From: Tom Lane <tgl@sss.pgh.pa.us>
To: Thomas Thai <thomas.t.thai@gmail.com>
cc: pgsql-bugs@lists.postgresql.org
Subject: Re: PostgreSQL 18 beta1 - Segmentation fault on custom type casting
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Comments: In-reply-to Thomas Thai <thomas.t.thai@gmail.com>
	message dated "Wed, 09 Jul 2025 21:35:56 -0700"
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Thomas Thai <thomas.t.thai@gmail.com> writes:
> PostgreSQL 18 beta crashes with a segmentation fault when casting strings
> to custom types. The crash occurs specifically in PostgreSQL's type-casting
> system, not in extension code.

The reason you're having a problem is that this is not a valid way to
store a custom type.  The representation has to be a single blob of
memory [1], and it has to be represented in a way that lets
type-independent code determine its length.  Typically that means
following the varlena-header rules.

			regards, tom lane

[1] Well, there is a notion of an "expanded" representation that
doesn't have to be a flat blob.  But you're not following the
rules for that either.