Interfacing with C
NOTE! Have to do some spell checking, verify text, code and filenames.
Foreword
This is a tiny basic tutorial on how to write a simple library in C and then use it in FreeBASIC. The tutorial should be possible to follow without to much knowledge of C or FreeBASIC. After doing this tutorial you should be able to compile a static and a dynamically linked C library. Translate the necessary header files to FreeBASIC header files and understand how to use the libraries in a FreeBASIC project.
What is a library
Prerequisite
This tutorial was written and tested with FreeBASIC 0.16b and the latest Current release of MinGW32 at the time. As a note Dev-cpp uses MinGW32 as it's compiler tool chain. You also get code::blocks with a mingw32 bundle.
Formal description of the task at hand
To demonstrate usage of a C library in FreeBASIC we need to create the simplest possible library with a few functions. A test file in C to demonstrate that our library works as intended. Then we have to translate the library header file to a FreeBASIC header file (*.bi) and finally create a test project in FreeBASIC using the library.
Creating the files
So our file list will look like this:
myClib.c: C file implementing our library.
myClib.h: C header file describing the libraries interface.
myClibCTest.c: C file implementing our test program in C.
myClib.bi: FreeBASIC header file. A translation of myClib.h.
myClibFBTest.bas: FreeBASIC
make.cmd: A sample shell script compiling the library and test files.
The C file to become a static library. myClib.c

    (C)
    /* A function adding two integers and returning the result */
    #include "myClib.h"
    int SampleAddInt(int i1, int i2)
    {
    	return i1 + i2;
    }

    /* A function doing nothing ;) */
    void SampleFunction1()
    {
    	/* insert code here */

    }

    /* A function always returning zero */
    int SampleFunction2()
    {
    	/* insert code here */

    	return 10;
    }

The header file myClib.h

    (C)
    int  SampleAddInt(int i1, int i2);
    void SampleFunction1();
    int  SampleFunction2();

A C test project to verify that the static lib is C compatible. myClibCTest.c:

    (C)
    #include
    #include
    #include "myClib.h"
    int main(int argc, char *argv[])
    {
      printf("SampleAddInt(5, 5):=%d\n", SampleAddInt(5, 5));
      system("PAUSE");
      return 0;
    }

Translating the C header file to a FreeBASIC header file
myClib.bi: To interface the static library and automatically include it (#inclib "myClib" ) i have this file.
`
''include file for libmyClib.a
#ifndef __myClib_bi__
#define __myClib_bi__
#inclib "myClib"
Declare Function SampleAddInt cdecl Alias "SampleAddInt" (ByVal i1 As Integer, ByVal i2 As Integer) As Integer
Declare Sub SampleFunction1 cdecl Alias "SampleFunction1" ()
Declare Function SampleFunction2 cdecl Alias "SampleFunction2" () As Integer
#endif
`
And finally the FreeBASIC file using the library
myClibFBTest.bas:
`
''Testing functions in myClib.bi
#include "myClib.bi"
''
Print "SampleAddInt(10, 10):=", SampleAddInt(10, 10)
'' Just a dumy call
SampleFunction1()
''
Print "SampleFunction2():=", SampleFunction2()
`
The make file: make.cmd
I have created a batch file to compile all the files. Including a sample in C using the static library. Note the config lines at the top which has to be modified to suite your setup.

    (cmd)
    @REM TODO: Set PATH's for this session.
    SET PATH=C:\mingw32\bin;c:\mingw32\mingw32\bin
    SET MINGW_INCLUDE="C:/MinGW32/include"
    SET MINGW_LIB="C:/MinGW32/lib"

    @REM
    @REM fbc testing SET fbc="C:\portableapps\FreeBASIC\fbc.exe"
    SET fbc="C:\FreeBasic16b\fbc.exe"
    @echo *** Verify pat's to compilers
    @pause
    @echo off

    @REM
    @REM Remove old files
    DEL /F *.o  *.a myClibFBTest.exe

    @REM
    @REM Create static lib from c source
    gcc.exe -c myClib.c -o myClib.o -I%MINGW_INCLUDE%

    @REM
    @REM ar: creating libstatictest.a
    ar r libmyClib.a myClib.o

    @REM
    @REM No nead for ranlib anymore? ar is supposed to take care of it
    ranlib libmyClib.a

    @REM
    @REM Create a test with a C file

    gcc.exe -c myClibCTest.c -o myClibCTest.o -I%MINGW_INCLUDE%
    gcc.exe myClibCTest.o -o "myClibCTest.exe" -L%MINGW_LIB% libmyClib.a

    echo =====================================
    echo RUnning C sample
    echo =====================================
    myClibCTest.exe

    echo =====================================
    echo Creating FreeBASIC sample
    echo =====================================
    REM I thought this explicit reference is unnecessary as I use #inclib
    SET fbcop= -I myClib
    SET fbcfl="myClibFBTest.bas"
    %fbc% %fbcop% %fbcfl%
    echo =====================================
    echo RUnning FreeBASIC sample
    echo =====================================
    myClibFBTest.exe
    @pause

Encountered error messages and their solutions
undefined reference to
Trying to link against the static C library without using the cdecl alias "functionname" in the FreeBASIC header file results in errors like this.

    (cmd)
    C:\code>"C:\FreeBasic16b\fbc.exe"     "myClibFBTest.bas"
    myClibFBTest.o:fake:(.text+0x3d): undefined reference to `SAMPLEADDINT@8'
    myClibFBTest.o:fake:(.text+0x4a): undefined reference to `SAMPLEFUNCTION1@0'
    myClibFBTest.o:fake:(.text+0x67): undefined reference to `SAMPLEFUNCTION2@0'
    Press any key to continue . . .

To resolve this you will have to locate function declarations in a *.bi file that look like this:
`
Declare Function SampleAddInt(ByVal i1 As Integer, ByVal i2 As Integer) As Integer
`
And change it to something like this:
`
Declare Function SampleAddInt cdecl Alias "SampleAddInt" (ByVal i1 As Integer, ByVal i2 As Integer) As Integer
`
Appendix A: links
The basis for this tutorial is several threads in the forum.
When it evolves and can stand alone the links to the threads might be removed.
Some interesting links containing information on interfacing libraries created in FreeBASIC and used by other languages or visa versa.
How do I compile a C project as a static lib for inclusion..