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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 in PL/Tcl, but there are some restrictions that
     must be understood.
    </P
><P
>     For security reasons, PL/Tcl executes functions called by any one SQL
     role in a separate Tcl interpreter for that role.  This prevents
     accidental or malicious interference by one user with the behavior of
     another user's PL/Tcl functions.  Each such interpreter will have its own
     values for any <SPAN
CLASS="QUOTE"
>"global"</SPAN
> Tcl variables.  Thus, two PL/Tcl
     functions will share the same global variables if and only if they are
     executed by the same SQL role.  In an application wherein a single
     session executes code under multiple SQL roles (via <TT
CLASS="LITERAL"
>SECURITY
     DEFINER</TT
> functions, use of <TT
CLASS="COMMAND"
>SET ROLE</TT
>, etc) you may need to
     take explicit steps to ensure that PL/Tcl functions can share data.  To
     do that, make sure that functions that should communicate are owned by
     the same user, and mark them <TT
CLASS="LITERAL"
>SECURITY DEFINER</TT
>.  You must of
     course take care that such functions can't be used to do anything
     unintended.
    </P
><P
>     All PL/TclU functions used in a session execute in the same Tcl
     interpreter, which of course is distinct from the interpreter(s)
     used for PL/Tcl functions.  So global data is automatically shared
     between PL/TclU functions.  This is not considered a security risk
     because all PL/TclU functions execute at the same trust level,
     namely that of a database superuser.
    </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.  (Note that the <TT
CLASS="LITERAL"
>GD</TT
> arrays are only
     global within a particular interpreter, so they do not bypass the
     security restrictions mentioned above.)
    </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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