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>39.4. Global Data in PL/Tcl</A
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>     Sometimes it
     is useful to have some global data that is held between two
     calls to a function or is shared between different functions.
     This is easily done since
     all PL/Tcl functions executed in one session share the same
     safe Tcl interpreter.  So, any global Tcl variable is accessible to
     all PL/Tcl function calls and will persist for the duration of the
     SQL session.  (Note that <SPAN
CLASS="APPLICATION"
>PL/TclU</SPAN
> functions likewise share
     global data, but they are in a different Tcl interpreter and cannot
     communicate with PL/Tcl functions.)
    </P
><P
>     To help protect PL/Tcl functions from unintentionally interfering
     with each other, a global
     array is made available to each function via the <CODE
CLASS="FUNCTION"
>upvar</CODE
>
     command. The global name of this variable is the function's internal
     name, and the local name is <TT
CLASS="LITERAL"
>GD</TT
>.  It is recommended that
     <TT
CLASS="LITERAL"
>GD</TT
> be used
     for persistent private data of a function.  Use regular Tcl global
     variables only for values that you specifically intend to be shared among
     multiple functions.
    </P
><P
>     An example of using <TT
CLASS="LITERAL"
>GD</TT
> appears in the
     <CODE
CLASS="FUNCTION"
>spi_execp</CODE
> example below.
    </P
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