libctf: add CTF format specification

It's been a long time since most of this was written: it's long past
time to put it in the binutils source tree.  It's believed correct and
complete insofar as it goes: it documents format v3 (the current
version) but not the libctf API or any earlier versions.  (The
earlier versions can be read by libctf but not generated by it, and you
are highly unlikely ever to see an example of any of them.)

libctf/ChangeLog
2021-11-08  Nick Alcock  <nick.alcock@oracle.com>

	* doc/ctf-spec.texi: New file.
	* configure.ac (MAKEINFO): Add.
	(BUILD_INFO): Likewise.
	(AC_CONFIG_FILES) [doc/Makefile]: Add.
	* Makefile.am [BUILD_INFO] (SUBDIRS): Add doc/.
	* doc/Makefile.am: New file.
	* doc/Makefile.in: Likewise.
	* configure: Regenerated.
	* Makefile.in: Likewise.

3 files changed, 2578 insertions, 0 deletions

diff --git a/libctf/doc/Makefile.am b/libctf/doc/Makefile.am
new file mode 100644
index 0000000..fb6b0a8
--- /dev/null
+++ b/libctf/doc/Makefile.am
@@ -0,0 +1,33 @@
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+# the Free Software Foundation; either version 2 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program; see the file COPYING.  If not see
+# <http://www.gnu.org/licenses/>.
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+ctf-spec/index.html: ctf-spec.texi
+	$(AM_V_at)$(MAKEINFOHTML) $(AM_MAKEINFOHTMLFLAGS) $(MAKEINFOFLAGS) \
+	  --split=node -I$(srcdir) $(srcdir)/ctf-spec.texi
diff --git a/libctf/doc/Makefile.in b/libctf/doc/Makefile.in
new file mode 100644
index 0000000..6e4833c
--- /dev/null
+++ b/libctf/doc/Makefile.in
@@ -0,0 +1,808 @@
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diff --git a/libctf/doc/ctf-spec.texi b/libctf/doc/ctf-spec.texi
new file mode 100644
index 0000000..78c7aed
--- /dev/null
+++ b/libctf/doc/ctf-spec.texi
@@ -0,0 +1,1737 @@
+\input texinfo       @c                    -*- Texinfo -*-
+@setfilename ctf-spec.info
+@settitle The CTF File Format
+@ifnottex
+@validatemenus off
+@xrefautomaticsectiontitle on
+@end ifnottex
+@synindex fn cp
+@synindex tp cp
+@synindex vr cp
+
+@copying
+Copyright @copyright{} 2021 Free Software Foundation, Inc.
+
+Permission is granted to copy, distribute and/or modify this document
+under the terms of the GNU General Public License, Version 3 or any
+later version published by the Free Software Foundation.  A copy of the
+license is included in the section entitled ``GNU General Public
+License''.
+
+@end copying
+
+@dircategory Software development
+@direntry
+* CTF: (ctf-spec).         The CTF file format.
+@end direntry
+
+@titlepage
+@title The CTF File Format
+@subtitle Version 3
+@author Nick Alcock
+
+@page
+@vskip 0pt plus 1filll
+@insertcopying
+@end titlepage
+@contents
+
+@ifnottex
+@node Top
+@top The CTF file format
+
+This manual describes version 3 of the CTF file format, which is
+intended to model the C type system in a fashion that C programs can
+consume at runtime.
+@end ifnottex
+
+@node Overview
+@unnumbered Overview
+@cindex Overview
+
+The CTF file format compactly describes C types and the association
+between function and data symbols and types: if embedded in ELF objects,
+it can exploit the ELF string table to reduce duplication further.
+There is no real concept of namespacing: only top-level types are
+described, not types scoped to within single functions.
+
+CTF dictionaries can be @dfn{children} of other dictionaries, in a
+one-level hierarchy: child dictionaries can refer to types in the
+parent, but the opposite is not sensible (since if you refer to a child
+type in the parent, the actual type you cited would vary depending on
+what child was attached).  This parent/child definition is recorded in
+the child, but only as a recommendation: users of the API have to attach
+parents to children explicitly, and can choose to attach a child to any
+parent they like, or to none, though doing so might lead to unpleasant
+consequences like dangling references to types.  @xref{Type indexes and
+type IDs}.  Type lookups in child dicts that are not associated with a
+parent at all will fail with @code{ECTF_NOPARENT} if a parent type was
+needed.
+
+The associated API to generate, merge together, and query this file
+format will be described in the accompanying @code{libctf} manual once
+it is written.  There is no API to modify dictionaries once they've been
+written out: CTF is a write-once file format.  (However, it is always
+possible to dynamically create a new child dictionary on the fly and
+attach it to a pre-existing, read-only parent.)
+
+There are two major pieces to CTF: the @dfn{archive} and the
+@dfn{dictionary}.  Some relatives and ancestors of CTF call dictionaries
+@dfn{containers}: the archive format is unique to this variant of CTF.
+(Much of the source code still uses the old term.)
+
+The archive file format is a very simple mmappable archive used to group
+multiple dictionaries together into groups: it is expected to slowly go
+away and be replaced by other mechanisms, but right now it is an
+important part of the file format, used to group dictionaries containing
+types with conflicting definitions in different TUs with the overarching
+dictionary used to store all other types.  (Even when archives go away,
+the @code{libctf} API used to access them will remain, and access the
+other mechanisms that replace it instead.)
+
+The CTF dictionary consists of a @dfn{preamble}, which does not vary
+between versions of the CTF file format, and a @dfn{header} and some
+number of @dfn{sections}, which can vary between versions.
+
+The rest of this specification describes the format of these sections,
+first for the latest version of CTF, then for all earlier versions
+supported by @code{libctf}: the earlier versions are defined in terms of
+their differences from the next later one.  We describe each part of the
+format first by reproducing the C structure which defines that part,
+then describing it at greater length in terms of file offsets.
+
+The description of the file format ends with a description of relevant
+limits that apply to it.  These limits can vary between file format
+versions.
+
+This document is quite young, so for now the C code in @file{ctf.h}
+should be presumed correct when this document conflicts with it.
+
+@node CTF archive
+@chapter CTF archives
+@cindex archive, CTF archive
+
+The CTF archive format maps names to CTF dictionaries.  The names may
+contain any character other than \0, but for now archives containing
+slashes in the names may not extract correctly.  It is possible to
+insert multiple members with the same name, but these are quite hard to
+access reliably (you have to iterate through all the members rather than
+opening by name) so this is not recommended.
+
+CTF archives are not themselves compressed: the constituent components,
+CTF dictionaries, can be compressed.  (@xref{CTF header}).
+
+CTF archives usually contain a collection of related dictionaries, one
+parent and many children of that parent.  CTF archives can have a member
+with a @dfn{default name}, @code{.ctf} (which can be represented as
+@code{NULL} in the API).  If present, this member is usually the parent
+of all the children, but it is possible for CTF producers to emit
+parents with different names if they wish (usually for backward-
+compatibility purposes).
+
+@code{.ctf} sections in ELF objects consist of a single CTF dictionary
+rather than an archive of dictionaries if and only if the section
+contains no types with identical names but conflicting definitions: if
+two conflicting definitions exist, the deduplicator will place the type
+most commonly referred to by other types in the parent and will place
+the other type in a child named after the translation unit it is found
+in, and will emit a CTF archive containing both dictionaries instead of
+a raw dictionary.  All types that refer to such conflicting types are
+also placed in the per-translation-unit child.
+
+The definition of an archive in @file{ctf.h} is as follows:
+
+@verbatim
+struct ctf_archive
+{
+  uint64_t ctfa_magic;
+  uint64_t ctfa_model;
+  uint64_t ctfa_nfiles;
+  uint64_t ctfa_names;
+  uint64_t ctfa_ctfs;
+};
+
+typedef struct ctf_archive_modent
+{
+  uint64_t name_offset;
+  uint64_t ctf_offset;
+} ctf_archive_modent_t;
+@end verbatim
+
+(Note one irregularity here: the @code{ctf_archive_t} is not a typedef
+to @code{struct ctf_archive}, but a different typedef, private to
+@code{libctf}, so that things that are not really archives can be made
+to appear as if they were.)
+
+All the above items are always in little-endian byte order, regardless
+of the machine endianness.
+
+The archive header has the following fields:
+
+@tindex struct ctf_archive
+@multitable {Offset} {@code{uint64_t ctfa_nfiles}} {The data model for this archive: an arbitrary integer}
+@headitem Offset @tab Name @tab Description
+@item 0x00
+@tab @code{uint64_t ctfa_magic}
+@vindex ctfa_magic
+@vindex struct ctf_archive, ctfa_magic
+@tab The magic number for archives, @code{CTFA_MAGIC}: 0x8b47f2a4d7623eeb.
+@tindex CTFA_MAGIC
+
+@item 0x08
+@tab @code{uint64_t ctfa_model}
+@vindex ctfa_model
+@vindex struct ctf_archive, ctfa_model
+@tab The data model for this archive: an arbitrary integer that serves no
+purpose but to be handed back by the libctf API. @xref{Data models}.
+
+@item 0x10
+@tab @code{uint64_t ctfa_nfiles}
+@vindex ctfa_nfiles
+@vindex struct ctf_archive, ctfa_nfiles
+@tab The number of CTF dictionaries in this archive.
+
+@item 0x18
+@tab @code{uint64_t ctfa_names}
+@vindex ctfa_names
+@vindex struct ctf_archive, ctfa_names
+@tab Offset of the name table, in bytes from the start of the archive.
+The name table is an array of @code{struct ctf_archive_modent_t[ctfa_nfiles]}.
+
+@item 0x20
+@tab @code{uint64_t ctfa_ctfs}
+@vindex ctfa_ctfs
+@vindex struct ctf_archive, ctfa_ctfs
+@tab Offset of the CTF table.  Each element starts with a @code{uint64_t} size,
+followed by a CTF dictionary.
+
+@end multitable
+
+The array pointed to by @code{ctfa_names} is an array of entries of
+@code{ctf_archive_modent}:
+
+@tindex struct ctf_archive_modent
+@tindex ctf_archive_modent_t
+@multitable {Offset} {@code{uint64_t name_offset}} {Offset of this name, in bytes from the start}
+@headitem Offset @tab Name @tab Description
+@item 0x00
+@tab @code{uint64_t name_offset}
+@vindex name_offset
+@vindex struct ctf_archive_modent, name_offset
+@vindex ctf_archive_modent_t, name_offset
+@tab Offset of this name, in bytes from the start of the archive.
+
+@item 0x08
+@tab @code{uint64_t ctf_offset}
+@vindex ctf_offset
+@vindex struct ctf_archive_modent, ctf_offset
+@vindex ctf_archive_modent_t, ctf_offset
+@tab Offset of this CTF dictionary, in bytes from the start of the archive.
+
+@end multitable
+
+The @code{ctfa_names} array is sorted into ASCIIbetical order by name
+(i.e. by the result of dereferencing the @code{name_offset}).
+
+The archive file also contains a name table and a table of CTF
+dictionaries: these are pointed to by the structures above.  The name
+table is a simple strtab which is not required to be sorted; the
+dictionary array is described above in the entry for @code{ctfa_ctfs}.
+
+The relative order of these various parts is not defined, except that
+the header naturally always comes first.
+
+@node CTF dictionaries
+@chapter CTF dictionaries
+@cindex dictionary, CTF dictionary
+
+CTF dictionaries consist of a header, starting with a premable, and a
+number of sections.
+
+@node CTF Preamble
+@section CTF Preamble
+
+The preamble is the only part of the CTF dictionary whose format cannot
+vary between versions.  It is never compressed.  It is correspondingly
+simple:
+
+@verbatim
+typedef struct ctf_preamble
+{
+  unsigned short ctp_magic;
+  unsigned char ctp_version;
+  unsigned char ctp_flags;
+} ctf_preamble_t;
+@end verbatim
+
+@code{#define}s are provided under the names @code{cth_magic},
+@code{cth_version} and @code{cth_flags} to make the fields of the
+@code{ctf_preamble_t} appear to be part of the @code{ctf_header_t}, so
+consuming programs rarely need to consider the existence of the preamble
+as a separate structure.
+
+@tindex struct ctf_preamble
+@tindex ctf_preamble_t
+@multitable {Offset} {@code{unsigned char ctp_version}} {The magic number for CTF dictionaries}
+@headitem Offset @tab Name @tab Description
+@item 0x00
+@tab @code{unsigned short ctp_magic}
+@vindex ctp_magic
+@vindex cth_magic
+@vindex ctf_preamble_t, ctp_magic
+@vindex struct ctf_preamble, ctp_magic
+@vindex ctf_header_t, cth_magic
+@vindex struct ctf_header, cth_magic
+@tab The magic number for CTF dictionaries, @code{CTF_MAGIC}: 0xdff2.
+@tindex CTF_MAGIC
+
+@item 0x02
+@tab @code {unsigned char ctp_version}
+@vindex ctp_version
+@vindex cth_version
+@vindex ctf_preamble_t, ctp_version
+@vindex struct ctf_preamble, ctp_version
+@vindex ctf_header_t, cth_version
+@vindex struct ctf_header, cth_version
+@tab The version number of this CTF dictionary.
+
+@item 0x03
+@tab @code{ctp_flags}
+@vindex ctp_flags
+@vindex cth_flags
+@vindex ctf_preamble_t, ctp_flags
+@vindex struct ctf_preamble, ctp_flags
+@vindex ctf_header_t, cth_flags
+@vindex struct ctf_header, cth_flags
+@tab Flags for this CTF file. @xref{CTF file-wide flags}.
+@end multitable
+
+@cindex alignment
+Every element of a dictionary must be naturally aligned unless otherwise
+specified.  (This restriction will be lifted in later versions.)
+
+@cindex endianness
+CTF dictionaries are stored in the native endianness of the system that
+generates them: the consumer (e.g., @code{libctf}) can detect whether to
+endian-flip a CTF dictionary by inspecting the @code{ctp_magic}. (If it
+appears as 0xf2df, endian-flipping is needed.)
+
+The version of the CTF dictionary can be determined by inspecting
+@code{ctp_version}.  The following versions are currently valid, and
+@code{libctf} can read all of them:
+
+@tindex CTF_VERSION_3
+@cindex CTF versions, versions
+@multitable {@code{CTF_VERSION_1_UPGRADED_3}} {Number} {First version, rare.  Very similar to Solaris CTF.}
+@headitem Version @tab Number @tab Description
+@item @code{CTF_VERSION_1}
+@tab 1 @tab First version, rare. Very similar to Solaris CTF.
+
+@item @code{CTF_VERSION_1_UPGRADED_3}
+@tab 2 @tab First version, upgraded to v3 or higher and written out again.
+Name may change. Very rare.
+
+@item @code{CTF_VERSION_2}
+@tab 3 @tab Second version, with many range limits lifted.
+
+@item @code{CTF_VERSION_3}
+@tab 4  @tab Third and current version, documented here.
+@end multitable
+
+This section documents @code{CTF_VERSION_3}.
+
+@vindex ctp_flags
+@node CTF file-wide flags
+@subsection CTF file-wide flags
+
+The preamble contains bitflags in its @code{ctp_flags} field that
+describe various file-wide properties.  Some of the flags are valid only
+for particular file-format versions, which means the flags can be used
+to fix file-format bugs.  Consumers that see unknown flags should
+accordingly assume that the dictionary is not comprehensible, and
+refuse to open them.
+
+The following flags are currently defined.  Many are bug workarounds,
+valid only in CTFv3, and will not be valid in any future versions: the
+same values may be reused for other flags in v4+.
+
+@multitable {@code{CTF_F_NEWFUNCINFO}} {Versions} {Value} {The external strtab is in @code{.dynstr} and the}
+@headitem Flag @tab Versions @tab Value @tab Meaning
+@tindex CTF_F_COMPRESS
+@item @code{CTF_F_COMPRESS} @tab All @tab 0x1 @tab Compressed with zlib
+@tindex CTF_F_NEWFUNCINFO
+@item @code{CTF_F_NEWFUNCINFO} @tab 3 only @tab 0x2
+@tab ``New-format'' func info section.
+@tindex CTF_F_IDXSORTED
+@item @code{CTF_F_IDXSORTED} @tab 3+ @tab 0x4 @tab The index section is
+in sorted order
+@tindex CTF_F_DYNSTR
+@item @code{CTF_F_DYNSTR} @tab 3 only @tab 0x8 @tab The external strtab is
+in @code{.dynstr} and the symtab used is @code{.dynsym}.
+@xref{The string section}
+@end multitable
+
+@code{CTF_F_NEWFUNCINFO} and @code{CTF_F_IDXSORTED} relate to the
+function info and data object sections. @xref{The symtypetab sections}.
+
+Further flags (and further compression methods) wil be added in future.
+
+@node CTF header
+@section CTF header
+@cindex CTF header
+@cindex Sections, header
+
+The CTF header is the first part of a CTF dictionary, including the
+preamble.  All parts of it other than the preamble (@pxref{CTF Preamble})
+can vary between CTF file versions and are never compressed.  It
+contains things that apply to the dictionary as a whole, and a table of
+the sections into which the rest of the dictionary is divided.  The
+sections tile the file: each section runs from the offset given until
+the start of the next section.  Only the last section cannot follow this
+rule, so the header has a length for it instead.
+
+All section offsets, here and in the rest of the CTF file, are relative to the
+@emph{end} of the header.  (This is annoyingly different to how offsets in CTF
+archives are handled.)
+
+This is the first structure to include offsets into the string table, which are
+not straight references because CTF dictionaries can include references into the
+ELF string table to save space, as well as into the string table internal to the
+CTF dictionary.  @xref{The string section} for more on these.  Offset 0 is
+always the null string.
+
+@verbatim
+typedef struct ctf_header
+{
+  ctf_preamble_t cth_preamble;
+  uint32_t cth_parlabel;
+  uint32_t cth_parname;
+  uint32_t cth_cuname;
+  uint32_t cth_lbloff;
+  uint32_t cth_objtoff;
+  uint32_t cth_funcoff;
+  uint32_t cth_objtidxoff;
+  uint32_t cth_funcidxoff;
+  uint32_t cth_varoff;
+  uint32_t cth_typeoff;
+  uint32_t cth_stroff;
+  uint32_t cth_strlen;
+} ctf_header_t;
+@end verbatim
+
+In detail:
+
+@tindex struct ctf_header
+@tindex ctf_header_t
+@multitable {Offset} {@code{ctf_preamble_t cth_preamble}} {The parent label, if deduplication happened against}
+@headitem Offset @tab Name @tab Description
+@item 0x00
+@tab @code{ctf_preamble_t cth_preamble}
+@vindex cth_preamble
+@vindex struct ctf_header, cth_preamble
+@vindex ctf_header_t, cth_preamble
+@tab The preamble (conceptually embedded in the header). @xref{CTF Preamble}
+
+@item 0x04
+@tab @code{uint32_t cth_parlabel}
+@vindex cth_parlabel
+@vindex struct ctf_header, cth_parlabel
+@vindex ctf_header_t, cth_parlabel
+@tab The parent label, if deduplication happened against a specific label: a
+strtab offset. @xref{The label section}. Currently unused and always 0, but may
+be used in future when semantics are attached to the label section.
+
+@item 0x08
+@tab @code{uint32_t cth_parname}
+@vindex cth_parname
+@vindex struct ctf_header, cth_parname
+@vindex ctf_header_t, cth_parname
+@tab The name of the parent dictionary deduplicated against: a strtab offset.
+Interpretation is up to the consumer (usually a CTF archive member name).  0
+(the null string) if this is not a child dictionary.
+
+@item 0x1c
+@tab @code{uint32_t cth_cuname}
+@vindex cth_cuname
+@vindex struct ctf_header, cth_cuname
+@vindex ctf_header_t, cth_cuname
+@tab The name of the compilation unit, for consumers like GDB that want to
+know the name of CUs associated with single CUs: a strtab offset.  0 if this
+dictionary describes types from many CUs.
+
+@item 0x10
+@tab @code{uint32_t cth_lbloff}
+@vindex cth_lbloff
+@vindex struct ctf_header, cth_lbloff
+@vindex ctf_header_t, cth_lbloff
+@tab The offset of the label section, which tiles the type space into
+named regions.  @xref{The label section}.
+
+@item 0x14
+@tab @code{uint32_t cth_objtoff}
+@vindex cth_objtoff
+@vindex struct ctf_header, cth_objtoff
+@vindex ctf_header_t, cth_objtoff
+@tab The offset of the data object symtypetab section, which maps ELF data symbols to
+types.  @xref{The symtypetab sections}.
+
+@item 0x18
+@tab @code{uint32_t cth_funcoff}
+@vindex cth_funcoff
+@vindex struct ctf_header, cth_funcoff
+@vindex ctf_header_t, cth_funcoff
+@tab The offset of the function info symtypetab section, which maps ELF function
+symbols to a return type and arg types.  @xref{The symtypetab sections}.
+
+@item 0x1c
+@tab @code{uint32_t cth_objtidxoff}
+@vindex cth_objtidxoff
+@vindex struct ctf_header, cth_objtidxoff
+@vindex ctf_header_t, cth_objtidxoff
+@tab The offset of the object index section, which maps ELF object symbols to
+entries in the data object section.  @xref{The symtypetab sections}.
+
+@item 0x20
+@tab @code{uint32_t cth_funcidxoff}
+@vindex cth_funcidxoff
+@vindex struct ctf_header, cth_funcidxoff
+@vindex ctf_header_t, cth_funcidxoff
+@tab The offset of the function info index section, which maps ELF function
+symbols to entries in the function info section. @xref{The symtypetab sections}.
+
+@item 0x24
+@tab @code{uint32_t cth_varoff}
+@vindex cth_varoff
+@vindex struct ctf_header, cth_varoff
+@vindex ctf_header_t, cth_varoff
+@tab The offset of the variable section, which maps string names to types.
+@xref{The variable section}.
+
+@item 0x28
+@tab @code{uint32_t cth_typeoff}
+@vindex cth_typeoff
+@vindex struct ctf_header, cth_typeoff
+@vindex ctf_header_t, cth_typeoff
+@tab The offset of the type section, the core of CTF, which describes types
+ using variable-length array elements. @xref{The type section}.
+
+@item 0x2c
+@tab @code{uint32_t cth_stroff}
+@vindex cth_stroff
+@vindex struct ctf_header, cth_stroff
+@vindex ctf_header_t, cth_stroff
+@tab The offset of the string section. @xref{The string section}.
+
+@item 0x30
+@tab @code{uint32_t cth_strlen}
+@vindex cth_strlen
+@vindex struct ctf_header, cth_strlen
+@vindex ctf_header_t, cth_strlen
+@tab The length of the string section (not an offset!).  The CTF file ends
+at this point.
+
+@end multitable
+
+Everything from this point on (until the end of the file at @code{cth_stroff} +
+@code{cth_strlen}) is compressed with zlib if @code{CTF_F_COMPRESS} is set in
+the preamble's @code{ctp_flags}.
+
+@node The type section
+@section The type section
+@cindex Type section
+@cindex Sections, type
+
+This section is the most important section in CTF, describing all the top-level
+types in the program.  It consists of an array of type structures, each of which
+describes a type of some @dfn{kind}: each kind of type has some amount of
+variable-length data associated with it (some kinds have none).  The amount of
+variable-length data associated with a given type can be determined by
+inspecting the type, so the reading code can walk through the types in sequence
+at opening time.
+
+Each type structure is one of a set of overlapping structures in a discriminated
+union of sorts: the variable-length data for each type immediately follows the
+type's type structure.  Here's the largest of the overlapping structures, which
+is only needed for huge types and so is very rarely seen:
+
+@verbatim
+typedef struct ctf_type
+{
+  uint32_t ctt_name;
+  uint32_t ctt_info;
+  __extension__
+  union
+  {
+    uint32_t ctt_size;
+    uint32_t ctt_type;
+  };
+  uint32_t ctt_lsizehi;
+  uint32_t ctt_lsizelo;
+} ctf_type_t;
+@end verbatim
+
+Here's the much more common smaller form:
+
+@verbatim
+typedef struct ctf_stype
+{
+  uint32_t ctt_name;
+  uint32_t ctt_info;
+  __extension__
+  union
+  {
+    uint32_t ctt_size;
+    uint32_t ctt_type;
+  };
+} ctf_type_t;
+@end verbatim
+
+If @code{ctt_size} is the #define @code{CTF_LSIZE_SENT}, 0xffffffff, this type
+is described by a @code{ctf_type_t}: otherwise, a @code{ctf_stype_t}.
+@tindex CTF_LSIZE_SENT
+
+Here's what the fields mean:
+
+@tindex struct ctf_type
+@tindex struct ctf_stype
+@tindex ctf_type_t
+@tindex ctf_stype_t
+@multitable {0x1c (@code{ctf_type_t}} {@code{uint32_t ctt_lsizehi}} {The size of this type, if this type is of a kind for}
+@headitem Offset @tab Name @tab Description
+@item 0x00
+@tab @code{uint32_t ctt_name}
+@vindex ctt_name
+@tab Strtab offset of the type name, if any (0 if none).
+
+@item 0x04
+@tab @code{uint32_t ctt_info}
+@vindex ctt_info
+@vindex struct ctf_type, ctt_info
+@vindex ctf_type_t, ctt_info
+@vindex struct ctf_stype, ctt_info
+@vindex ctf_stype_t, ctt_info
+@tab The @dfn{info word}, containing information on the kind of this type, its
+variable-length data and whether it is visible to name lookup. See @xref{The
+info word}.
+
+@item 0x08
+@tab @code{uint32_t ctt_size}
+@vindex ctt_size
+@vindex struct ctf_type, ctt_size
+@vindex ctf_type_t, ctt_size
+@vindex struct ctf_stype, ctt_size
+@vindex ctf_stype_t, ctt_size
+@tab The size of this type, if this type is of a kind for which a size needs
+to be recorded (constant-size types don't need one).  If this is
+@code{CTF_LSIZE_SENT}, this type is a huge type described by @code{ctf_type_t}.
+
+@item 0x08
+@tab @code{uint32_t ctt_type}
+@vindex ctt_type
+@vindex struct ctf_stype, ctt_type
+@vindex ctf_stype_t, ctt_type
+@tab The type this type refers to, if this type is of a kind which refers to
+other types (like a pointer).  All such types are fixed-size, and no types that
+are variable-size refer to other types, so @code{ctt_size} and @code{ctt_type}
+overlap.  All type kinds that use @code{ctt_type} are described by
+@code{ctf_stype_t}, not @code{ctf_type_t}. @xref{Type indexes and type IDs}.
+
+@item 0x0c (@code{ctf_type_t} only)
+@tab @code{uint32_t ctt_lsizehi}
+@vindex ctt_lsizehi
+@vindex struct ctf_type, ctt_lsizehi
+@vindex ctf_type_t, ctt_lsizehi
+@tab The high 32 bits of the size of a very large type. The @code{CTF_TYPE_LSIZE} macro
+can be used to get a 64-bit size out of this field and the next one.
+@code{CTF_SIZE_TO_LSIZE_HI} splits the @code{ctt_lsizehi} out of it again.
+@findex CTF_TYPE_LSIZE
+@findex CTF_SIZE_TO_LSIZE_HI
+
+@item 0x10 (@code{ctf_type_t} only)
+@tab @code{uint32_t ctt_lsizelo}
+@vindex ctt_lsizelo
+@vindex struct ctf_type, ctt_lsizelo
+@vindex ctf_type_t, ctt_lsizelo
+@tab The low 32 bits of the size of a very large type.
+@code{CTF_SIZE_TO_LSIZE_LO} splits the @code{ctt_lsizelo} out of a 64-bit size.
+@findex CTF_SIZE_TO_LSIZE_LO
+@end multitable
+
+Two aspects of this need further explanation: the info word, and what exactly a
+type ID is and how you determine it.  (Information on the various type-kind-
+dependent things, like whether @code{ctt_size} or @code{ctt_type} is used,
+is described in the section devoted to each kind.)
+
+@node The info word
+@subsection The info word, ctt_info
+
+The info word is a bitfield split into three parts.  From MSB to LSB:
+
+@multitable {Bit offset} {@code{isroot}} {Length of variable-length data for this type (some kinds only).}
+@headitem Bit offset @tab Name @tab Description
+@item 26--31
+@tab @code{kind}
+@tab Type kind: @pxref{Type kinds}.
+
+@item 25
+@tab @code{isroot}
+@tab 1 if this type is visible to name lookup
+
+@item 0--24
+@tab @code{vlen}
+@tab Length of variable-length data for this type (some kinds only).
+The variable-length data directly follows the @code{ctf_type_t} or
+@code{ctf_stype_t}.  This is a kind-dependent array length value,
+not a length in bytes.  Some kinds have no variable-length data, or
+fixed-size variable-length data, and do not use this value.
+@end multitable
+
+The most mysterious of these is undoubtedly @code{isroot}.  This indicates
+whether types with names (nonzero @code{ctt_name}) are visible to name lookup:
+if zero, this type is considered a @dfn{non-root type} and you can't look it up
+by name at all.  Multiple types with the same name in the same C namespace
+(struct, union, enum, other) can exist in a single dictionary, but only one of
+them may have a nonzero value for @code{isroot}.  @code{libctf} validates this
+at open time and refuses to open dictionaries that violate this constraint.
+
+Historically, this feature was introduced for the encoding of bitfields
+(@pxref{Integer types}): for instance, int bitfields will all be named
+@code{int} with different widths or offsets, but only the full-width one at
+offset zero is wanted when you look up the type named @code{int}.  With the
+introduction of slices (@pxref{Slices}) as a more general bitfield encoding
+mechanism, this is less important, but we still use non-root types to handle
+conflicts if the linker API is used to fuse multiple translation units into one
+dictionary and those translation units contain types with the same name and
+conflicting definitions.  (We do not discuss this further here, because the
+linker never does this: only specialized type mergers do, like that used for the
+Linux kernel.  The libctf documentation will describe this in more detail.)
+@c XXX update when libctf docs are written.
+
+The @code{CTF_TYPE_INFO} macro can be used to compose an info word from
+a @code{kind}, @code{isroot}, and @code{vlen}; @code{CTF_V2_INFO_KIND},
+@code{CTF_V2_INFO_ISROOT} and @code{CTF_V2_INFO_VLEN} pick it apart again.
+@findex CTF_TYPE_INFO
+@findex CTF_V2_INFO_KIND
+@findex CTF_V2_INFO_ISROOT
+@findex CTF_V2_INFO_VLEN
+
+@node Type indexes and type IDs
+@subsection Type indexes and type IDs
+@cindex Type indexes
+@cindex Type IDs
+@cindex Type, IDs of
+@cindex Type, indexes of
+@cindex ctf_id_t
+
+@cindex Parent range
+@cindex Child range
+@cindex Type IDs, ranges
+Types are referred to within the CTF file via @dfn{type IDs}.  A type ID is a
+number from 0 to @math{2^32}, from a space divided in half.  Types @math{2^31-1}
+and below are in the @dfn{parent range}: these IDs are used for dictionaries
+that have not had any other dictionary @code{ctf_import}ed into it as a parent.
+Both completely standalone dictionaries and parent dictionaries with children
+hanging off them have types in this range.  Types @math{2^31} and above are in
+the @dfn{child range}: only types in child dictionaries are in this range.
+
+These IDs appear in @code{ctf_type_t.ctt_type} (@pxref{The type section}), but
+the types themselves have no visible ID: quite intentionally, because adding an
+ID uses space, and every ID is different so they don't compress well.  The IDs
+are implicit: at open time, the consumer walks through the entire type section
+and counts the types in the type section. The type section is an array of
+variable-length elements, so each entry could be considered as having an index,
+starting from 1. We count these indexes and associate each with its
+corresponding @code{ctf_type_t} or @code{ctf_stype_t}.
+
+Lookups of types with IDs in the parent space look in the parent dictionary if
+this dictionary has one associated with it; lookups of types with IDs in the
+child space error out if the dictionary does not have a parent, and otherwise
+convert the ID into an index by shaving off the top bit and look up the index
+in the child.
+
+These properties mean that the same dictionary can be used as a parent of child
+dictionaries and can also be used directly with no children at all, but a
+dictionary created as a child dictionary must always be associated with a parent
+--- usually, the same parent --- because its references to its own types have
+the high bit turned on and this is only flipped off again if this is a child
+dictionary.  (This is not a problem, because if you @emph{don't} associate the
+child with a parent, any references within it to its parent types will fail, and
+there are almost certain to be many such references, or why is it a child at
+all?)
+
+This does mean that consumers should keep a close eye on the distinction between
+type IDs and type indexes: if you mix them up, everything will appear to work as
+long as you're only using parent dictionaries or standalone dictionaries, but as
+soon as you start using children, everything will fail horribly.
+
+Type index zero, and type ID zero, are used to indicate that this type cannot be
+represented in CTF as currently constituted: they are emitted by the compiler,
+but all type chains that terminate in the unknown type are erased at link time
+(structure fields that use them just vanish, etc).  So you will probably never
+see a use of type zero outside the symtypetab sections, where they serve as
+sentinels of sorts, to indicate symbols with no associated type.
+
+The macros @code{CTF_V2_TYPE_TO_INDEX} and @code{CTF_V2_INDEX_TO_TYPE} may help
+in translation between types and indexes: @code{CTF_V2_TYPE_ISPARENT} and
+@code{CTF_V2_TYPE_ISCHILD} can be used to tell whether a given ID is in the
+parent or child range.
+@findex CTF_V2_TYPE_TO_INDEX
+@findex CTF_V2_INDEX_TO_TYPE
+@findex CTF_V2_TYPE_ISPARENT
+@findex CTF_V2_TYPE_ISCHILD
+
+It is quite possible and indeed common for type IDs to point forward in the
+dictionary, as well as backward.
+
+@node Type kinds
+@subsection Type kinds
+@cindex Type kinds
+@cindex Type, kinds of
+
+Every type in CTF is of some @dfn{kind}. Each kind is some variety of C type:
+all structures are a single kind, as are all unions, all pointers, all arrays,
+all integers regardless of their bitfield width, etc.  The kind of a type is
+given in the @code{kind} field of the @code{ctt_info} word (@pxref{The info
+word}).
+
+The space of type kinds is only a quarter full so far, so there is plenty of
+room for expansion.  It is likely that in future versions of the file format,
+types with smaller kinds will be more efficiently encoded than types with larger
+kinds, so their numerical value will actually start to matter in future. (So
+these IDs will probably change their numerical values in a later release of this
+format, to move more frequently-used kinds like structures and cv-quals towards
+the top of the space, and move rarely-used kinds like integers downwards. Yes,
+integers are rare: how many kinds of @code{int} are there in a program? They're
+just very frequently @emph{referenced}.)
+
+Here's the set of kinds so far.  Each kind has a @code{#define} associated with
+it, also given here.
+
+@multitable {Kind} {@code{CTF_K_VOLATILE}} {Indicates a type that cannot be represented in CTF, or that} {@xref{Pointers typedefs and cvr-quals}}
+@headitem Kind @tab Macro @tab Purpose
+@item 0
+@tab @code{CTF_K_UNKNOWN}
+@tab Indicates a type that cannot be represented in CTF, or that is being skipped.
+It is very similar to type ID 0, except that you can have @emph{multiple}, distinct types
+of kind @code{CTF_K_UNKNOWN}.
+@tindex CTF_K_UNKNOWN
+
+@item 1
+@tab @code{CTF_K_INTEGER}
+@tab An integer type. @xref{Integer types}.
+
+@item 2
+@tab @code{CTF_K_FLOAT}
+@tab A floating-point type. @xref{Floating-point types}.
+
+@item 3
+@tab @code{CTF_K_POINTER}
+@tab A pointer. @xref{Pointers typedefs and cvr-quals}.
+
+@item 4
+@tab @code{CTF_K_ARRAY}
+@tab An array. @xref{Arrays}.
+
+@item 5
+@tab @code{CTF_K_FUNCTION}
+@tab A function pointer. @xref{Function pointers}.
+
+@item 6
+@tab @code{CTF_K_STRUCT}
+@tab A structure. @xref{Structs and unions}.
+
+@item 7
+@tab @code{CTF_K_UNION}
+@tab A union. @xref{Structs and unions}.
+
+@item 8
+@tab @code{CTF_K_ENUM}
+@tab An enumerated type. @xref{Enums}.
+
+@item 9
+@tab @code{CTF_K_FORWARD}
+@tab A forward. @xref{Forward declarations}.
+
+@item 10
+@tab @code{CTF_K_TYPEDEF}
+@tab A typedef. @xref{Pointers typedefs and cvr-quals}.
+
+@item 11
+@tab @code{CTF_K_VOLATILE}
+@tab A volatile-qualified type. @xref{Pointers typedefs and cvr-quals}.
+
+@item 12
+@tab @code{CTF_K_CONST}
+@tab A const-qualified type. @xref{Pointers typedefs and cvr-quals}.
+
+@item 13
+@tab @code{CTF_K_RESTRICT}
+@tab A restrict-qualified type. @xref{Pointers typedefs and cvr-quals}.
+
+@item 14
+@tab @code{CTF_K_SLICE}
+@tab A slice, a change of the bit-width or offset of some other type. @xref{Slices}.
+@end multitable
+
+Now we cover all type kinds in turn. Some are more complicated than others.
+
+@node Integer types
+@subsection Integer types
+@cindex Integer types
+@cindex Types, integer
+@tindex int
+@tindex long
+@tindex long long
+@tindex short
+@tindex char
+@tindex bool
+@tindex unsigned int
+@tindex unsigned long
+@tindex unsigned long long
+@tindex unsigned short
+@tindex unsigned char
+@tindex signed int
+@tindex signed long
+@tindex signed long long
+@tindex signed short
+@tindex signed char
+@cindex CTF_K_INTEGER
+
+Integral types are all represented as types of kind @code{CTF_K_INTEGER}.  These
+types fill out @code{ctt_size} in the @code{ctf_stype_t} with the size in bytes
+of the integral type in question.  They are always represented by
+@code{ctf_stype_t}, never @code{ctf_type_t}. Their variable-length data is one
+@code{uint32_t} in length: @code{vlen} in the info word should be disregarded
+and is always zero.
+
+The variable-length data for integers has multiple items packed into it much
+like the info word does.
+
+@multitable {Bit offset} {Encoding} {The integer encoding and desired display representation.}
+@headitem Bit offset @tab Name @tab Description
+@item 24--31
+@tab Encoding
+@tab The desired display representation of this integer. You can extract this
+field with the @code{CTF_INT_ENCODING} macro.  See below.
+@findex CTF_INT_ENCODING
+
+@item 16--23
+@tab Offset
+@tab The offset of this integral type in bits from the start of its enclosing
+structure field, adjusted for endianness: @pxref{Structs and unions}.  You can
+extract this field with the @code{CTF_INT_OFFSET} macro.
+@findex CTF_INT_OFFSET
+
+@item 0--15
+@tab Bit-width
+@tab The width of this integral type in bits.  You can extract this field with
+the @code{CTF_INT_BITS} macro.
+@findex CTF_INT_BITS
+@end multitable
+
+If you choose, bitfields can be represented using the things above as a sort of
+integral type with the @code{isroot} bit flipped off and the offset and bits
+values set in the vlen word: you can populate it with the @code{CTF_INT_DATA}
+macro.  (But it may be more convenient to represent them using slices of a
+full-width integer: @pxref{Slices}.)
+@findex CTF_INT_DATA
+
+Integers that are bitfields usually have a @code{ctt_size} rounded up to the
+nearest power of two in bytes, for natural alignment (e.g. a 17-bit integer
+would have a @code{ctt_size} of 4).  However, not all types are naturally
+aligned on all architectures: packed structures may in theory use integral
+bitfields with different @code{ctt_size}, though this is rarely observed.
+
+The @dfn{encoding} for integers is a bit-field comprised of the values below,
+which consumers can use to decide how to display values of this type:
+
+@multitable {Offset} {@code{CTF_INT_VARARGS}} {If set, this is a char type.  It is platform-dependent whether unadorned}
+@headitem Offset @tab Name @tab Description
+@item 0x01
+@tab @code{CTF_INT_SIGNED}
+@tab If set, this is a signed int: if false, unsigned.
+@tindex CTF_INT_SIGNED
+
+@item 0x02
+@tab @code{CTF_INT_CHAR}
+@tab If set, this is a char type.  It is platform-dependent whether unadorned
+@code{char} is signed or not: the @code{CTF_CHAR} macro produces an integral
+type suitable for the definition of @code{char} on this platform.
+@tindex CTF_INT_CHAR
+@findex CTF_CHAR
+
+@item 0x04
+@tab @code{CTF_INT_BOOL}
+@tab If set, this is a boolean type.  (It is theoretically possible to turn this
+and @code{CTF_INT_CHAR} on at the same time, but it is not clear what this would
+mean.)
+@tindex CTF_INT_BOOL
+
+@item 0x08
+@tab @code{CTF_INT_VARARGS}
+@tab If set, this is a varargs-promoted value in a K&R function definition.
+This is not currently produced or consumed by anything that we know of: it is set
+aside for future use.
+@end multitable
+
+The GCC ``@code{Complex int}'' and fixed-point extensions are not yet supported:
+references to such types will be emitted as type 0.
+
+@node Floating-point types
+@subsection Floating-point types
+@cindex Floating-point types
+@cindex Types, floating-point
+@tindex float
+@tindex double
+@tindex signed float
+@tindex signed double
+@tindex unsigned float
+@tindex unsigned double
+@tindex Complex, float
+@tindex Complex, double
+@tindex Complex, signed float
+@tindex Complex, signed double
+@tindex Complex, unsigned float
+@tindex Complex, unsigned double
+@cindex CTF_K_FLOAT
+
+Floating-point types are all represented as types of kind @code{CTF_K_FLOAT}.
+Like integers, These types fill out @code{ctt_size} in the @code{ctf_stype_t}
+with the size in bytes of the floating-point type in question.  They are always
+represented by @code{ctf_stype_t}, never @code{ctf_type_t}.
+
+This part of CTF shows many rough edges in the more obscure corners of
+floating-point handling, and is likely to change in format v4.
+
+The variable-length data for floats has multiple items packed into it just like
+integers do:
+
+@multitable {Bit offset} {Encoding} {The floating-;point encoding and desired display representation.}
+@headitem Bit offset @tab Name @tab Description
+@item 24--31
+@tab Encoding
+@tab The desired display representation of this float. You can extract this
+field with the @code{CTF_FP_ENCODING} macro.  See below.
+@findex CTF_FP_ENCODING
+
+@item 16--23
+@tab Offset
+@tab The offset of this floating-point type in bits from the start of its enclosing
+structure field, adjusted for endianness: @pxref{Structs and unions}.  You can
+extract this field with the @code{CTF_FP_OFFSET} macro.
+@findex CTF_FP_OFFSET
+
+@item 0--15
+@tab Bit-width
+@tab The width of this floating-point type in bits.  You can extract this field with
+the @code{CTF_FP_BITS} macro.
+@findex CTF_FP_BITS
+@end multitable
+
+The purpose of the floating-point offset and bit-width is somewhat opaque, since
+there are no such things as floating-point bitfields in C: the bit-width should
+be filled out with the full width of the type in bits, and the offset should
+always be zero.  It is likely that these fields will go away in the future.  As
+with integers, you can use @code{CTF_FP_DATA} to assemble one of these vlen
+items from its component parts.
+@findex CTF_INT_DATA
+
+The @dfn{encoding} for floats is not a bitfield but a simple value indicating
+the display representation.  Many of these are unused, relate to
+Solaris-specific compiler extensions, and will be recycled in future: some are
+unused and will become used in future.
+
+@multitable {Offset} {@code{CTF_FP_LDIMAGRY}} {This is a @code{float} interval type, a Solaris-specific extension.}
+@headitem Offset @tab Name @tab Description
+@item 1
+@tab @code{CTF_FP_SINGLE}
+@tab This is a single-precision IEEE 754 @code{float}.
+@tindex CTF_FP_SINGLE
+@item 2
+@tab @code{CTF_FP_DOUBLE}
+@tab This is a double-precision IEEE 754 @code{double}.
+@tindex CTF_FP_DOUBLE
+@item 3
+@tab @code{CTF_FP_CPLX}
+@tab This is a @code{Complex float}.
+@tindex CTF_FP_CPLX
+@item 4
+@tab @code{CTF_FP_DCPLX}
+@tab This is a @code{Complex double}.
+@tindex CTF_FP_DCPLX
+@item 5
+@tab @code{CTF_FP_LDCPLX}
+@tab This is a @code{Complex long double}.
+@tindex CTF_FP_LDCPLX
+@item 6
+@tab @code{CTF_FP_LDOUBLE}
+@tab This is a @code{long double}.
+@tindex CTF_FP_LDOUBLE
+@item 7
+@tab @code{CTF_FP_INTRVL}
+@tab This is a @code{float} interval type, a Solaris-specific extension.
+Unused: will be recycled.
+@tindex CTF_FP_INTRVL
+@cindex Unused bits
+@item 8
+@tab @code{CTF_FP_DINTRVL}
+@tab This is a @code{double} interval type, a Solaris-specific extension.
+Unused: will be recycled.
+@tindex CTF_FP_DINTRVL
+@cindex Unused bits
+@item 9
+@tab @code{CTF_FP_LDINTRVL}
+@tab This is a @code{long double} interval type, a Solaris-specific extension.
+Unused: will be recycled.
+@tindex CTF_FP_LDINTRVL
+@cindex Unused bits
+@item 10
+@tab @code{CTF_FP_IMAGRY}
+@tab This is a the imaginary part of a @code{Complex float}. Not currently
+generated. May change.
+@tindex CTF_FP_IMAGRY
+@cindex Unused bits
+@item 11
+@tab @code{CTF_FP_DIMAGRY}
+@tab This is a the imaginary part of a @code{Complex double}. Not currently
+generated. May change.
+@tindex CTF_FP_DIMAGRY
+@cindex Unused bits
+@item 12
+@tab @code{CTF_FP_LDIMAGRY}
+@tab This is a the imaginary part of a @code{Complex long double}. Not currently
+generated. May change.
+@tindex CTF_FP_LDIMAGRY
+@cindex Unused bits
+@end multitable
+
+The use of the complex floating-point encodings is obscure: it is possible that
+@code{CTF_FP_CPLX} is meant to be used for only the real part of complex types,
+and @code{CTF_FP_IMAGRY} et al for the imaginary part -- but for now, we are
+emitting @code{CTF_FP_CPLX} to cover the entire type, with no way to get at its
+constituent parts. There appear to be no uses of these encodings anywhere, so
+they are quite likely to change incompatibly in future.
+
+@node Slices
+@subsection Slices
+@cindex Slices
+@cindex Types, slices of integral
+@tindex CTF_K_SLICE
+
+Slices, with kind @code{CTF_K_SLICE}, are an unusual CTF construct: they do not
+directly correspond to any C type, but are a way to model other types in a more
+convenient fashion for CTF generators.
+
+A slice is like a pointer or other reference type in that they are always
+represented by @code{ctf_stype_t}: but unlike pointers and other reference
+types, they populate the @code{ctt_size} field just like integral types do, and
+come with an attached encoding and transform the encoding of the underlying
+type.  The underlying type is described in the variable-length data, similarly
+to structure and union fields: see below.  Requests for the type size should
+also chase down to the referenced type.
+
+Slices are always nameless: @code{ctt_name} is always zero for them.
+
+(The @code{libctf} API behaviour is unusual as well, and justifies the existence
+of slices: @code{ctf_type_kind} never returns @code{CTF_K_SLICE} but always the
+underlying type kind, so that consumers never need to know about slices: they
+can tell if an apparent integer is actually a slice if they need to by calling
+@code{ctf_type_reference}, which will uniquely return the underlying integral
+type rather than erroring out with @code{ECTF_NOTREF} if this is actually a
+slice. So slices act just like an integer with an encoding, but more closely
+mirror DWARF and other debugging information formats by allowing CTF file
+creators to represent a bitfield as a slice of an underlying integral type.)
+@findex Slices, effect on ctf_type_kind
+@findex Slices, effect on ctf_type_reference
+@findex libctf, effect of slices
+
+The vlen in the info word for a slice should be ignored and is always zero. The
+variable-length data for a slice is a single @code{ctf_slice_t}:
+
+@verbatim
+typedef struct ctf_slice
+{
+  uint32_t cts_type;
+  unsigned short cts_offset;
+  unsigned short cts_bits;
+} ctf_slice_t;
+@end verbatim
+
+@tindex struct ctf_slice
+@tindex ctf_slice_t
+@multitable {Offset} {@code{unsigned short cts_offset}} {The type this slice is a slice of.  Must be an}
+@headitem Offset @tab Name @tab Description
+@item 0x0
+@tab @code{uint32_t cts_type}
+@vindex cts_type
+@vindex struct ctf_slice, cts_type
+@vindex ctf_slice_t, cts_type
+@tab The type this slice is a slice of.  Must be an integral type (or a
+floating-point type, but this nonsensical option will go away in v4.)
+
+@item 0x4
+@tab @code{unsigned short cts_offset}
+@vindex cts_offset
+@vindex struct ctf_slice, cts_offset
+@vindex ctf_slice_t, cts_offset
+@tab The offset of this integral type in bits from the start of its enclosing
+structure field, adjusted for endianness: @pxref{Structs and unions}. Identical
+semantics to the @code{CTF_INT_OFFSET} field: @pxref{Integer types}.  This field
+is much too long, because the maximum possible offset of an integral type would
+easily fit in a char: this field is bigger just for the sake of alignment.  This
+will change in v4.
+
+@item 0x6
+@tab @code{unsigned short cts_bits}
+@vindex cts_bits
+@vindex struct ctf_slice, cts_bits
+@vindex ctf_slice_t, cts_bits
+@tab The bit-width of this integral type. Identical semantics to the
+@code{CTF_INT_BITS} field: @pxref{Integer types}.  As above, this field is
+really too large and will shrink in v4.
+@end multitable
+
+@node Pointers typedefs and cvr-quals
+@subsection Pointers, typedefs, and cvr-quals
+@cindex Pointers
+@cindex Typedefs
+@cindex cvr-quals
+@tindex typedef
+@tindex const
+@tindex volatile
+@tindex restrict
+@tindex CTF_K_POINTER
+@tindex CTF_K_TYPEDEF
+@tindex CTF_K_CONST
+@tindex CTF_K_VOLATILE
+@tindex CTF_K_RESTRICT
+
+Pointers, @code{typedef}s, and @code{const}, @code{volatile} and @code{restrict}
+qualifiers are represented identically except for their type kind (though they
+may be treated differently by consuming libraries like @code{libctf}, since
+pointers affect assignment-compatibility in ways cvr-quals do not, and they may
+have different alignment requirements, etc).
+
+All of these are represented by @code{ctf_stype_t}, have no variable data at
+all, and populate @code{ctt_type} with the type ID of the type they point
+to. These types can stack: a @code{CTF_K_RESTRICT} can point to a
+@code{CTF_K_CONST} which can point to a @code{CTF_K_POINTER} etc.
+
+They are all unnamed: @code{ctt_name} is 0.
+
+The size of @code{CTF_K_POINTER} is derived from the data model (@pxref{Data
+models}), i.e. in practice, from the target machine ABI, and is not explicitly
+represented.  The size of other kinds in this set should be determined by
+chasing ctf_types as necessary until a non-typedef/const/volatile/restrict is
+found, and using that.
+
+@node Arrays
+@subsection Arrays
+@cindex Arrays
+
+Arrays are encoded as types of kind @code{CTF_K_ARRAY} in a @code{ctf_stype_t}.
+Both size and kind for arrays are zero.  The variable-length data is a
+@code{ctf_array_t}: @code{vlen} in the info word should be disregarded and is
+always zero.
+
+@verbatim
+typedef struct ctf_array
+{
+  uint32_t cta_contents;
+  uint32_t cta_index;
+  uint32_t cta_nelems;
+} ctf_array_t;
+@end verbatim
+
+@tindex struct ctf_array
+@tindex ctf_array_t
+@multitable {Offset} {@code{unsigned short cta_contents}} {The type of the array index: a type ID of an}
+@headitem Offset @tab Name @tab Description
+@item 0x0
+@tab @code{uint32_t cta_contents}
+@vindex cta_contents
+@vindex struct ctf_array, cta_contents
+@vindex ctf_array_t, cta_contents
+@tab The type of the array elements: a type ID.
+
+@item 0x4
+@tab @code{uint32_t cta_index}
+@vindex cta_index
+@vindex struct ctf_array, cta_index
+@vindex ctf_array_t, cta_index
+@tab The type of the array index: a type ID of an integral type.
+If this is a variable-length array, the index type ID will be 0
+(but the actual index type of this array is probably @code{int}).
+Probably redundant and may be dropped in v4.
+
+@item 0x8
+@tab @code{uint32_t cta_nelems}
+@vindex cta_nelems
+@vindex struct ctf_array, cta_nelems
+@vindex ctf_array_t, cta_nelems
+@tab The number of array elements.  0 for VLAs, and also for
+the historical variety of VLA which has explicit zero dimensions (which will
+have a nonzero @code{cta_index}.)
+@end multitable
+
+The size of an array can be computed by simple multiplication of the size of the
+@code{cta_contents} type by the @code{cta_nelems}.
+
+@node Function pointers
+@subsection Function pointers
+@cindex Function pointers
+@cindex Pointers, to functions
+
+Function pointers are explicitly represented in the CTF type section by a type
+of kind @code{CTF_K_FUNCTION}, always encoded with a @code{ctf_stype_t}.  The
+@code{ctt_type} is the function return type ID.  The @code{vlen} in the info
+word is the number of arguments, each of which is a type ID, a @code{uint32_t}:
+if the last argument is 0, this is a varargs function and the number of
+arguments is one less than indicated by the vlen.
+
+If the number of arguments is odd, a single @code{uint32_t} of padding is
+inserted to maintain alignment.
+
+@node Enums
+@subsection Enums
+@cindex Enums
+@tindex enum
+@tindex CTF_K_ENUM
+
+Enumerated types are represented as types of kind @code{CTF_K_ENUM} in a
+@code{ctf_stype_t}.  The @code{ctt_size} is always the size of an int from the
+data model (enum bitfields are implemented via slices).  The @code{vlen} is a
+count of enumerations, each of which is represented by a @code{ctf_enum_t} in
+the vlen:
+
+@verbatim
+typedef struct ctf_enum
+{
+  uint32_t cte_name;
+  int32_t cte_value;
+} ctf_enum_t;
+@end verbatim
+
+@tindex struct ctf_enum
+@tindex ctf_enum_t
+@multitable {Offset} {@code{int32_t cte_value}} {Strtab offset of the enumeration name.}
+@headitem Offset @tab Name @tab Description
+@item 0x0
+@tab @code{uint32_t cte_name}
+@vindex cte_name
+@vindex struct ctf_enum, cte_name
+@vindex ctf_enum_t, cte_name
+@tab Strtab offset of the enumeration name.  Must not be 0.
+
+@item 0x4
+@tab @code{int32_t cte_value}
+@vindex cte_value
+@vindex struct ctf_enum, cte_value
+@vindex ctf_enum_t, cte_value
+@tab The enumeration value.
+
+@end multitable
+
+Enumeration values larger than @math{2^32} are not yet supported and are omitted
+from the enumeration.  (v4 will lift this restriction by encoding the value
+differently.)
+
+Forward declarations of enums are not implemented with this kind: @pxref{Forward
+declarations}.
+
+Enumerated type names, as usual in C, go into their own namespace, and do not
+conflict with non-enums, structs, or unions with the same name.
+
+@node Structs and unions
+@subsection Structs and unions
+@cindex Structures
+@cindex Unions
+@tindex struct
+@tindex union
+@tindex CTF_K_STRUCT
+@tindex CTF_K_UNION
+
+Structures and unions are represnted as types of kind @code{CTF_K_STRUCT} and
+@code{CTF_K_UNION}: their representation is otherwise identical, and it is
+perfectly allowed for ``structs'' to contain overlapping fields etc, so we will
+treat them together for the rest of this section.
+
+They fill out @code{ctt_size}, and use @code{ctf_type_t} in preference to
+@code{ctf_stype_t} if the structure size is greater than @code{CTF_MAX_SIZE}
+(0xfffffffe).
+@tindex CTF_MAX_LSIZE
+
+The vlen for structures and unions is a count of structure fields, but the type
+used to represent a structure field (and thus the size of the variable-length
+array element representing the type) depends on the size of the structure: truly
+huge structures, greater than @code{CTF_LSTRUCT_THRESH} bytes in length, use a
+different type.  (@code{CTF_LSTRUCT_THRESH} is 536870912, so such structures are
+vanishingly rare: in v4, this representation will change somewhat for greater
+compactness. It's inherited from v1, where the limits were much lower.)
+@tindex CTF_LSTRUCT_THRESH
+
+Most structures can get away with using @code{ctf_member_t}:
+
+@verbatim
+typedef struct ctf_member_v2
+{
+  uint32_t ctm_name;
+  uint32_t ctm_offset;
+  uint32_t ctm_type;
+} ctf_member_t;
+@end verbatim
+
+Huge structures that are represented by @code{ctf_type_t} rather than
+@code{ctf_stype_t} have to use @code{ctf_lmember_t}, which splits the offset as
+@code{ctf_type_t} splits the size:
+
+@verbatim
+typedef struct ctf_lmember_v2
+{
+  uint32_t ctlm_name;
+  uint32_t ctlm_offsethi;
+  uint32_t ctlm_type;
+  uint32_t ctlm_offsetlo;
+} ctf_lmember_t;
+@end verbatim
+
+Here's what the fields of @code{ctf_member} mean:
+
+@tindex struct ctf_member_v2
+@tindex ctf_member_t
+@multitable {Offset} {@code{uint32_t ctm_offset}} {The offset of this field @emph{in bits}.  (Usually, for bitfields, this is}
+@headitem Offset @tab Name @tab Description
+@item 0x00
+@tab @code{uint32_t ctm_name}
+@vindex ctm_name
+@vindex struct ctf_member_v2, ctm_name
+@vindex ctf_member_t, ctm_name
+@tab Strtab offset of the field name.
+
+@item 0x04
+@tab @code{uint32_t ctm_offset}
+@vindex ctm_offset
+@vindex struct ctf_member_v2, ctm_offset
+@vindex ctf_member_t, ctm_offset
+@tab The offset of this field @emph{in bits}.  (Usually, for bitfields, this is
+machine-word-aligned and the individual field has an offset in bits, but
+the format allows for the offset to be encoded in bits here.)
+
+@item 0x08
+@tab @code{uint32_t ctm_type}
+@vindex ctm_type
+@vindex struct ctf_member_v2, ctm_type
+@vindex ctf_member_t, ctm_type
+@tab The type ID of the type of the field.
+@end multitable
+
+Here's what the fields of the very similar @code{ctf_lmember} mean:
+
+@tindex struct ctf_lmember_v2
+@tindex ctf_lmember_t
+@multitable {Offset} {@code{uint32_t ctlm_offsethi}} {The offset of this field @emph{in bits}.  (Usually, for bitfields, this is}
+@headitem Offset @tab Name @tab Description
+@item 0x00
+@tab @code{uint32_t ctlm_name}
+@vindex ctlm_name
+@vindex struct ctf_lmember_v2, ctlm_name
+@vindex ctf_lmember_t, ctlm_name
+@tab Strtab offset of the field name.
+
+@item 0x04
+@tab @code{uint32_t ctlm_offsethi}
+@vindex ctlm_offsethi
+@vindex struct ctf_lmember_v2, ctlm_offsethi
+@vindex ctf_lmember_t, ctlm_offsethi
+@tab The high 32 bits of the offset of this field in bits.
+
+@item 0x08
+@tab @code{uint32_t ctlm_type}
+@vindex ctm_type
+@vindex struct ctf_lmember_v2, ctlm_type
+@vindex ctf_member_t, ctlm_type
+@tab The type ID of the type of the field.
+
+@item 0x0c
+@tab @code{uint32_t ctlm_offsetlo}
+@vindex ctlm_offsetlo
+@vindex struct ctf_lmember_v2, ctlm_offsetlo
+@vindex ctf_lmember_t, ctlm_offsetlo
+@tab The low 32 bits of the offset of this field in bits.
+@end multitable
+
+Macros @code{CTF_LMEM_OFFSET}, @code{CTF_OFFSET_TO_LMEMHI} and
+@code{CTF_OFFSET_TO_LMEMLO} serve to extract and install the values of the
+@code{ctlm_offset} fields, much as with the split size fields in
+@code{ctf_type_t}.
+
+Unnamed structure and union fields are simply implemented by collapsing the
+unnamed field's members into the containing structure or union: this does mean
+that a structure containing an unnamed union can end up being a ``structure''
+with multiple members at the same offset.  (A future format revision may
+collapse @code{CTF_K_STRUCT} and @code{CTF_K_UNION} into the same kind and
+decide among them based on whether their members do in fact overlap.)
+
+Structure and union type names, as usual in C, go into their own namespace,
+just as enum type names do.
+
+Forward declarations of structures and unions are not implemented with this
+kind: @pxref{Forward declarations}.
+
+@node Forward declarations
+@subsection Forward declarations
+@cindex Forwards
+@tindex enum
+@tindex struct
+@tindex union
+@tindex CTF_K_FORWARD
+
+When the compiler encounters a forward declaration of a struct, union, or enum,
+it emits a type of kind @code{CTF_K_FORWARD}.  If it later encounters a non-
+forward declaration of the same thing, it marks the forward as non-root-visible:
+before link time, therefore, non-root-visible forwards indicate that a
+non-forward is coming.
+
+After link time, forwards are fused with their corresponding non-forwards by the
+deduplicator where possible.  They are kept if there is no non-forward
+definition (maybe it's not visible from any TU at all) or if @code{multiple}
+conflicting structures with the same name might match it.  Otherwise, all other
+forwards are converted to structures, unions, or enums as appropriate, even
+across TUs if only one structure could correspond to the forward (after all,
+all types across all TUs land in the same dictionary unless they conflict,
+so promoting forwards to their concrete type seems most helpful).
+
+A forward has a rather strange representation: it is encoded with a
+@code{ctf_stype_t} but the @code{ctt_type} is populated not with a type (if it's
+a forward, we don't have an underlying type yet: if we did, we'd have promoted
+it and this wouldn't be a forward any more) but with the @code{kind} of the
+forward.  This means that we can distinguish forwards to structs, enums and
+unions reliably and ensure they land in the appropriate namespace even before
+the actual struct, union or enum is found.
+
+@node The symtypetab sections
+@section The symtypetab sections
+@cindex Symtypetab section
+@cindex Sections, symtypetab
+@cindex Function info section
+@cindex Sections, function info
+@cindex Data object section
+@cindex Sections, data object
+@cindex Function info index section
+@cindex Sections, function info index
+@cindex Data object index section
+@cindex Sections, data object index
+@tindex CTF_F_IDXSORTED
+@tindex CTF_F_DYNSTR
+@cindex Bug workarounds, CTF_F_DYNSTR
+
+These are two very simple sections with identical formats, used by consumers to
+map from ELF function and data symbols directly to their types. So they are
+usually populated only in CTF sections that are embedded in ELF objects.
+
+Their format is very simple: an array of type IDs. Which symbol each type ID
+corresponds to depends on whether the optional @emph{index section} associated
+with this symtypetab section has any content.
+
+If the index section is nonempty, it is an array of @code{uint32_t} string table
+offsets, each giving the name of the symbol whose type is at the same offset in
+the corresponding non-index section: users can look up symbols in such a table
+by name.  The index section and corresponding symtypetab section is usually
+ASCIIbetically sorted (indicated by the @code{CTF_F_IDXSORTED} flag in the
+header): if it's sorted, it can be bsearched for a symbol name rather than
+having to use a slower linear search.
+
+If the data object index section is empty, the entries in the data object and
+function info sections are associated 1:1 with ELF symbols of type
+@code{STT_OBJECT} (for data object) or @code{STT_FUNC} (for function info) with
+a nonzero value: the linker shuffles the symtypetab sections to correspond with
+the order of the symbols in the ELF file.  Symbols with no name, undefined
+symbols and symbols named ``@code{_START_}'' and ``@code{_END_}'' are skipped
+and never appear in either section.  Symbols that have no corresponding type are
+represented by type ID 0. The section may have fewer entries than the symbol
+table, in which case no later entries have associated types.  This format is
+more compact than an indexed form if most entries have types (since there is no
+need to record any symbol names), but if the producer and consumer disagree even
+slightly about which symbols are omitted, the types of all further symbols will
+be wrong!
+
+The compiler always emits indexed symtypetab tables, because there is no symbol
+table yet. The linker will always have to read them all in and always works
+through them from start to end, so there is no benefit having the compiler sort
+them either. The linker (actually, @code{libctf}'s linking machinery) will
+automatically sort unsorted indexed sections, and convert indexed sections that
+contain a lot of pads into the more compact, unindexed form.
+
+If child dicts are in use, only symbols that use types actually mentioned in the
+child appear in the child's symtypetab: symbols that use only types in the
+parent appear in the parent's symtypetab instead. So the child's symtypetab will
+almost always be very sparse, and thus will usually use the indexed form even in
+fully linked objects. (It is, of course, impossible for symbols to exist that
+use types from multiple child dicts at once, since it's impossible to declare a
+function in C that uses types that are only visible in two different, disjoint
+translation units.)
+
+@node The variable section
+@section The variable section
+@cindex Variable section
+@cindex Sections, variable
+
+The variable section is a simple array mapping names (strtab entries) to type
+IDs, intended to provide a replacement for the data object section in dynamic
+situations in which there is no static ELF strtab but the consumer instead hands
+back names.  The section is sorted into ASCIIbetical order by name for rapid
+lookup, like the CTF archive name table.
+
+The section is an array of these structures:
+
+@verbatim
+typedef struct ctf_varent
+{
+  uint32_t ctv_name;
+  uint32_t ctv_type;
+} ctf_varent_t;
+@end verbatim
+
+@tindex struct ctf_varent
+@tindex ctf_varent_t
+@multitable {Offset} {@code{uint32_t ctv_name}} {Strtab offset of the name}
+@headitem Offset @tab Name @tab Description
+@item 0x00
+@tab @code{uint32_t ctv_name}
+@vindex ctv_name
+@vindex struct ctf_varent, ctv_name
+@vindex ctf_varent_t, ctv_name
+@tab Strtab offset of the name
+
+@item 0x04
+@tab @code{uint32_t ctv_type}
+@vindex ctv_type
+@vindex struct ctf_varent, ctv_type
+@vindex ctf_varent_t, ctv_type
+@tab Type ID of this type
+@end multitable
+
+There is no analogue of the function info section yet: v4 will probably drop
+this section in favour of a way to put both indexed (thus, named) and nonindexed
+symbols into the symtypetab sections at the same time.
+
+@node The label section
+@section The label section
+@cindex Label section
+@cindex Sections, label
+
+The label section is a currently-unused facility allowing the tiling of the type
+space with names taken from the strtab.  The section is an array of these
+structures:
+
+@verbatim
+typedef struct ctf_lblent
+{
+  uint32_t ctl_label;
+  uint32_t ctl_type;
+} ctf_lblent_t;
+@end verbatim
+
+@tindex struct ctf_lblent
+@tindex ctf_lblent_t
+@multitable {Offset} {@code{uint32_t ctl_label}} {Strtab offset of the label}
+@headitem Offset @tab Name @tab Description
+@item 0x00
+@tab @code{uint32_t ctl_label}
+@vindex ctl_label
+@vindex struct ctf_lblent, ctl_label
+@vindex ctf_lblent_t, ctl_label
+@tab Strtab offset of the label
+
+@item 0x04
+@tab @code{uint32_t ctl_type}
+@vindex ctl_type
+@vindex struct ctf_lblent, ctl_type
+@vindex ctf_lblent_t, ctl_type
+@tab Type ID of the last type covered by this label
+@end multitable
+
+Semantics will be attached to labels soon, probably in v4 (the plan is to use
+them to allow multiple disjoint namespaces in a single CTF file, removing many
+uses of CTF archives, in particular in the @code{.ctf} section in ELF objects).
+
+@node The string section
+@section The string section
+@cindex String section
+@cindex Sections, string
+
+This section is a simple ELF-format strtab, starting with a zero byte (thus
+ensuring that the string with offset 0 is the null string, as assumed elsewhere
+in this spec).  The strtab is usually ASCIIbetically sorted to somewhat improve
+compression efficiency.
+
+Where the strtab is unusual is the @emph{references} to it.  CTF has two
+string tables, the internal strtab and an external strtab associated
+with the CTF dictionary at open time: usually, this is the ELF dynamic
+strtab (@code{.dynstr}) of a CTF dictionary embedded in an ELF file.  We
+distinguish between these strtabs by the most significant bit, bit 31,
+of the 32-bit strtab references: if it is 0, the offset is in the
+internal strtab: if 1, the offset is in the external strtab.
+
+@tindex CTF_F_DYNSTR
+@cindex Bug workarounds, CTF_F_DYNSTR
+There is a bug workaround in this area: in format v3 (the first version
+to have working support for external strtabs), the external strtab is
+@code{.strtab} unless the @code{CTF_F_DYNSTR} flag is set on the
+dictionary (@pxref{CTF file-wide flags}).  Format v4 will introduce a
+header field that explicitly names the external strtab, making this flag
+unnecessary.
+
+@node Data models
+@section Data models
+@cindex Data models
+
+The data model is a simple integer which indicates the ABI in use on this
+platform. Right now, it is very simple, distinguishing only between 32- and
+64-bit types: a model of 1 indicates ILP32, 2 indicats LP64. The mapping from
+ABI integer to type sizes is hardwired into @code{libctf}: currently, we use
+this to hardwire the size of pointers, function pointers, and enumerated types,
+
+This is a very kludgy corner of CTF and will probably be replaced with explicit
+header fields to record this sort of thing in future.
+
+@node Limits of CTF
+@section Limits of CTF
+@cindex Limits
+
+The following limits are imposed by various aspects of CTF version 3:
+
+@table @code
+@item CTF_MAX_TYPE
+Maximum type identifier (maximum number of types accessible with parent and
+child containers in use): 0xfffffffe
+@item CTF_MAX_PTYPE
+Maximum type identifier in a parent dictioanry: maximum number of types in any
+one dictionary: 0x7fffffff
+@item CTF_MAX_NAME
+Maximum offset into a string table: 0x7fffffff
+@item CTF_MAX_VLEN
+Maximum number of members in a struct, union, or enum: maximum number of
+function args: 0xffffff
+@item CTF_MAX_SIZE
+Maximum size of a @code{ctf_stype_t} in bytes before we fall back to
+@code{ctf_type_t}: 0xfffffffe bytes
+@end table
+
+Other maxima without associated macros:
+@itemize
+@item
+Maximum value of an enumerated type: 2^32
+@item
+Maximum size of an array element: 2^32
+@end itemize
+
+These maxima are generally considered to be too low, because C programs can and
+do exceed them: they will be lifted in format v4.
+
+@node Index
+@unnumbered Index
+
+@printindex cp
+
+@bye