3 The C Part

Program 2.1 shows a first attempt to provide access to the getenv C library function as explained above. In the following we will go through it in detail and successively improve it.

Every C program that will be linked to Oz must include the header file mozart.h (line 1 in Program 2.1), which is located in the include subdirectory of the Oz installation directory. It contains the definition of the data structures and functions used to interface C to Oz. All these data structures start with OZ_ such that they will not clash with the user's name space.

To declare a C function that can be used from Oz you have to enclose its declaration into the macros OZ_BI_define(name,inarity,outarity) and OZ_BI_end . The following code fragment declares a C-function BIgetenv for inclusion into Oz, which has one input an one output argument:


Oz represents data like strings, integers, floats, etc. different than C. For this reason mozart.h provides type testing functions and routines to convert from the C into the Oz representation and vice versa. All Oz values are summarized in one abstract C data type called OZ_Term .

To signal whether the call to a C function was successful or not it must return a value of type OZ_Return, which may be OZ_FAILED (in which case failure will occur), OZ_ENTAILED to signal successful completion. OZ_Return also contains several other values, which are not visible to the user. They are only used for internal purposes, for example to handle suspension or raising exceptions.


It is important that your C function returns one of the above values. Not returning a value will inevitably crash the executing Oz engine!

In line 5 we use the macro OZ_declareAtom(n,name): it checks whether the n-th input argument is an atom and declares a new C++ variable of type char* with name name. So line 5 declares envVarName which holds the C++ string representation of the first and only input argument.

Values that can be passed as input arguments can of course be logic variables! Do not confuse this with output arguments: Output arguments are handled only after the C function returns!

In line 7 we declare a C++ variable envValue which will temporarily hold the return value of BIgetenv.

In lines 9--11 of Program 2.1 we check whether an environment variable of the requested name exists and return OZ_FAILED if not.

Line 19 loads the result into the output argument of BIgetenv using the macro OZ_RETURN_ATOM: it converts its argument to an Oz atom assigns the first output register to that value and leaves the function with OZ_ENTAILED signalling that the call to BIgetenv was success full.

Lines 16--23 in Program 2.1 are needed for the linking step and will be explained in Section 4.3.

Michael Mehl, Tobias Müller, Christian Schulte and Ralf Scheidhauer
Version 1.4.0 (20080702)