standard Library. You just need to study the library code and any documentation

See also:-


  1. Hi,
    thanks for this great article!

    I tried this and it works perfect. Now I want to use a external library (libmodbus) within the shared library for Gambas and I'm getting an error ("The program has returned the value: 127") not sure whats happening here... I then added the timesTwo() function to that shared library to see if this works and it does.

    As soon as I call the initializer for the modbus RTU communication I'm getting the messag 127.

    Any idea what I doing wrong?

    1. Error 127 sounds like a path issue, but I'd really like to see your code.
      Its not easy to post code here, so can I ask you to go over to the Gambas ONE forum ( and post your code & question there. I will then have a look and see what I can suggest to help you.

    2. Create a shared library with this:
      gcc -shared -o mathlib2.o mathlib1.o