## 71.1. System Catalog Declaration Rules

 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 `CATALOG` macro, which so far as the C compiler is concerned is just shorthand for `typedef struct FormData_*`catalogname`*`. Each field in the struct gives rise to a catalog column. Fields can be annotated using the BKI property macros described in `genbki.h`, for example to define a default value for a field or mark it as nullable or not nullable. The `CATALOG` line can also be annotated, with some other BKI property macros described in `genbki.h`, to define other properties of the catalog as a whole, such as whether it is a shared relation.

 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 `pg_type`.`typrelid` to be NULL, it would fail when some piece of code tried to reference `typetup->typrelid` (or worse, `typetup->typelem`, because that follows `typrelid`). This would result in random errors or even segmentation violations.

 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 `#ifdef CATALOG_VARLEN` ... `#endif` (where `CATALOG_VARLEN` 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 `pg_attribute`. The bootstrap code will automatically mark catalog columns as `NOT NULL` 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 `BKI_FORCE_NOT_NULL` and `BKI_FORCE_NULL` annotations as needed.

 Frontend code should not include any `pg_xxx.h` 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 `src/backend/catalog/` files.) Instead, frontend code may include the corresponding generated `pg_xxx_d.h` header, which will contain OID `#define`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 `#ifdef EXPOSE_TO_CLIENT_CODE` ... `#endif` around that section to instruct `genbki.pl` to copy that section to the `pg_xxx_d.h` header.

 A few of the catalogs are so fundamental that they can't even be created by the BKI `create` 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 *bootstrap* 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 `pg_class` and `pg_type`, and those entries will need to be updated for subsequent changes to the catalog's structure. (Bootstrap catalogs also need pre-loaded entries in `pg_attribute`, but fortunately `genbki.pl` handles that chore nowadays.) Avoid making new catalogs be bootstrap catalogs if at all possible.