44.1. Overview

44.1. Overview
Prev	Up	Chapter 44. PL/Tcl — Tcl Procedural Language	Home	Next

+ 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. +

+ One compelling good 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 elog(). PL/Tcl + provides no way to access internals of the database server or to + gain OS-level access under the permissions of the + PostgreSQL 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. +

+ The other notable implementation restriction is that Tcl functions + cannot be used to create input/output functions for new data + types. +

+ 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 PL/Tcl called PL/TclU + (for untrusted Tcl). This is exactly the same language except that a full + Tcl interpreter is used. If PL/TclU is used, it must be + installed as an untrusted procedural language so that only + database superusers can create functions in it. The writer of a PL/TclU + 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. +

+ The shared object code for the PL/Tcl and + PL/TclU call handlers is automatically built and + installed in the PostgreSQL library + directory if Tcl support is specified in the configuration step of + the installation procedure. To install PL/Tcl + and/or PL/TclU in a particular database, use the + CREATE EXTENSION command, for example + CREATE EXTENSION pltcl or + CREATE EXTENSION pltclu. +