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></A
>Chapter 36. PL/Tcl - Tcl Procedural Language</H1
><DIV
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><DL
><DT
><B
>Table of Contents</B
></DT
><DT
>36.1. <A
HREF="pltcl.html#PLTCL-OVERVIEW"
>Overview</A
></DT
><DT
>36.2. <A
HREF="pltcl-functions.html"
>PL/Tcl Functions and Arguments</A
></DT
><DT
>36.3. <A
HREF="pltcl-data.html"
>Data Values in PL/Tcl</A
></DT
><DT
>36.4. <A
HREF="pltcl-global.html"
>Global Data in PL/Tcl</A
></DT
><DT
>36.5. <A
HREF="pltcl-dbaccess.html"
>Database Access from PL/Tcl</A
></DT
><DT
>36.6. <A
HREF="pltcl-trigger.html"
>Trigger Procedures in PL/Tcl</A
></DT
><DT
>36.7. <A
HREF="pltcl-unknown.html"
>Modules and the <CODE
CLASS="FUNCTION"
>unknown</CODE
> command</A
></DT
><DT
>36.8. <A
HREF="pltcl-procnames.html"
>Tcl Procedure Names</A
></DT
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><P
> PL/Tcl is a loadable procedural language for the
<SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> database system
that enables the <A
HREF="http://www.tcl.tk/"
TARGET="_top"
>Tcl</A
>
language to be used to write functions and
trigger procedures.
</P
><DIV
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><H1
CLASS="SECT1"
><A
NAME="PLTCL-OVERVIEW"
>36.1. Overview</A
></H1
><P
> PL/Tcl offers most of the capabilities a function
writer has in the C language, except for some restrictions.
</P
><P
> The good restriction is that everything is executed in a safe
Tcl interpreter. In addition to the limited command set of safe Tcl, only
a few commands are available to access the database via SPI and to raise
messages via <CODE
CLASS="FUNCTION"
>elog()</CODE
>. There is no way to access internals of the
database server or to gain OS-level access under the permissions of the
<SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> server process, as a C function can do.
Thus, any unprivileged database user may be
permitted to use this language.
</P
><P
> The other, implementation restriction is that Tcl functions cannot
be used to create input/output functions for new data types.
</P
><P
> Sometimes it is desirable to write Tcl functions that are not restricted
to safe Tcl. For example, one might want a Tcl function that sends
email. To handle these cases, there is a variant of <SPAN
CLASS="APPLICATION"
>PL/Tcl</SPAN
> called <TT
CLASS="LITERAL"
>PL/TclU</TT
>
(for untrusted Tcl). This is the exact same language except that a full
Tcl interpreter is used. <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>If <SPAN
CLASS="APPLICATION"
>PL/TclU</SPAN
> is used, it must be
installed as an untrusted procedural language</I
></SPAN
> so that only
database superusers can create functions in it. The writer of a <SPAN
CLASS="APPLICATION"
>PL/TclU</SPAN
>
function must take care that the function cannot be used to do anything
unwanted, since it will be able to do anything that could be done by
a user logged in as the database administrator.
</P
><P
> The shared object for the <SPAN
CLASS="APPLICATION"
>PL/Tcl</SPAN
> and <SPAN
CLASS="APPLICATION"
>PL/TclU</SPAN
> call handlers is
automatically built and installed in the
<SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
>
library directory if Tcl support is specified
in the configuration step of the installation procedure. To install
<SPAN
CLASS="APPLICATION"
>PL/Tcl</SPAN
> and/or <SPAN
CLASS="APPLICATION"
>PL/TclU</SPAN
> in a particular database, use the
<TT
CLASS="COMMAND"
>createlang</TT
> program, for example
<TT
CLASS="LITERAL"
>createlang pltcl <TT
CLASS="REPLACEABLE"
><I
>dbname</I
></TT
></TT
> or
<TT
CLASS="LITERAL"
>createlang pltclu <TT
CLASS="REPLACEABLE"
><I
>dbname</I
></TT
></TT
>.
</P
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