libctf/doc/ctf-spec.texi

\input texinfo       @c                    -*- Texinfo -*-
@setfilename ctf-spec.info
@settitle The CTF File Format
@ifnottex
@xrefautomaticsectiontitle on
@end ifnottex
@synindex fn cp
@synindex tp cp
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@copying
Copyright @copyright{} 2021-2022 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
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@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