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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>74.1. System Catalog Declaration Rules</title><link rel="stylesheet" type="text/css" href="stylesheet.css" /><link rev="made" href="pgsql-docs@lists.postgresql.org" /><meta name="generator" content="DocBook XSL Stylesheets Vsnapshot" /><link rel="prev" href="bki.html" title="Chapter 74. System Catalog Declarations and Initial Contents" /><link rel="next" href="system-catalog-initial-data.html" title="74.2. System Catalog Initial Data" /></head><body id="docContent" class="container-fluid col-10"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">74.1. System Catalog Declaration Rules</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="bki.html" title="Chapter 74. System Catalog Declarations and Initial Contents">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="bki.html" title="Chapter 74. System Catalog Declarations and Initial Contents">Up</a></td><th width="60%" align="center">Chapter 74. System Catalog Declarations and Initial Contents</th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 15.6 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="system-catalog-initial-data.html" title="74.2. System Catalog Initial Data">Next</a></td></tr></table><hr /></div><div class="sect1" id="SYSTEM-CATALOG-DECLARATIONS"><div class="titlepage"><div><div><h2 class="title" style="clear: both">74.1. System Catalog Declaration Rules</h2></div></div></div><p>
The key part of a catalog header file is a C structure definition
describing the layout of each row of the catalog. This begins with
a <code class="literal">CATALOG</code> macro, which so far as the C compiler is
concerned is just shorthand for <code class="literal">typedef struct
FormData_<em class="replaceable"><code>catalogname</code></em></code>.
Each field in the struct gives rise to a catalog column.
Fields can be annotated using the BKI property macros described
in <code class="filename">genbki.h</code>, for example to define a default value
for a field or mark it as nullable or not nullable.
The <code class="literal">CATALOG</code> line can also be annotated, with some
other BKI property macros described in <code class="filename">genbki.h</code>, to
define other properties of the catalog as a whole, such as whether
it is a shared relation.
</p><p>
The system catalog cache code (and most catalog-munging code in general)
assumes that the fixed-length portions of all system catalog tuples are
in fact present, because it maps this C struct declaration onto them.
Thus, all variable-length fields and nullable fields must be placed at
the end, and they cannot be accessed as struct fields.
For example, if you tried to
set <code class="structname">pg_type</code>.<code class="structfield">typrelid</code>
to be NULL, it would fail when some piece of code tried to reference
<code class="literal">typetup->typrelid</code> (or worse,
<code class="literal">typetup->typelem</code>, because that follows
<code class="structfield">typrelid</code>). This would result in
random errors or even segmentation violations.
</p><p>
As a partial guard against this type of error, variable-length or
nullable fields should not be made directly visible to the C compiler.
This is accomplished by wrapping them in <code class="literal">#ifdef
CATALOG_VARLEN</code> ... <code class="literal">#endif</code> (where
<code class="literal">CATALOG_VARLEN</code> is a symbol that is never defined).
This prevents C code from carelessly trying to access fields that might
not be there or might be at some other offset.
As an independent guard against creating incorrect rows, we
require all columns that should be non-nullable to be marked so
in <code class="structname">pg_attribute</code>. The bootstrap code will
automatically mark catalog columns as <code class="literal">NOT NULL</code>
if they are fixed-width and are not preceded by any nullable or
variable-width column.
Where this rule is inadequate, you can force correct marking by using
<code class="literal">BKI_FORCE_NOT_NULL</code>
and <code class="literal">BKI_FORCE_NULL</code> annotations as needed.
</p><p>
Frontend code should not include any <code class="filename">pg_xxx.h</code>
catalog header file, as these files may contain C code that won't compile
outside the backend. (Typically, that happens because these files also
contain declarations for functions
in <code class="filename">src/backend/catalog/</code> files.)
Instead, frontend code may include the corresponding
generated <code class="filename">pg_xxx_d.h</code> header, which will contain
OID <code class="literal">#define</code>s and any other data that might be of use
on the client side. If you want macros or other code in a catalog header
to be visible to frontend code, write <code class="literal">#ifdef
EXPOSE_TO_CLIENT_CODE</code> ... <code class="literal">#endif</code> around that
section to instruct <code class="filename">genbki.pl</code> to copy that section
to the <code class="filename">pg_xxx_d.h</code> header.
</p><p>
A few of the catalogs are so fundamental that they can't even be created
by the <acronym class="acronym">BKI</acronym> <code class="literal">create</code> command that's
used for most catalogs, because that command needs to write information
into these catalogs to describe the new catalog. These are
called <em class="firstterm">bootstrap</em> catalogs, and defining one takes
a lot of extra work: you have to manually prepare appropriate entries for
them in the pre-loaded contents of <code class="structname">pg_class</code>
and <code class="structname">pg_type</code>, and those entries will need to be
updated for subsequent changes to the catalog's structure.
(Bootstrap catalogs also need pre-loaded entries
in <code class="structname">pg_attribute</code>, but
fortunately <code class="filename">genbki.pl</code> handles that chore nowadays.)
Avoid making new catalogs be bootstrap catalogs if at all possible.
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