\subsection{Data Number}
In some points of its life SICS has to write data files. The file names 
usually consist of a header, a serial number and an indicator for the 
year. The serial number must be unique and steadliy increasing. In order to
achieve this, the serial number are written into a file after any change.
Incrementing the serial number thus involves the following steps:
\begin{itemize}
\item Open file and read current number.
\item Increment number
\item Write File and close
\end{itemize}
This little task is implemented in this module.

The interface to this looks like:
\begin{flushleft} \small
\begin{minipage}{\linewidth} \label{scrap1}
$\langle$dni {\footnotesize ?}$\rangle\equiv$
\vspace{-1ex}
\begin{list}{}{} \item
\mbox{}\verb@@\\
\mbox{}\verb@    typedef struct __DataNumber *pDataNumber;@\\
\mbox{}\verb@@\\
\mbox{}\verb@    pDataNumber CreateDataNumber(char *pFilename);@\\
\mbox{}\verb@    void        DeleteDataNumber(void *pData);@\\
\mbox{}\verb@@\\
\mbox{}\verb@    int IncrementDataNumber(pDataNumber self, int *iYear);@\\
\mbox{}\verb@@\\
\mbox{}\verb@    int DecrementDataNumber(pDataNumber self);@\\
\mbox{}\verb@@\\
\mbox{}\verb@    int DNWrapper(SConnection *pCon, SicsInterp *pSics, void *pData,@\\
\mbox{}\verb@                  int argc, char *argv[]);@\\
\mbox{}\verb@@\\
\mbox{}\verb@    int DEWrapper(SConnection *pCon, SicsInterp *pSics, void *pData,@\\
\mbox{}\verb@                  int argc, char *argv[]);@\\
\mbox{}\verb@@\\
\mbox{}\verb@    int DNFactory(SConnection *pCon, SicsInterp *pSics, void *pData,@\\
\mbox{}\verb@                  int argc, char *argv[]);@\\
\mbox{}\verb@@$\diamond$
\end{list}
\vspace{-1ex}
\footnotesize\addtolength{\baselineskip}{-1ex}
\begin{list}{}{\setlength{\itemsep}{-\parsep}\setlength{\itemindent}{-\leftmargin}}
\item Macro referenced in scrap ?.
\end{list}
\end{minipage}\\[4ex]
\end{flushleft}
\begin{description}
\item [CreateDataNumber] creates a data number data structure. It checks if
the file requested as parameter exists and asserts if not. Returns a pointer
on success, NULL else.
\item [DeleteDataNumber] deletes a data number structure form memory. 
\item [IncrementDataNumber] is the main working function of this module.
It performs the steps listed above. It returns a new id for the data number
in case of success, a negative value otherwise. iYear is filled with a value
for the year. 
\item[DecrementDataNumber] decrements the data number and is used for 
 implementing the killdata function. Whis is the invalidation of a 
 data file by overwriting it.
\item[DNWrapper] is the wrapper function which makes DataNumber accessible
from within SICS. 
\item[DEWrapper] is the wrapper for the killdata functionality.
\item [DNFactory] is the SICS factory function which creates a Data Number
object from the initialisation file. Only parameter is the filename.
\end{description}

\begin{flushleft} \small
\begin{minipage}{\linewidth} \label{scrap2}
\verb@"danu.h"@ {\footnotesize ? }$\equiv$
\vspace{-1ex}
\begin{list}{}{} \item
\mbox{}\verb@@\\
\mbox{}\verb@/*-----------------------------------------------------------------------@\\
\mbox{}\verb@                        D A T A N U M B E R@\\
\mbox{}\verb@@\\
\mbox{}\verb@  A module to provide a unique data number for data file writing.@\\
\mbox{}\verb@@\\
\mbox{}\verb@  Mark Koennecke, Juli 1997@\\
\mbox{}\verb@  @\\
\mbox{}\verb@  copyright: see implementation file.@\\
\mbox{}\verb@@\\
\mbox{}\verb@---------------------------------------------------------------------------*/@\\
\mbox{}\verb@#ifndef SICSDATANUMBER@\\
\mbox{}\verb@#define SICSDATANUMBER@\\
\mbox{}\verb@@\\
\mbox{}\verb@@$\langle$dni {\footnotesize ?}$\rangle$\verb@@\\
\mbox{}\verb@@\\
\mbox{}\verb@#endif@\\
\mbox{}\verb@@$\diamond$
\end{list}
\vspace{-2ex}
\end{minipage}\\[4ex]
\end{flushleft}