Types In Python (Debugging with GDB)
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23.3.2.4 Types In Python
GDB represents types from the inferior using the class gdb.Type
.
The following type-related functions are available in the gdb
module:
- Function: gdb.lookup_type (name [, block])
This function looks up a type by its
name
, which must be a string.If
block
is given, thenname
is looked up in that scope. Otherwise, it is searched for globally.Ordinarily, this function will return an instance of
gdb.Type
. If the named type cannot be found, it will throw an exception.
If the type is a structure or class type, or an enum type, the fields of that type can be accessed using the Python dictionary syntax. For example, if some_type
is a gdb.Type
instance holding a structure type, you can access its foo
field with:
bar = some_type['foo']
bar
will be a gdb.Field
object; see below under the description of the Type.fields
method for a description of the gdb.Field
class.
An instance of Type
has the following attributes:
- Variable
- Type.alignof
- The alignment of this type, in bytes. Type alignment comes from the debugging information; if it was not specified, then GDB will use the relevant ABI to try to determine the alignment. In some cases, even this is not possible, and zero will be returned.
- Variable
- Type.code
- The type code for this type. The type code will be one of the
TYPE_CODE_
constants defined below.
- Variable: Type.dynamic
A boolean indicating whether this type is dynamic. In some situations, such as Rust
enum
types or Ada variant records, the concrete type of a value may vary depending on its contents. That is, the declared type of a variable, or the type returned bygdb.lookup_type
may be dynamic; while the type of the variable’s value will be a concrete instance of that dynamic type.For example, consider this code:
int n; int array[n];
Here, at least conceptually (whether your compiler actually does this is a separate issue), examining
gdb.lookup_symbol("array", ...).type
/@w could yield agdb.Type
which reports a size ofNone
. This is the dynamic type.However, examining
gdb.parse_and_eval("array").type
would yield a concrete type, whose length would be known.
- Variable
- Type.name
- The name of this type. If this type has no name, then
None
is returned.
- Variable
- Type.sizeof
- The size of this type, in target
char
units. Usually, a target’schar
type will be an 8-bit byte. However, on some unusual platforms, this type may have a different size. A dynamic type may not have a fixed size; in this case, this attribute’s value will beNone
.
- Variable
- Type.tag
- The tag name for this type. The tag name is the name after
struct
,union
, orenum
in C and C++; not all languages have this concept. If this type has no tag name, thenNone
is returned.
- Variable
- Type.objfile
- The
gdb.Objfile
that this type was defined in, orNone
if there is no associated objfile.
The following methods are provided:
- Function: Type.fields ()
Return the fields of this type. The behavior depends on the type code:
- For structure and union types, this method returns the fields.
- Range types have two fields, the minimum and maximum values.
- Enum types have one field per enum constant.
- Function and method types have one field per parameter. The base types of C++ classes are also represented as fields.
- Array types have one field representing the array’s range.
- If the type does not fit into one of these categories, a
TypeError
is raised.
Each field is a
gdb.Field
object, with some pre-defined attributes:bitpos
This attribute is not available for
enum
orstatic
(as in C++) fields. The value is the position, counting in bits, from the start of the containing type. Note that, in a dynamic type, the position of a field may not be constant. In this case, the value will beNone
. Also, a dynamic type may have fields that do not appear in a corresponding concrete type.enumval
This attribute is only available for
enum
fields, and its value is the enumeration member’s integer representation.name
The name of the field, or
None
for anonymous fields.artificial
This is
True
if the field is artificial, usually meaning that it was provided by the compiler and not the user. This attribute is always provided, and isFalse
if the field is not artificial.is_base_class
This is
True
if the field represents a base class of a C++ structure. This attribute is always provided, and isFalse
if the field is not a base class of the type that is the argument offields
, or if that type was not a C++ class.bitsize
If the field is packed, or is a bitfield, then this will have a non-zero value, which is the size of the field in bits. Otherwise, this will be zero; in this case the field’s size is given by its type.
type
The type of the field. This is usually an instance of
Type
, but it can beNone
in some situations.parent_type
The type which contains this field. This is an instance of
gdb.Type
.
- Function
- Type.array (n1 [, n2])
- Return a new
gdb.Type
object which represents an array of this type. If one argument is given, it is the inclusive upper bound of the array; in this case the lower bound is zero. If two arguments are given, the first argument is the lower bound of the array, and the second argument is the upper bound of the array. An array’s length must not be negative, but the bounds can be.
- Function: Type.vector (n1 [, n2])
Return a new
gdb.Type
object which represents a vector of this type. If one argument is given, it is the inclusive upper bound of the vector; in this case the lower bound is zero. If two arguments are given, the first argument is the lower bound of the vector, and the second argument is the upper bound of the vector. A vector’s length must not be negative, but the bounds can be.The difference between an
array
and avector
is that arrays behave like in C: when used in expressions they decay to a pointer to the first element whereas vectors are treated as first class values.
- Function
- Type.const ()
- Return a new
gdb.Type
object which represents aconst
-qualified variant of this type.
- Function
- Type.volatile ()
- Return a new
gdb.Type
object which represents avolatile
-qualified variant of this type.
- Function
- Type.unqualified ()
- Return a new
gdb.Type
object which represents an unqualified variant of this type. That is, the result is neitherconst
norvolatile
.
- Function
- Type.range ()
- Return a Python
Tuple
object that contains two elements: the low bound of the argument type and the high bound of that type. If the type does not have a range, GDB will raise agdb.error
exception (see Exception Handling).
- Function
- Type.reference ()
- Return a new
gdb.Type
object which represents a reference to this type.
- Function
- Type.pointer ()
- Return a new
gdb.Type
object which represents a pointer to this type.
- Function
- Type.strip_typedefs ()
- Return a new
gdb.Type
that represents the real type, after removing all layers of typedefs.
- Function: Type.target ()
Return a new
gdb.Type
object which represents the target type of this type.For a pointer type, the target type is the type of the pointed-to object. For an array type (meaning C-like arrays), the target type is the type of the elements of the array. For a function or method type, the target type is the type of the return value. For a complex type, the target type is the type of the elements. For a typedef, the target type is the aliased type.
If the type does not have a target, this method will throw an exception.
- Function: Type.template_argument (n [, block])
If this
gdb.Type
is an instantiation of a template, this will return a newgdb.Value
orgdb.Type
which represents the value of then
th template argument (indexed starting at 0).If this
gdb.Type
is not a template type, or if the type has fewer thann
template arguments, this will throw an exception. Ordinarily, only C++ code will have template types.If
block
is given, thenname
is looked up in that scope. Otherwise, it is searched for globally.
- Function
- Type.optimized_out ()
- Return
gdb.Value
instance of this type whose value is optimized out. This allows a frame decorator to indicate that the value of an argument or a local variable is not known.
Each type has a code, which indicates what category this type falls into. The available type categories are represented by constants defined in the gdb
module:
gdb.TYPE_CODE_PTR
The type is a pointer.
gdb.TYPE_CODE_ARRAY
The type is an array.
gdb.TYPE_CODE_STRUCT
The type is a structure.
gdb.TYPE_CODE_UNION
The type is a union.
gdb.TYPE_CODE_ENUM
The type is an enum.
gdb.TYPE_CODE_FLAGS
A bit flags type, used for things such as status registers.
gdb.TYPE_CODE_FUNC
The type is a function.
gdb.TYPE_CODE_INT
The type is an integer type.
gdb.TYPE_CODE_FLT
A floating point type.
gdb.TYPE_CODE_VOID
The special type void
.
gdb.TYPE_CODE_SET
A Pascal set type.
gdb.TYPE_CODE_RANGE
A range type, that is, an integer type with bounds.
gdb.TYPE_CODE_STRING
A string type. Note that this is only used for certain languages with language-defined string types; C strings are not represented this way.
gdb.TYPE_CODE_BITSTRING
A string of bits. It is deprecated.
gdb.TYPE_CODE_ERROR
An unknown or erroneous type.
gdb.TYPE_CODE_METHOD
A method type, as found in C++.
gdb.TYPE_CODE_METHODPTR
A pointer-to-member-function.
gdb.TYPE_CODE_MEMBERPTR
A pointer-to-member.
gdb.TYPE_CODE_REF
A reference type.
gdb.TYPE_CODE_RVALUE_REF
A C++11 rvalue reference type.
gdb.TYPE_CODE_CHAR
A character type.
gdb.TYPE_CODE_BOOL
A boolean type.
gdb.TYPE_CODE_COMPLEX
A complex float type.
gdb.TYPE_CODE_TYPEDEF
A typedef to some other type.
gdb.TYPE_CODE_NAMESPACE
A C++ namespace.
gdb.TYPE_CODE_DECFLOAT
A decimal floating point type.
gdb.TYPE_CODE_INTERNAL_FUNCTION
A function internal to GDB. This is the type used to represent convenience functions.
Further support for types is provided in the gdb.types
Python module (see gdb.types).
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