View source for Cross Compiling C and Fortran

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Recently I had to mix two large models written in C and Fortran (95 standard) to build a single monolithic program. In theory cross compilation is easy -- in practice its far from it. Your run off the mill 'hello world' implemented in C and called from Fortran (or vice versa) is one thing, but marrying off two large programs (numerous code files in each) written in two languages is a whole different story. Following is an account of my experience: 

;Summary: What worked for me 1) Sun Studio 11 works brilliantly. It's only available on Linux & Solaris platforms (not on Windows), but that is what got me though. (Free as in beer) 2) Intel compilers are also good. But, getting them to work effectively is a real pain. Download multiple gigabyte Microsoft Visual Studio 90 day trial from Microsoft (Even if you don't want to use their compilers!), then download C and Fortran compiler trial versions from Intel!! 3) Eclipse CDT/Photran (no that is not a spelling mistake!) is nice. But, you must know your way around configuring eclipse -- its a massive system that includes everything plus the Kitchen Sink. But, the good news is it is Free and Open Source (Free as in speech). 

==First the Theory==

To repeat, cross compiling should simply work -- in theory. But in practice, there are numerous points to get straight before anything works. The following two web sites helped me a great deal for learning the 'rules'. 
# [http://www.yolinux.com/TUTORIALS/LinuxTutorialMixingFortranAndC.html Using C/C++ and Fortran together]
# [http://www.math.utah.edu/software/c-with-fortran.html Using C and C++ with Fortran]

Most important things I learned: 

* Fortran compiler and C compiler does [[wikipedia:name mangling|name mangling]] in very different ways. It totally depends on the specific compiler and flags used. One common pattern is
<table border=".5">

              <tr  >
                <th>FORTRAN</th>
                <th>C</th>

              </tr>
              <tr>
                <td>CALL FOO( K, value)</td>
                <td>void foo_(int * bar, double * bat)</td>
              </tr>
</table>

* Fortran does not know the difference between UPPER and lower case. Often you have to tell the compiler to use lowercase and make sure all your c functions are written in lowercase. 
* Fortran always pass arguments by reference. And in C these look as memory addresses. (Notice the dereference on the C side in above table.)
* C function returns does not mean a thing to Fortran. Values should be sent using (pointer) arguments. 

<table border=".5">

              <tr  >
                <th>C -- incorrect</th>
                <th>C -- Correct</th>
                 <th>Fortran -- Calling</th>
              </tr>
              <tr>
                <td>double foo_(int * k)</td>
                <td>void foo_(int * k, double * p)</td>
                <td>CALL FOO(valin, valout) </td>
              </tr>
</table>

For example look at the C function get_swmm_dt and how it is called from Fortran.
;C:
<source lang="c" line start="10" highlight="5">
void get_swmm_dt_(double * dt){
	*dt=RouteStep;
}
</source>
;Fortran:
<source lang="fortran" line start="100" highlight="5">

real*8 swmmdt 
...
call get_swmm_dt(swmmdt)
</source>