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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> <HTML ><HEAD ><TITLE >Global Data in PL/Tcl</TITLE ><META NAME="GENERATOR" CONTENT="Modular DocBook HTML Stylesheet Version 1.79"><LINK REV="MADE" HREF="mailto:pgsql-docs@postgresql.org"><LINK REL="HOME" TITLE="PostgreSQL 9.2.24 Documentation" HREF="index.html"><LINK REL="UP" TITLE="PL/Tcl - Tcl Procedural Language" HREF="pltcl.html"><LINK REL="PREVIOUS" TITLE="Data Values in PL/Tcl" HREF="pltcl-data.html"><LINK REL="NEXT" TITLE="Database Access from PL/Tcl" HREF="pltcl-dbaccess.html"><LINK REL="STYLESHEET" TYPE="text/css" HREF="stylesheet.css"><META HTTP-EQUIV="Content-Type" CONTENT="text/html; charset=ISO-8859-1"><META NAME="creation" CONTENT="2017-11-06T22:43:11"></HEAD ><BODY CLASS="SECT1" ><DIV CLASS="NAVHEADER" ><TABLE SUMMARY="Header navigation table" WIDTH="100%" BORDER="0" CELLPADDING="0" CELLSPACING="0" ><TR ><TH COLSPAN="5" ALIGN="center" VALIGN="bottom" ><A HREF="index.html" >PostgreSQL 9.2.24 Documentation</A ></TH ></TR ><TR ><TD WIDTH="10%" ALIGN="left" VALIGN="top" ><A TITLE="Data Values in PL/Tcl" HREF="pltcl-data.html" ACCESSKEY="P" >Prev</A ></TD ><TD WIDTH="10%" ALIGN="left" VALIGN="top" ><A HREF="pltcl.html" ACCESSKEY="U" >Up</A ></TD ><TD WIDTH="60%" ALIGN="center" VALIGN="bottom" >Chapter 40. PL/Tcl - Tcl Procedural Language</TD ><TD WIDTH="20%" ALIGN="right" VALIGN="top" ><A TITLE="Database Access from PL/Tcl" HREF="pltcl-dbaccess.html" ACCESSKEY="N" >Next</A ></TD ></TR ></TABLE ><HR ALIGN="LEFT" WIDTH="100%"></DIV ><DIV CLASS="SECT1" ><H1 CLASS="SECT1" ><A NAME="PLTCL-GLOBAL" >40.4. Global Data in PL/Tcl</A ></H1 ><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 ></DIV ><DIV CLASS="NAVFOOTER" ><HR ALIGN="LEFT" WIDTH="100%"><TABLE SUMMARY="Footer navigation table" WIDTH="100%" BORDER="0" CELLPADDING="0" CELLSPACING="0" ><TR ><TD WIDTH="33%" ALIGN="left" VALIGN="top" ><A HREF="pltcl-data.html" ACCESSKEY="P" >Prev</A ></TD ><TD WIDTH="34%" ALIGN="center" VALIGN="top" ><A HREF="index.html" ACCESSKEY="H" >Home</A ></TD ><TD WIDTH="33%" ALIGN="right" VALIGN="top" ><A HREF="pltcl-dbaccess.html" ACCESSKEY="N" >Next</A ></TD ></TR ><TR ><TD WIDTH="33%" ALIGN="left" VALIGN="top" >Data Values in PL/Tcl</TD ><TD WIDTH="34%" ALIGN="center" VALIGN="top" ><A HREF="pltcl.html" ACCESSKEY="U" >Up</A ></TD ><TD WIDTH="33%" ALIGN="right" VALIGN="top" >Database Access from PL/Tcl</TD ></TR ></TABLE ></DIV ></BODY ></HTML >