Debugging with GDB: Parameters In Python

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#### 23.3.2.21 Parameters In Python

You can implement new GDB parameters using Python. A new parameter is implemented as an instance of the `gdb.Parameter` class.

Parameters are exposed to the user via the `set` and `show` commands. See Help.

There are many parameters that already exist and can be set in GDB. Two examples are: `set follow fork` and `set charset`. Setting these parameters influences certain behavior in GDB. Similarly, you can define parameters that can be used to influence behavior in custom Python scripts and commands.

Function: Parameter.__init__ (name, command-class, parameter-class [, enum-sequence])

  The object initializer for `Parameter` registers the new parameter with GDB. This initializer is normally invoked from the subclass’ own `__init__` method.

  name is the name of the new parameter. If name consists of multiple words, then the initial words are looked for as prefix parameters. An example of this can be illustrated with the `set print` set of parameters. If name is `print foo`, then `print` will be searched as the prefix parameter. In this case the parameter can subsequently be accessed in GDB as `set print foo`.

  If name consists of multiple words, and no prefix parameter group can be found, an exception is raised.

  command-class should be one of the ‘`COMMAND_`’ constants (see Commands In Python). This argument tells GDB how to categorize the new parameter in the help system.

  parameter-class should be one of the ‘`PARAM_`’ constants defined below. This argument tells GDB the type of the new parameter; this information is used for input validation and completion.

  If parameter-class is `PARAM_ENUM`, then enum-sequence must be a sequence of strings. These strings represent the possible values for the parameter.

  If parameter-class is not `PARAM_ENUM`, then the presence of a fourth argument will cause an exception to be thrown.

  The help text for the new parameter is taken from the Python documentation string for the parameter’s class, if there is one. If there is no documentation string, a default value is used.

Variable: Parameter.set_doc

  If this attribute exists, and is a string, then its value is used as the help text for this parameter’s `set` command. The value is examined when `Parameter.__init__` is invoked; subsequent changes have no effect.

Variable: Parameter.show_doc

  If this attribute exists, and is a string, then its value is used as the help text for this parameter’s `show` command. The value is examined when `Parameter.__init__` is invoked; subsequent changes have no effect.

Variable: Parameter.value

  The `value` attribute holds the underlying value of the parameter. It can be read and assigned to just as any other attribute. GDB does validation when assignments are made.

There are two methods that may be implemented in any `Parameter` class. These are:

Function: Parameter.get_set_string (self)

  If this method exists, GDB will call it when a parameter’s value has been changed via the `set` API (for example, `set foo off`). The `value` attribute has already been populated with the new value and may be used in output. This method must return a string. If the returned string is not empty, GDB will present it to the user.

  If this method raises the `gdb.GdbError` exception (see Exception Handling), then GDB will print the exception’s string and the `set` command will fail. Note, however, that the `value` attribute will not be reset in this case. So, if your parameter must validate values, it should store the old value internally and reset the exposed value, like so:

      class ExampleParam (gdb.Parameter):
         def __init__ (self, name):
            super (ExampleParam, self).__init__ (name,
                         gdb.COMMAND_DATA,
                         gdb.PARAM_BOOLEAN)
            self.value = True
            self.saved_value = True
         def validate(self):
            return False
         def get_set_string (self):
            if not self.validate():
              self.value = self.saved_value
              raise gdb.GdbError('Failed to validate')
            self.saved_value = self.value
            return ""

Function: Parameter.get_show_string (self, svalue)

  GDB will call this method when a parameter’s `show` API has been invoked (for example, `show foo`). The argument `svalue` receives the string representation of the current value. This method must return a string.

When a new parameter is defined, its type must be specified. The available types are represented by constants defined in the `gdb` module:

`gdb.PARAM_BOOLEAN`

  The value is a plain boolean. The Python boolean values, `True` and `False` are the only valid values.

`gdb.PARAM_AUTO_BOOLEAN`

  The value has three possible states: true, false, and ‘`auto`’. In Python, true and false are represented using boolean constants, and ‘`auto`’ is represented using `None`.

`gdb.PARAM_UINTEGER`

  The value is an unsigned integer. The value of 0 should be interpreted to mean “unlimited”.

`gdb.PARAM_INTEGER`

  The value is a signed integer. The value of 0 should be interpreted to mean “unlimited”.

`gdb.PARAM_STRING`

  The value is a string. When the user modifies the string, any escape sequences, such as ‘`\t`’, ‘`\f`’, and octal escapes, are translated into corresponding characters and encoded into the current host charset.

`gdb.PARAM_STRING_NOESCAPE`

  The value is a string. When the user modifies the string, escapes are passed through untranslated.

`gdb.PARAM_OPTIONAL_FILENAME`

  The value is a either a filename (a string), or `None`.

`gdb.PARAM_FILENAME`

  The value is a filename. This is just like `PARAM_STRING_NOESCAPE`, but uses file names for completion.

`gdb.PARAM_ZINTEGER`

  The value is an integer. This is like `PARAM_INTEGER`, except 0 is interpreted as itself.

`gdb.PARAM_ZUINTEGER`

  The value is an unsigned integer. This is like `PARAM_INTEGER`, except 0 is interpreted as itself, and the value cannot be negative.

`gdb.PARAM_ZUINTEGER_UNLIMITED`

  The value is a signed integer. This is like `PARAM_ZUINTEGER`, except the special value -1 should be interpreted to mean “unlimited”. Other negative values are not allowed.

`gdb.PARAM_ENUM`

  The value is a string, which must be one of a collection string constants provided when the parameter is created.

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