# Part 2 - Wrtiting code to interface with MESA¶

Now that we can get things compiling outside of MESA, we’ll take a look inside the sample_eos.f file to see how it interfaces with MESA’s EOS module. The first thing you should notice is the include statements at the top.

use eos_def
use eos_lib
use chem_def
use chem_lib
use const_lib
use crlibm_lib

Each module has a library corresponding to a def (mostly module-level variables declarations, eg. the vector of results you get from querying the EOS module) and a lib (mostly subroutines that to the actual work of the module). If you want to use a module’s variables in your code you’ll need a use *_def statement at the beginning of your code, and if you want to use the subroutines or functions in a module, you’ll need a use *_lib for the appropriate module.

Why do you think we’re using const_lib, but not const_def here?

If you want to take a look at the things you have access to in a module, you can look in the two files (eg. in the EOS module)

$MESA_DIR/eos/public/eos_def.f90$MESA_DIR/eos/public/eos_lib.f90

For example, notice the integer num_eos_basic_results that defines the size of the EOS result arrays res, d_dlnd, d_dlnT, d_dabar, d_dzbar is not defined in the local code - it’s definition lies in eos_def.f90.

The basic structure of the sample_eos.f code is to initialize all the modules. Most modules, even const, need an explicit call to an initialization subroutine that sets up all the module data, allocates arrays, etc. There are corresponding shutdown subroutines that should be called at the end of your program in order to free the allocated memory.

## Mini-exercise 1¶

Take a look at the result arrays that are returned by the subroutine eosDT_get (look at its documentation to figure out what’s going on). Some of the data is printed out to the screen already (Pgas, grad_ad, and c_P). Add statements to print out a few other quantities, the specific entropy and eta (the electron degeneracy parameter), as well as dlnS_dlnT.

## Mini-exercise 2¶

Try the same thing with the net module. Make a directory next to your eos_local directory and put the sample_net code in it, compile it, and run it. hint: you can use nearly the same makefile you used for the EOS code, but you’ll also have to change the $(FCfixed) to$(FCfree) in the last line.