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>40.4. Global Data in PL/Tcl</A
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><P
> 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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