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>40.1. Overview</A
></H1
><P
> PL/Tcl offers most of the capabilities a function writer has in
the C language, with a few restrictions, and with the addition of
the powerful string processing libraries that are available for
Tcl.
</P
><P
> One compelling <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>good</I
></SPAN
> restriction is that
everything is executed from within the safety of the context of a
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
>. PL/Tcl
provides 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, unprivileged database users can be trusted
to use this language; it does not give them unlimited authority.
</P
><P
> The other notable 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 exactly the 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 code 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"
>CREATE EXTENSION</TT
> command or 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
>.
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