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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">“<span class="quote">global</span>”</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 <code class="literal">SECURITY
- DEFINER</code> functions, use of <code class="command">SET ROLE</code>, 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 <code class="literal">SECURITY DEFINER</code>. 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 <code class="literal">GD</code>. It is recommended that
- <code class="literal">GD</code> 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 <code class="literal">GD</code> arrays are only
- global within a particular interpreter, so they do not bypass the
- security restrictions mentioned above.)
- </p><p>
- An example of using <code class="literal">GD</code> appears in the
- <code class="function">spi_execp</code> example below.
- </p></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="pltcl-data.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="pltcl.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="pltcl-dbaccess.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">43.3. Data Values in PL/Tcl </td><td width="20%" align="center"><a accesskey="h" href="index.html">Home</a></td><td width="40%" align="right" valign="top"> 43.5. Database Access from PL/Tcl</td></tr></table></div></body></html>
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