Next: Types In Python, Previous: Exception Handling, Up: Python API [Contents][Index]

#### 23.2.2.3 Values From Inferior

GDB provides values it obtains from the inferior program in an object of type `gdb.Value`. GDB uses this object for its internal bookkeeping of the inferior’s values, and for fetching values when necessary.

Inferior values that are simple scalars can be used directly in Python expressions that are valid for the value’s data type. Here’s an example for an integer or floating-point value `some_val`:

    bar = some_val + 2

As result of this, `bar` will also be a `gdb.Value` object whose values are of the same type as those of `some_val`. Valid Python operations can also be performed on `gdb.Value` objects representing a `struct` or `class` object. For such cases, the overloaded operator (if present), is used to perform the operation. For example, if `val1` and `val2` are `gdb.Value` objects representing instances of a `class` which overloads the `+` operator, then one can use the `+` operator in their Python script as follows:

    val3 = val1 + val2

The result of the operation `val3` is also a `gdb.Value` object corresponding to the value returned by the overloaded `+` operator. In general, overloaded operators are invoked for the following operations: `+` (binary addition), `-` (binary subtraction), `*` (multiplication), `/`, `%`, `<<`, `>>`, `|`, `&`, `^`.

Inferior values that are structures or instances of some class can be accessed using the Python dictionary syntax. For example, if `some_val` is a `gdb.Value` instance holding a structure, you can access its `foo` element with:

    bar = some_val['foo']

Again, `bar` will also be a `gdb.Value` object. Structure elements can also be accessed by using `gdb.Field` objects as subscripts (see Types In Python, for more information on `gdb.Field` objects). For example, if `foo_field` is a `gdb.Field` object corresponding to element `foo` of the above structure, then `bar` can also be accessed as follows:

    bar = some_val[foo_field]

A `gdb.Value` that represents a function can be executed via inferior function call. Any arguments provided to the call must match the function’s prototype, and must be provided in the order specified by that prototype.

For example, `some_val` is a `gdb.Value` instance representing a function that takes two integers as arguments. To execute this function, call it like so:

    result = some_val (10,20)

Any values returned from a function call will be stored as a `gdb.Value`.

The following attributes are provided:

Variable: Value.address

  If this object is addressable, this read-only attribute holds a `gdb.Value` object representing the address. Otherwise, this attribute holds `None`.

Variable: Value.is_optimized_out

  This read-only boolean attribute is true if the compiler optimized out this value, thus it is not available for fetching from the inferior.

Variable: Value.type

  The type of this `gdb.Value`. The value of this attribute is a `gdb.Type` object (see Types In Python).

Variable: Value.dynamic_type

  The dynamic type of this `gdb.Value`. This uses C`++` run-time type information (RTTI) to determine the dynamic type of the value. If this value is of class type, it will return the class in which the value is embedded, if any. If this value is of pointer or reference to a class type, it will compute the dynamic type of the referenced object, and return a pointer or reference to that type, respectively. In all other cases, it will return the value’s static type.

  Note that this feature will only work when debugging a C`++` program that includes RTTI for the object in question. Otherwise, it will just return the static type of the value as in `ptype foo` (see ptype).

Variable: Value.is_lazy

  The value of this read-only boolean attribute is `True` if this `gdb.Value` has not yet been fetched from the inferior. GDB does not fetch values until necessary, for efficiency. For example:

      myval = gdb.parse_and_eval ('somevar')

  The value of `somevar` is not fetched at this time. It will be fetched when the value is needed, or when the `fetch_lazy` method is invoked.

The following methods are provided:

Function: Value.__init__ (val)

  Many Python values can be converted directly to a `gdb.Value` via this object initializer. Specifically:

  Python boolean

    A Python boolean is converted to the boolean type from the current language.

  Python integer

    A Python integer is converted to the C `long` type for the current architecture.

  Python long

    A Python long is converted to the C `long long` type for the current architecture.

  Python float

    A Python float is converted to the C `double` type for the current architecture.

  Python string

    A Python string is converted to a target string in the current target language using the current target encoding. If a character cannot be represented in the current target encoding, then an exception is thrown.

  `gdb.Value`

    If `val` is a `gdb.Value`, then a copy of the value is made.

  `gdb.LazyString`

    If `val` is a `gdb.LazyString` (see Lazy Strings In Python), then the lazy string’s `value` method is called, and its result is used.

Function: Value.cast (type)

  Return a new instance of `gdb.Value` that is the result of casting this instance to the type described by type, which must be a `gdb.Type` object. If the cast cannot be performed for some reason, this method throws an exception.

Function: Value.dereference ()

  For pointer data types, this method returns a new `gdb.Value` object whose contents is the object pointed to by the pointer. For example, if `foo` is a C pointer to an `int`, declared in your C program as

      int *foo;

  then you can use the corresponding `gdb.Value` to access what `foo` points to like this:

      bar = foo.dereference ()

  The result `bar` will be a `gdb.Value` object holding the value pointed to by `foo`.

  A similar function `Value.referenced_value` exists which also returns `gdb.Value` objects corresonding to the values pointed to by pointer values (and additionally, values referenced by reference values). However, the behavior of `Value.dereference` differs from `Value.referenced_value` by the fact that the behavior of `Value.dereference` is identical to applying the C unary operator `*` on a given value. For example, consider a reference to a pointer `ptrref`, declared in your C`++` program as

      typedef int *intptr;
      ...
      int val = 10;
      intptr ptr = &val;
      intptr &ptrref = ptr;

  Though `ptrref` is a reference value, one can apply the method `Value.dereference` to the `gdb.Value` object corresponding to it and obtain a `gdb.Value` which is identical to that corresponding to `val`. However, if you apply the method `Value.referenced_value`, the result would be a `gdb.Value` object identical to that corresponding to `ptr`.

      py_ptrref = gdb.parse_and_eval ("ptrref")
      py_val = py_ptrref.dereference ()
      py_ptr = py_ptrref.referenced_value ()

  The `gdb.Value` object `py_val` is identical to that corresponding to `val`, and `py_ptr` is identical to that corresponding to `ptr`. In general, `Value.dereference` can be applied whenever the C unary operator `*` can be applied to the corresponding C value. For those cases where applying both `Value.dereference` and `Value.referenced_value` is allowed, the results obtained need not be identical (as we have seen in the above example). The results are however identical when applied on `gdb.Value` objects corresponding to pointers (`gdb.Value` objects with type code `TYPE_CODE_PTR`) in a C/C`++` program.

Function: Value.referenced_value ()

  For pointer or reference data types, this method returns a new `gdb.Value` object corresponding to the value referenced by the pointer/reference value. For pointer data types, `Value.dereference` and `Value.referenced_value` produce identical results. The difference between these methods is that `Value.dereference` cannot get the values referenced by reference values. For example, consider a reference to an `int`, declared in your C`++` program as

      int val = 10;
      int &ref = val;

  then applying `Value.dereference` to the `gdb.Value` object corresponding to `ref` will result in an error, while applying `Value.referenced_value` will result in a `gdb.Value` object identical to that corresponding to `val`.

      py_ref = gdb.parse_and_eval ("ref")
      er_ref = py_ref.dereference ()       # Results in error
      py_val = py_ref.referenced_value ()  # Returns the referenced value

  The `gdb.Value` object `py_val` is identical to that corresponding to `val`.

Function: Value.reference_value ()

  Return a `gdb.Value` object which is a reference to the value encapsulated by this instance.

Function: Value.const_value ()

  Return a `gdb.Value` object which is a `const` version of the value encapsulated by this instance.

Function: Value.dynamic_cast (type)

  Like `Value.cast`, but works as if the C`++` `dynamic_cast` operator were used. Consult a C`++` reference for details.

Function: Value.reinterpret_cast (type)

  Like `Value.cast`, but works as if the C`++` `reinterpret_cast` operator were used. Consult a C`++` reference for details.

Function: Value.string ([encoding[, errors[, length]]])

  If this `gdb.Value` represents a string, then this method converts the contents to a Python string. Otherwise, this method will throw an exception.

  Values are interpreted as strings according to the rules of the current language. If the optional length argument is given, the string will be converted to that length, and will include any embedded zeroes that the string may contain. Otherwise, for languages where the string is zero-terminated, the entire string will be converted.

  For example, in C-like languages, a value is a string if it is a pointer to or an array of characters or ints of type `wchar_t`, `char16_t`, or `char32_t`.

  If the optional encoding argument is given, it must be a string naming the encoding of the string in the `gdb.Value`, such as `"ascii"`, `"iso-8859-6"` or `"utf-8"`. It accepts the same encodings as the corresponding argument to Python’s `string.decode` method, and the Python codec machinery will be used to convert the string. If encoding is not given, or if encoding is the empty string, then either the `target-charset` (see Character Sets) will be used, or a language-specific encoding will be used, if the current language is able to supply one.

  The optional errors argument is the same as the corresponding argument to Python’s `string.decode` method.

  If the optional length argument is given, the string will be fetched and converted to the given length.

Function: Value.lazy_string ([encoding [, length]])

  If this `gdb.Value` represents a string, then this method converts the contents to a `gdb.LazyString` (see Lazy Strings In Python). Otherwise, this method will throw an exception.

  If the optional encoding argument is given, it must be a string naming the encoding of the `gdb.LazyString`. Some examples are: ‘`ascii`’, ‘`iso-8859-6`’ or ‘`utf-8`’. If the encoding argument is an encoding that GDB does recognize, GDB will raise an error.

  When a lazy string is printed, the GDB encoding machinery is used to convert the string during printing. If the optional encoding argument is not provided, or is an empty string, GDB will automatically select the encoding most suitable for the string type. For further information on encoding in GDB please see Character Sets.

  If the optional length argument is given, the string will be fetched and encoded to the length of characters specified. If the length argument is not provided, the string will be fetched and encoded until a null of appropriate width is found.

Function: Value.fetch_lazy ()

  If the `gdb.Value` object is currently a lazy value (`gdb.Value.is_lazy` is `True`), then the value is fetched from the inferior. Any errors that occur in the process will produce a Python exception.

  If the `gdb.Value` object is not a lazy value, this method has no effect.

  This method does not return a value.

Next: Types In Python, Previous: Exception Handling, Up: Python API [Contents][Index]