Before using pgtcl commands, you must load libpgtcl into your Tcl application. This is normally done with the Tcl load command. Here is an example:
load libpgtcl[info sharedlibextension]
The use of info sharedlibextension is recommended in preference to hard-wiring .so or .sl into the program.
The load command will fail unless the system's dynamic loader knows where to look for the libpgtcl shared library file. You may need to work with ldconfig, or set the environment variable LD_LIBRARY_PATH, or use some equivalent facility for your platform to make it work. Refer to the PostgreSQL installation instructions for more information.
libpgtcl in turn depends on libpq, so the dynamic loader must also be able to find the libpq shared library. In practice this is seldom an issue, since both of these shared libraries are normally stored in the same directory, but it can be a stumbling block in some configurations.
If you use a custom executable for your application, you might choose to statically bind libpgtcl into the executable and thereby avoid the load command and the potential problems of dynamic linking. See the source code for pgtclsh for an example.