Debugging with GDB: Target Description Format

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### G.2 Target Description Format

A target description annex is an XML document which complies with the Document Type Definition provided in the GDB sources in `gdb/features/gdb-target.dtd`. This means you can use generally available tools like `xmllint` to check that your feature descriptions are well-formed and valid. However, to help people unfamiliar with XML write descriptions for their targets, we also describe the grammar here.

Target descriptions can identify the architecture of the remote target and (for some architectures) provide information about custom register sets. They can also identify the OS ABI of the remote target. GDB can use this information to autoconfigure for your target, or to warn you if you connect to an unsupported target.

Here is a simple target description:

    <target version="1.0">
      <architecture>i386:x86-64</architecture>
    </target>

This minimal description only says that the target uses the x86-64 architecture.

A target description has the following overall form, with [ ] marking optional elements and … marking repeatable elements. The elements are explained further below.

    <?xml version="1.0"?>
    <!DOCTYPE target SYSTEM "gdb-target.dtd">
    <target version="1.0">
      [architecture]
      [osabi]
      [compatible]
      [feature…]
    </target>

The description is generally insensitive to whitespace and line breaks, under the usual common-sense rules. The XML version declaration and document type declaration can generally be omitted (GDB does not require them), but specifying them may be useful for XML validation tools. The ‘`version`’ attribute for ‘`<target>`’ may also be omitted, but we recommend including it; if future versions of GDB use an incompatible revision of `gdb-target.dtd`, they will detect and report the version mismatch.

#### G.2.1 Inclusion

It can sometimes be valuable to split a target description up into several different annexes, either for organizational purposes, or to share files between different possible target descriptions. You can divide a description into multiple files by replacing any element of the target description with an inclusion directive of the form:

    <xi:include href="document"/>

When GDB encounters an element of this form, it will retrieve the named XML document, and replace the inclusion directive with the contents of that document. If the current description was read using ‘`qXfer`’, then so will be the included document; document will be interpreted as the name of an annex. If the current description was read from a file, GDB will look for document as a file in the same directory where it found the original description.

#### G.2.2 Architecture

An ‘`<architecture>`’ element has this form:

      <architecture>arch</architecture>

arch is one of the architectures from the set accepted by `set architecture` (see Specifying a Debugging Target).

#### G.2.3 OS ABI

This optional field was introduced in GDB version 7.0. Previous versions of GDB ignore it.

An ‘`<osabi>`’ element has this form:

      <osabi>abi-name</osabi>

abi-name is an OS ABI name from the same selection accepted by `set osabi` (see Configuring the Current ABI).

#### G.2.4 Compatible Architecture

This optional field was introduced in GDB version 7.0. Previous versions of GDB ignore it.

A ‘`<compatible>`’ element has this form:

      <compatible>arch</compatible>

arch is one of the architectures from the set accepted by `set architecture` (see Specifying a Debugging Target).

A ‘`<compatible>`’ element is used to specify that the target is able to run binaries in some other than the main target architecture given by the ‘`<architecture>`’ element. For example, on the Cell Broadband Engine, the main architecture is `powerpc:common` or `powerpc:common64`, but the system is able to run binaries in the `spu` architecture as well. The way to describe this capability with ‘`<compatible>`’ is as follows:

      <architecture>powerpc:common</architecture>
      <compatible>spu</compatible>

#### G.2.5 Features

Each ‘`<feature>`’ describes some logical portion of the target system. Features are currently used to describe available CPU registers and the types of their contents. A ‘`<feature>`’ element has this form:

    <feature name="name">
      [type…]
      reg…
    </feature>

Each feature’s name should be unique within the description. The name of a feature does not matter unless GDB has some special knowledge of the contents of that feature; if it does, the feature should have its standard name. See Standard Target Features.

#### G.2.6 Types

Any register’s value is a collection of bits which GDB must interpret. The default interpretation is a two’s complement integer, but other types can be requested by name in the register description. Some predefined types are provided by GDB (see Predefined Target Types), and the description can define additional composite types.

Each type element must have an ‘`id`’ attribute, which gives a unique (within the containing ‘`<feature>`’) name to the type. Types must be defined before they are used.

Some targets offer vector registers, which can be treated as arrays of scalar elements. These types are written as ‘`<vector>`’ elements, specifying the array element type, type, and the number of elements, count:

    <vector id="id" type="type" count="count"/>

If a register’s value is usefully viewed in multiple ways, define it with a union type containing the useful representations. The ‘`<union>`’ element contains one or more ‘`<field>`’ elements, each of which has a name and a type:

    <union id="id">
      <field name="name" type="type"/>
      …
    </union>

If a register’s value is composed from several separate values, define it with a structure type. There are two forms of the ‘`<struct>`’ element; a ‘`<struct>`’ element must either contain only bitfields or contain no bitfields. If the structure contains only bitfields, its total size in bytes must be specified, each bitfield must have an explicit start and end, and bitfields are automatically assigned an integer type. The field’s start should be less than or equal to its end, and zero represents the least significant bit.

    <struct id="id" size="size">
      <field name="name" start="start" end="end"/>
      …
    </struct>

If the structure contains no bitfields, then each field has an explicit type, and no implicit padding is added.

    <struct id="id">
      <field name="name" type="type"/>
      …
    </struct>

If a register’s value is a series of single-bit flags, define it with a flags type. The ‘`<flags>`’ element has an explicit size and contains one or more ‘`<field>`’ elements. Each field has a name, a start, and an end. Only single-bit flags are supported.

    <flags id="id" size="size">
      <field name="name" start="start" end="end"/>
      …
    </flags>

#### G.2.7 Registers

Each register is represented as an element with this form:

    <reg name="name"
         bitsize="size"
         [regnum="num"]
         [save-restore="save-restore"]
         [type="type"]
         [group="group"]/>

The components are as follows:

name

  The register’s name; it must be unique within the target description.

bitsize

  The register’s size, in bits.

regnum

  The register’s number. If omitted, a register’s number is one greater than that of the previous register (either in the current feature or in a preceding feature); the first register in the target description defaults to zero. This register number is used to read or write the register; e.g. it is used in the remote `p` and `P` packets, and registers appear in the `g` and `G` packets in order of increasing register number.

save-restore

  Whether the register should be preserved across inferior function calls; this must be either `yes` or `no`. The default is `yes`, which is appropriate for most registers except for some system control registers; this is not related to the target’s ABI.

type

  The type of the register. It may be a predefined type, a type defined in the current feature, or one of the special types `int` and `float`. `int` is an integer type of the correct size for bitsize, and `float` is a floating point type (in the architecture’s normal floating point format) of the correct size for bitsize. The default is `int`.

group

  The register group to which this register belongs. It must be either `general`, `float`, or `vector`. If no group is specified, GDB will not display the register in `info registers`.

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