CTF(5) File Formats and Configurations CTF(5)

NAME

ctf - Compact C Type Format

SYNOPSIS

#include <sys/ctf.h>

DESCRIPTION

ctf is designed to be a compact representation of the C programming
language's type information focused on serving the needs of dynamic
tracing, debuggers, and other in-situ and post-mortem introspection tools.
ctf data is generally included in ELF objects and is tagged as SHT_PROGBITS
to ensure that the data is accessible in a running process and in
subsequent core dumps, if generated.

The ctf data contained in each file has information about the layout and
sizes of C types, including intrinsic types, enumerations, structures,
typedefs, and unions, that are used by the corresponding ELF object. The
ctf data may also include information about the types of global objects and
the return type and arguments of functions in the symbol table.

Because a ctf file is often embedded inside a file, rather than being a
standalone file itself, it may also be referred to as a ctf container.

On illumos systems, ctf data is consumed by multiple programs. It can be
used by the modular debugger, mdb(1), as well as by dtrace(8).
Programmatic access to ctf data can be obtained through libctf(3LIB).

The ctf file format is broken down into seven different sections. The
first section is the preamble and header, which describes the version of
the ctf file, links it has to other ctf files, and the sizes of the other
sections. The next section is the label section, which provides a way of
identifying similar groups of ctf data across multiple files. This is
followed by the object information section, which describes the type of
global symbols. The subsequent section is the function information
section, which describes the return types and arguments of functions. The
next section is the type information section, which describes the format
and layout of the C types themselves, and finally the last section is the
string section, which contains the names of types, enumerations, members,
and labels.

While strictly speaking, only the preamble and header are required, to be
actually useful, both the type and string sections are necessary.

A ctf file may contain all of the type information that it requires, or it
may optionally refer to another ctf file which holds the remaining types.
When a ctf file refers to another file, it is called the child and the file
it refers to is called the parent. A given file may only refer to one
parent. This process is called uniquification because it ensures each
child only has type information that is unique to it. A common example of
this is that most kernel modules in illumos are uniquified against the
kernel module genunix and the type information that comes from the IP
module. This means that a module only has types that are unique to itself
and the most common types in the kernel are not duplicated.

FILE FORMAT

This documents version two of the ctf file format. All applications and
tools currently produce and operate on this version.

The file format can be summarized with the following image, the following
sections will cover this in more detail.


+-------------+ 0t0
+--------| Preamble |
| +-------------+ 0t4
|+-------| Header |
|| +-------------+ 0t36 + cth_lbloff
||+------| Labels |
||| +-------------+ 0t36 + cth_objtoff
|||+-----| Objects |
|||| +-------------+ 0t36 + cth_funcoff
||||+----| Functions |
||||| +-------------+ 0t36 + cth_typeoff
|||||+---| Types |
|||||| +-------------+ 0t36 + cth_stroff
||||||+--| Strings |
||||||| +-------------+ 0t36 + cth_stroff + cth_strlen
|||||||
|||||||
|||||||
||||||| +-- magic - vers flags
||||||| | | | |
||||||| +------+------+------+------+
+---------| 0xcf | 0xf1 | 0x02 | 0x00 |
|||||| +------+------+------+------+
|||||| 0 1 2 3 4
||||||
|||||| + parent label + objects
|||||| | + parent name | + functions + strings
|||||| | | + label | | + types | + strlen
|||||| | | | | | | | |
|||||| +------+------+------+------+------+-------+-------+-------+
+--------| 0x00 | 0x00 | 0x00 | 0x08 | 0x36 | 0x110 | 0x5f4 | 0x611 |
||||| +------+------+------+------+------+-------+-------+-------+
||||| 0x04 0x08 0x0c 0x10 0x14 0x18 0x1c 0x20 0x24
|||||
||||| + Label name
||||| | + Label type
||||| | | + Next label
||||| | | |
||||| +-------+------+-----+
+-----------| 0x01 | 0x42 | ... |
|||| +-------+------+-----+
|||| cth_lbloff +0x4 +0x8 cth_objtoff
||||
||||
|||| Symidx 0t15 0t43 0t44
|||| +------+------+------+-----+
+----------| 0x00 | 0x42 | 0x36 | ... |
||| +------+------+------+-----+
||| cth_objtoff +0x2 +0x4 +0x6 cth_funcoff
|||
||| + CTF_TYPE_INFO + CTF_TYPE_INFO
||| | + Return type |
||| | | + arg0 |
||| +--------+------+------+-----+
+---------| 0x2c10 | 0x08 | 0x0c | ... |
|| +--------+------+------+-----+
|| cth_funcff +0x2 +0x4 +0x6 cth_typeoff
||
|| + ctf_stype_t for type 1
|| | integer + integer encoding
|| | | + ctf_stype_t for type 2
|| | | |
|| +--------------------+-----------+-----+
+--------| 0x19 * 0xc01 * 0x0 | 0x1000000 | ... |
| +--------------------+-----------+-----+
| cth_typeoff +0x08 +0x0c cth_stroff
|
| +--- str 0
| | +--- str 1 + str 2
| | | |
| v v v
| +----+---+---+---+----+---+---+---+---+---+----+
+---| \0 | i | n | t | \0 | f | o | o | _ | t | \0 |
+----+---+---+---+----+---+---+---+---+---+----+
0 1 2 3 4 5 6 7 8 9 10 11

Every ctf file begins with a preamble, followed by a header. The preamble
is defined as follows:

typedef struct ctf_preamble {
ushort_t ctp_magic; /* magic number (CTF_MAGIC) */
uchar_t ctp_version; /* data format version number (CTF_VERSION) */
uchar_t ctp_flags; /* flags (see below) */
} ctf_preamble_t;

The preamble is four bytes long and must be four byte aligned. This
preamble defines the version of the ctf file which defines the format of
the rest of the header. While the header may change in subsequent
versions, the preamble will not change across versions, though the
interpretation of its flags may change from version to version. The
ctp_magic member defines the magic number for the ctf file format. This
must always be 0xcff1. If another value is encountered, then the file
should not be treated as a ctf file. The ctp_version member defines the
version of the ctf file. The current version is 2. It is possible to
encounter an unsupported version. In that case, software should not try to
parse the format, as it may have changed. Finally, the ctp_flags member
describes aspects of the file which modify its interpretation. The
following flags are currently defined:

#define CTF_F_COMPRESS 0x01

The flag CTF_F_COMPRESS indicates that the body of the ctf file, all the
data following the header, has been compressed through the zlib library and
its deflate algorithm. If this flag is not present, then the body has not
been compressed and no special action is needed to interpret it. All
offsets into the data as described by header, always refer to the
uncompressed data.

In version two of the ctf file format, the header denotes whether whether
or not this ctf file is the child of another ctf file and also indicates
the size of the remaining sections. The structure for the header,
logically contains a copy of the preamble and the two have a combined size
of 36 bytes.

typedef struct ctf_header {
ctf_preamble_t cth_preamble;
uint_t cth_parlabel; /* ref to name of parent lbl uniq'd against */
uint_t cth_parname; /* ref to basename of parent */
uint_t cth_lbloff; /* offset of label section */
uint_t cth_objtoff; /* offset of object section */
uint_t cth_funcoff; /* offset of function section */
uint_t cth_typeoff; /* offset of type section */
uint_t cth_stroff; /* offset of string section */
uint_t cth_strlen; /* length of string section in bytes */
} ctf_header_t;

After the preamble, the next two members cth_parlablel and cth_parname, are
used to identify the parent. The value of both members are offsets into
the string section which point to the start of a null-terminated string.
For more information on the encoding of strings, see the subsection on
String Identifiers. If the value of either is zero, then there is no entry
for that member. If the member cth_parlabel is set, then the ctf_parname
member must be set, otherwise it will not be possible to find the parent.
If ctf_parname is set, it is not necessary to define cth_parlabel, as the
parent may not have a label. For more information on labels and their
interpretation, see The Label Section.

The remaining members (excepting cth_strlen) describe the beginning of the
corresponding sections. These offsets are relative to the end of the
header. Therefore, something with an offset of 0 is at an offset of
thirty-six bytes relative to the start of the ctf file. The difference
between members indicates the size of the section itself. Different
offsets have different alignment requirements. The start of the
cth_objotoff and cth_funcoff must be two byte aligned, while the sections
cth_lbloff and cth_typeoff must be four-byte aligned. The section
cth_stroff has no alignment requirements. To calculate the size of a given
section, excepting the string section, one should subtract the offset of
the section from the following one. For example, the size of the types
section can be calculated by subtracting cth_stroff from cth_typeoff.

Finally, the member cth_strlen describes the length of the string section
itself. From it, you can also calculate the size of the entire ctf file by
adding together the size of the ctf_header_t, the offset of the string
section in cth_stroff, and the size of the string section in cth_srlen.

Type Identifiers

Through the ctf data, types are referred to by identifiers. A given ctf
file supports up to 32767 (0x7fff) types. The first valid type identifier
is 0x1. When a given ctf file is a child, indicated by a non-zero entry
for the header's cth_parname, then the first valid type identifier is
0x8000 and the last is 0xffff. In this case, type identifiers 0x1 through
0x7fff are references to the parent.

The type identifier zero is a sentinel value used to indicate that there is
no type information available or it is an unknown type.

Throughout the file format, the identifier is stored in different sized
values; however, the minimum size to represent a given identifier is a
uint16_t. Other consumers of ctf information may use larger or opaque
identifiers.

String Identifiers

String identifiers are always encoded as four byte unsigned integers which
are an offset into a string table. The ctf format supports two different
string tables which have an identifier of zero or one. This identifier is
stored in the high-order bit of the unsigned four byte offset. Therefore,
the maximum supported offset into one of these tables is 0x7ffffffff.

Table identifier zero, always refers to the string section in the CTF file
itself. String table identifier one refers to an external string table
which is the ELF string table for the ELF symbol table associated with the
ctf container.

Type Encoding

Every ctf type begins with metadata encoded into a uint16_t. This encoded
information tells us three different pieces of information:
+o The kind of the type
+o Whether this type is a root type or not
+o The length of the variable data

The 16 bits that make up the encoding are broken down such that you have
five bits for the kind, one bit for indicating whether or not it is a root
type, and 10 bits for the variable length. This is laid out as follows:

+--------------------+
| kind | root | vlen |
+--------------------+
15 11 10 9 0

The current version of the file format defines 14 different kinds. The
interpretation of these different kinds will be discussed in the section
The Type Section. If a kind is encountered that is not listed below, then
it is not a valid ctf file. The kinds are defined as follows:

#define CTF_K_UNKNOWN 0
#define CTF_K_INTEGER 1
#define CTF_K_FLOAT 2
#define CTF_K_POINTER 3
#define CTF_K_ARRAY 4
#define CTF_K_FUNCTION 5
#define CTF_K_STRUCT 6
#define CTF_K_UNION 7
#define CTF_K_ENUM 8
#define CTF_K_FORWARD 9
#define CTF_K_TYPEDEF 10
#define CTF_K_VOLATILE 11
#define CTF_K_CONST 12
#define CTF_K_RESTRICT 13

Programs directly reference many types; however, other types are referenced
indirectly because they are part of some other structure. These types that
are referenced directly and used are called root types. Other types may be
used indirectly, for example, a program may reference a structure directly,
but not one of its members which has a type. That type is not considered a
root type. If a type is a root type, then it will have bit 10 set.

The variable length section is specific to each kind and is discussed in
the section The Type Section.

The following macros are useful for constructing and deconstructing the
encoded type information:


#define CTF_MAX_VLEN 0x3ff
#define CTF_INFO_KIND(info) (((info) & 0xf800) >> 11)
#define CTF_INFO_ISROOT(info) (((info) & 0x0400) >> 10)
#define CTF_INFO_VLEN(info) (((info) & CTF_MAX_VLEN))

#define CTF_TYPE_INFO(kind, isroot, vlen) \
(((kind) << 11) | (((isroot) ? 1 : 0) << 10) | ((vlen) & CTF_MAX_VLEN))

The Label Section

When consuming ctf data, it is often useful to know whether two different
ctf containers come from the same source base and version. For example,
when building illumos, there are many kernel modules that are built against
a single collection of source code. A label is encoded into the ctf files
that corresponds with the particular build. This ensures that if files on
the system were to become mixed up from multiple releases, that they are
not used together by tools, particularly when a child needs to refer to a
type in the parent. Because they are linked used the type identifiers, if
the wrong parent is used then the wrong type will be encountered.

Each label is encoded in the file format using the following eight byte
structure:

typedef struct ctf_lblent {
uint_t ctl_label; /* ref to name of label */
uint_t ctl_typeidx; /* last type associated with this label */
} ctf_lblent_t;

Each label has two different components, a name and a type identifier. The
name is encoded in the ctl_label member which is in the format defined in
the section String Identifiers. Generally, the names of all labels are
found in the internal string section.

The type identifier encoded in the member ctl_typeidx refers to the last
type identifier that a label refers to in the current file. Labels only
refer to types in the current file, if the ctf file is a child, then it
will have the same label as its parent; however, its label will only refer
to its types, not its parents.

It is also possible, though rather uncommon, for a ctf file to have
multiple labels. Labels are placed one after another, every eight bytes.
When multiple labels are present, types may only belong to a single label.

The Object Section

The object section provides a mapping from ELF symbols of type STT_OBJECT
in the symbol table to a type identifier. Every entry in this section is a
uint16_t which contains a type identifier as described in the section Type
Identifiers. If there is no information for an object, then the type
identifier 0x0 is stored for that entry.

To walk the object section, you need to have a corresponding symbol table
in the ELF object that contains the ctf data. Not every object is included
in this section. Specifically, when walking the symbol table. An entry is
skipped if it matches any of the following conditions:

+o The symbol type is not STT_OBJECT
+o The symbol's section index is SHN_UNDEF
+o The symbol's name offset is zero
+o The symbol's section index is SHN_ABS and the value of the symbol
is zero.
+o The symbol's name is _START_ or _END_. These are skipped because
they are used for scoping local symbols in ELF.

The following sample code shows an example of iterating the object section
and skipping the correct symbols:

#include <gelf.h>
#include <stdio.h>

/*
* Given the start of the object section in the CTF file, the number of symbols,
* and the ELF Data sections for the symbol table and the string table, this
* prints the type identifiers that correspond to objects. Note, a more robust
* implementation should ensure that they don't walk beyond the end of the CTF
* object section.
*/
static int
walk_symbols(uint16_t *objtoff, Elf_Data *symdata, Elf_Data *strdata,
long nsyms)
{
long i;
uintptr_t strbase = strdata->d_buf;

for (i = 1; i < nsyms; i++, objftoff++) {
const char *name;
GElf_Sym sym;

if (gelf_getsym(symdata, i, &sym) == NULL)
return (1);

if (GELF_ST_TYPE(sym.st_info) != STT_OBJECT)
continue;
if (sym.st_shndx == SHN_UNDEF || sym.st_name == 0)
continue;
if (sym.st_shndx == SHN_ABS && sym.st_value == 0)
continue;
name = (const char *)(strbase + sym.st_name);
if (strcmp(name, "_START_") == 0 || strcmp(name, "_END_") == 0)
continue;

(void) printf("Symbol %d has type %d0, i, *objtoff);
}

return (0);
}

The Function Section

The function section of the ctf file encodes the types of both the
function's arguments and the function's return type. Similar to The Object
Section, the function section encodes information for all symbols of type
STT_FUNCTION, excepting those that fit specific criteria. Unlike with
objects, because functions have a variable number of arguments, they start
with a type encoding as defined in Type Encoding, which is the size of a
uint16_t. For functions which have no type information available, they are
encoded as CTF_TYPE_INFO(CTF_K_UNKNOWN, 0, 0). Functions with arguments
are encoded differently. Here, the variable length is turned into the
number of arguments in the function. If a function is a varargs type
function, then the number of arguments is increased by one. Functions with
type information are encoded as: CTF_TYPE_INFO(CTF_K_FUNCTION, 0, nargs).

For functions that have no type information, nothing else is encoded, and
the next function is encoded. For functions with type information, the
next uint16_t is encoded with the type identifier of the return type of the
function. It is followed by each of the type identifiers of the arguments,
if any exist, in the order that they appear in the function. Therefore,
argument 0 is the first type identifier and so on. When a function has a
final varargs argument, that is encoded with the type identifier of zero.

Like The Object Section, the function section is encoded in the order of
the symbol table. It has similar, but slightly different considerations
from objects. While iterating the symbol table, if any of the following
conditions are true, then the entry is skipped and no corresponding entry
is written:

+o The symbol type is not STT_FUNCTION
+o The symbol's section index is SHN_UNDEF
+o The symbol's name offset is zero
+o The symbol's name is _START_ or _END_. These are skipped because
they are used for scoping local symbols in ELF.

The Type Section

The type section is the heart of the ctf data. It encodes all of the
information about the types themselves. The base of the type information
comes in two forms, a short form and a long form, each of which may be
followed by a variable number of arguments. The following definitions
describe the short and long forms:

#define CTF_MAX_SIZE 0xfffe /* max size of a type in bytes */
#define CTF_LSIZE_SENT 0xffff /* sentinel for ctt_size */
#define CTF_MAX_LSIZE UINT64_MAX

typedef struct ctf_stype {
uint_t ctt_name; /* reference to name in string table */
ushort_t ctt_info; /* encoded kind, variant length */
union {
ushort_t _size; /* size of entire type in bytes */
ushort_t _type; /* reference to another type */
} _u;
} ctf_stype_t;

typedef struct ctf_type {
uint_t ctt_name; /* reference to name in string table */
ushort_t ctt_info; /* encoded kind, variant length */
union {
ushort_t _size; /* always CTF_LSIZE_SENT */
ushort_t _type; /* do not use */
} _u;
uint_t ctt_lsizehi; /* high 32 bits of type size in bytes */
uint_t ctt_lsizelo; /* low 32 bits of type size in bytes */
} ctf_type_t;

#define ctt_size _u._size /* for fundamental types that have a size */
#define ctt_type _u._type /* for types that reference another type */

Type sizes are stored in bytes. The basic small form uses a ushort_t to
store the number of bytes. If the number of bytes in a structure would
exceed 0xfffe, then the alternate form, the ctf_type_t, is used instead.
To indicate that the larger form is being used, the member ctt_size is set
to value of CTF_LSIZE_SENT (0xffff). In general, when going through the
type section, consumers use the ctf_type_t structure, but pay attention to
the value of the member ctt_size to determine whether they should increment
their scan by the size of the ctf_stype_t or ctf_type_t. Not all kinds of
types use ctt_size. Those which do not, will always use the ctf_stype_t
structure. The individual sections for each kind have more information.

Types are written out in order. Therefore the first entry encountered has
a type id of 0x1, or 0x8000 if a child. The member ctt_name is encoded as
described in the section String Identifiers. The string that it points to
is the name of the type. If the identifier points to an empty string (one
that consists solely of a null terminator) then the type does not have a
name, this is common with anonymous structures and unions that only have a
typedef to name them, as well as, pointers and qualifiers.

The next member, the ctt_info, is encoded as described in the section Type
Encoding. The types kind tells us how to interpret the remaining data in
the ctf_type_t and any variable length data that may exist. The rest of
this section will be broken down into the interpretation of the various
kinds.

Encoding of Integers

Integers, which are of type CTF_K_INTEGER, have no variable length
arguments. Instead, they are followed by a four byte uint_t which
describes their encoding. All integers must be encoded with a variable
length of zero. The ctt_size member describes the length of the integer in
bytes. In general, integer sizes will be rounded up to the closest power
of two.

The integer encoding contains three different pieces of information:
+o The encoding of the integer
+o The offset in bits of the type
+o The size in bits of the type

This encoding can be expressed through the following macros:

#define CTF_INT_ENCODING(data) (((data) & 0xff000000) >> 24)
#define CTF_INT_OFFSET(data) (((data) & 0x00ff0000) >> 16)
#define CTF_INT_BITS(data) (((data) & 0x0000ffff))

#define CTF_INT_DATA(encoding, offset, bits) \
(((encoding) << 24) | ((offset) << 16) | (bits))

The following flags are defined for the encoding at this time:

#define CTF_INT_SIGNED 0x01
#define CTF_INT_CHAR 0x02
#define CTF_INT_BOOL 0x04
#define CTF_INT_VARARGS 0x08

By default, an integer is considered to be unsigned, unless it has the
CTF_INT_SIGNED flag set. If the flag CTF_INT_CHAR is set, that indicates
that the integer is of a type that stores character data, for example the
intrinsic C type char would have the CTF_INT_CHAR flag set. If the flag
CTF_INT_BOOL is set, that indicates that the integer represents a boolean
type. For example, the intrinsic C type _Bool would have the CTF_INT_BOOL
flag set. Finally, the flag CTF_INT_VARARGS indicates that the integer is
used as part of a variable number of arguments. This encoding is rather
uncommon.

Encoding of Floats

Floats, which are of type CTF_K_FLOAT, are similar to their integer
counterparts. They have no variable length arguments and are followed by a
four byte encoding which describes the kind of float that exists. The
ctt_size member is the size, in bytes, of the float. The float encoding
has three different pieces of information inside of it:

+o The specific kind of float that exists
+o The offset in bits of the float
+o The size in bits of the float

This encoding can be expressed through the following macros:

#define CTF_FP_ENCODING(data) (((data) & 0xff000000) >> 24)
#define CTF_FP_OFFSET(data) (((data) & 0x00ff0000) >> 16)
#define CTF_FP_BITS(data) (((data) & 0x0000ffff))

#define CTF_FP_DATA(encoding, offset, bits) \
(((encoding) << 24) | ((offset) << 16) | (bits))

Where as the encoding for integers was a series of flags, the encoding for
floats maps to a specific kind of float. It is not a flag-based value.
The kinds of floats correspond to both their size, and the encoding. This
covers all of the basic C intrinsic floating point types. The following
are the different kinds of floats represented in the encoding:

#define CTF_FP_SINGLE 1 /* IEEE 32-bit float encoding */
#define CTF_FP_DOUBLE 2 /* IEEE 64-bit float encoding */
#define CTF_FP_CPLX 3 /* Complex encoding */
#define CTF_FP_DCPLX 4 /* Double complex encoding */
#define CTF_FP_LDCPLX 5 /* Long double complex encoding */
#define CTF_FP_LDOUBLE 6 /* Long double encoding */
#define CTF_FP_INTRVL 7 /* Interval (2x32-bit) encoding */
#define CTF_FP_DINTRVL 8 /* Double interval (2x64-bit) encoding */
#define CTF_FP_LDINTRVL 9 /* Long double interval (2x128-bit) encoding */
#define CTF_FP_IMAGRY 10 /* Imaginary (32-bit) encoding */
#define CTF_FP_DIMAGRY 11 /* Long imaginary (64-bit) encoding */
#define CTF_FP_LDIMAGRY 12 /* Long double imaginary (128-bit) encoding */

Encoding of Arrays

Arrays, which are of type CTF_K_ARRAY, have no variable length arguments.
They are followed by a structure which describes the number of elements in
the array, the type identifier of the elements in the array, and the type
identifier of the index of the array. With arrays, the ctt_size member is
set to zero. The structure that follows an array is defined as:

typedef struct ctf_array {
ushort_t cta_contents; /* reference to type of array contents */
ushort_t cta_index; /* reference to type of array index */
uint_t cta_nelems; /* number of elements */
} ctf_array_t;

The cta_contents and cta_index members of the ctf_array_t are type
identifiers which are encoded as per the section Type Identifiers. The
member cta_nelems is a simple four byte unsigned count of the number of
elements. This count may be zero when encountering C99's flexible array
members.

Encoding of Functions

Function types, which are of type CTF_K_FUNCTION, use the variable length
list to be the number of arguments in the function. When the function has
a final member which is a varargs, then the argument count is incremented
by one to account for the variable argument. Here, the ctt_type member is
encoded with the type identifier of the return type of the function. Note
that the ctt_size member is not used here.

The variable argument list contains the type identifiers for the arguments
of the function, if any. Each one is represented by a uint16_t and encoded
according to the Type Identifiers section. If the function's last argument
is of type varargs, then it is also written out, but the type identifier is
zero. This is included in the count of the function's arguments.

Encoding of Structures and Unions

Structures and Unions, which are encoded with CTF_K_STRUCT and CTF_K_UNION
respectively, are very similar constructs in C. The main difference
between them is the fact that every member of a structure follows one
another, where as in a union, all members share the same memory. They are
also very similar in terms of their encoding in ctf. The variable length
argument for structures and unions represents the number of members that
they have. The value of the member ctt_size is the size of the structure
and union. There are two different structures which are used to encode
members in the variable list. When the size of a structure or union is
greater than or equal to the large member threshold, 8192, then a different
structure is used to encode the member, all members are encoded using the
same structure. The structure for members is as follows:

typedef struct ctf_member {
uint_t ctm_name; /* reference to name in string table */
ushort_t ctm_type; /* reference to type of member */
ushort_t ctm_offset; /* offset of this member in bits */
} ctf_member_t;

typedef struct ctf_lmember {
uint_t ctlm_name; /* reference to name in string table */
ushort_t ctlm_type; /* reference to type of member */
ushort_t ctlm_pad; /* padding */
uint_t ctlm_offsethi; /* high 32 bits of member offset in bits */
uint_t ctlm_offsetlo; /* low 32 bits of member offset in bits */
} ctf_lmember_t;

Both the ctm_name and ctlm_name refer to the name of the member. The name
is encoded as an offset into the string table as described by the section
String Identifiers. The members ctm_type and ctlm_type both refer to the
type of the member. They are encoded as per the section Type Identifiers.

The last piece of information that is present is the offset which describes
the offset in memory that the member begins at. For unions, this value
will always be zero because the start of unions in memory is always zero.
For structures, this is the offset in bits that the member begins at. Note
that a compiler may lay out a type with padding. This means that the
difference in offset between two consecutive members may be larger than the
size of the member. When the size of the overall structure is strictly
less than 8192 bytes, the normal structure, ctf_member_t, is used and the
offset in bits is stored in the member ctm_offset. However, when the size
of the structure is greater than or equal to 8192 bytes, then the number of
bits is split into two 32-bit quantities. One member, ctlm_offsethi,
represents the upper 32 bits of the offset, while the other member,
ctlm_offsetlo, represents the lower 32 bits of the offset. These can be
joined together to get a 64-bit sized offset in bits by shifting the member
ctlm_offsethi to the left by thirty two and then doing a binary or of
ctlm_offsetlo.

Encoding of Enumerations

Enumerations, noted by the type CTF_K_ENUM, are similar to structures.
Enumerations use the variable list to note the number of values that the
enumeration contains, which we'll term enumerators. In C, an enumeration
is always equivalent to the intrinsic type int, thus the value of the
member ctt_size is always the size of an integer which is determined based
on the current model. For illumos systems, this will always be 4, as an
integer is always defined to be 4 bytes large in both ILP32 and LP64,
regardless of the architecture.

The enumerators encoded in an enumeration have the following structure in
the variable list:

typedef struct ctf_enum {
uint_t cte_name; /* reference to name in string table */
int cte_value; /* value associated with this name */
} ctf_enum_t;

The member cte_name refers to the name of the enumerator's value, it is
encoded according to the rules in the section String Identifiers. The
member cte_value contains the integer value of this enumerator.

Encoding of Forward References

Forward references, types of kind CTF_K_FORWARD, in a ctf file refer to
types which may not have a definition at all, only a name. If the ctf file
is a child, then it may be that the forward is resolved to an actual type
in the parent, otherwise the definition may be in another ctf container or
may not be known at all. The only member of the ctf_type_t that matters
for a forward declaration is the ctt_name which points to the name of the
forward reference in the string table as described earlier. There is no
other information recorded for forward references.

Encoding of Pointers, Typedefs, Volatile, Const, and Restrict
Pointers, typedefs, volatile, const, and restrict are all similar in ctf.
They all refer to another type. In the case of typedefs, they provide an
alternate name, while volatile, const, and restrict change how the type is
interpreted in the C programming language. This covers the ctf kinds
CTF_K_POINTER, CTF_K_TYPEDEF, CTF_K_VOLATILE, CTF_K_RESTRICT, and
CTF_K_CONST.

These types have no variable list entries and use the member ctt_type to
refer to the base type that they modify.

Encoding of Unknown Types

Types with the kind CTF_K_UNKNOWN are used to indicate gaps in the type
identifier space. These entries consume an identifier, but do not define
anything. Nothing should refer to these gap identifiers.

Dependencies Between Types

C types can be imagined as a directed, cyclic, graph. Structures and
unions may refer to each other in a way that creates a cyclic dependency.
In cases such as these, the entire type section must be read in and
processed. Consumers must not assume that every type can be laid out in
dependency order; they cannot.

The String Section

The last section of the ctf file is the string section. This section
encodes all of the strings that appear throughout the other sections. It
is laid out as a series of characters followed by a null terminator.
Generally, all names are written out in ASCII, as most C compilers do not
allow and characters to appear in identifiers outside of a subset of ASCII.
However, any extended characters sets should be written out as a series of
UTF-8 bytes.

The first entry in the section, at offset zero, is a single null terminator
to reference the empty string. Following that, each C string should be
written out, including the null terminator. Offsets that refer to
something in this section should refer to the first byte which begins a
string. Beyond the first byte in the section being the null terminator,
the order of strings is unimportant.

Data Encoding and ELF Considerations
ctf data is generally included in ELF objects which specify information to
identify the architecture and endianness of the file. A ctf container
inside such an object must match the endianness of the ELF object. Aside
from the question of the endian encoding of data, there should be no other
differences between architectures. While many of the types in this
document refer to non-fixed size C integral types, they are equivalent in
the models ILP32 and LP64. If any other model is being used with ctf data
that has different sizes, then it must not use the model's sizes for those
integral types and instead use the fixed size equivalents based on an ILP32
environment.

When placing a ctf container inside of an ELF object, there are certain
conventions that are expected for the purposes of tooling being able to
find the ctf data. In particular, a given ELF object should only contain a
single ctf section. Multiple containers should be merged together into a
single one.

The ctf file should be included in its own ELF section. The section's name
must be `.SUNW_ctf'. The type of the section must be SHT_PROGBITS. The
section should have a link set to the symbol table and its address
alignment must be 4.

SEE ALSO

mdb(1), gelf(3ELF), libelf(3LIB), a.out(5), dtrace(8)

illumos December 2, 2023 illumos