# 2 A small Example

> `#include "mozart.h" // 1
> // 2
> OZ_BI_define(BIgetenv,1,1) // 3
> { // 4
> OZ_declareAtom(0,envVarName); // 5
> // 6
> char *envValue = getenv(envVarName); // 7
> // 8
> if (envValue == 0) /* not defined in environment */// 9
> return OZ_FAILED; //10
> //11
> OZ_RETURN_ATOM(envValue); //12
> } OZ_BI_end //13
> //14
> //15
> OZ_C_proc_interface oz_interface[] = { //16
> {"getenv",1,1,BIgetenv}, //17
> {0,0,0,0} //18
> }; //19
> //20
> OZ_C_proc_interface *oz_init_module() { //21
> return oz_interface; //22
> } //23`

Program 2.1: File `getenv.cc`: a C++ program to provide a getenv for Oz

Suppose we want to provide an Oz native module `Goodies` containing a single procedure `{Goodies.getenv VarA ValueA}` as an interface to the C library function `getenv(3)`: it constrains `ValueA` to an atom, which is the value of the environment variable `VarA`, where `VarA` is an atom. Thus `{Goodies.getenv 'HOME' X}` will constrain `X` to an atom representing the path to our home directory. To realize this we have to perform the following steps:

1. Write a piece of C code.

2. Create an object file from the C code.

3. Create a dynamic library from the object file(s).

4. Link the library into Oz.

These steps are explained in more detail below.