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23.3.2.21 GDB/MI Commands In Python

It is possible to add GDB/MI (see GDB/MI) commands implemented in Python. A GDB/MI command is implemented using an instance of the gdb.MICommand class, most commonly using a subclass.

Function: MICommand.__init__ (name)

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

name is the name of the command. It must be a valid name of a GDB/MI command, and in particular must start with a hyphen (-). Reusing the name of a built-in GDB/MI is not allowed, and a RuntimeError will be raised. Using the name of an GDB/MI command previously defined in Python is allowed, the previous command will be replaced with the new command.

Function: MICommand.invoke (arguments)

This method is called by GDB when the new MI command is invoked.

arguments is a list of strings. Note, that --thread and --frame arguments are handled by GDB itself therefore they do not show up in arguments.

If this method raises an exception, then it is turned into a GDB/MI ^error response. Only gdb.GdbError exceptions (or its sub-classes) should be used for reporting errors to users, any other exception type is treated as a failure of the invoke method, and the exception will be printed to the error stream according to the set python print-stack setting (see set python print-stack).

If this method returns None, then the GDB/MI command will return a ^done response with no additional values.

Otherwise, the return value must be a dictionary, which is converted to a GDB/MI result-record (see GDB/MI Output Syntax). The keys of this dictionary must be strings, and are used as variable names in the result-record, these strings must comply with the naming rules detailed below. The values of this dictionary are recursively handled as follows:

The strings used for variable names in the GDB/MI output must follow the following rules; the string must be at least one character long, the first character must be in the set [a-zA-Z], while every subsequent character must be in the set [-_a-zA-Z0-9].

An instance of MICommand has the following attributes:

Variable: MICommand.name

A string, the name of this GDB/MI command, as was passed to the __init__ method. This attribute is read-only.

Variable: MICommand.installed

A boolean value indicating if this command is installed ready for a user to call from the command line. Commands are automatically installed when they are instantiated, after which this attribute will be True.

If later, a new command is created with the same name, then the original command will become uninstalled, and this attribute will be False.

This attribute is read-write, setting this attribute to False will uninstall the command, removing it from the set of available commands. Setting this attribute to True will install the command for use. If there is already a Python command with this name installed, the currently installed command will be uninstalled, and this command installed in its place.

The following code snippet shows how a two trivial MI command can be implemented in Python:

class MIEcho(gdb.MICommand):
    """Echo arguments passed to the command."""

    def __init__(self, name, mode):
        self._mode = mode
        super(MIEcho, self).__init__(name)

    def invoke(self, argv):
        if self._mode == 'dict':
            return { 'dict': { 'argv' : argv } }
        elif self._mode == 'list':
            return { 'list': argv }
        else:
            return { 'string': ", ".join(argv) }


MIEcho("-echo-dict", "dict")
MIEcho("-echo-list", "list")
MIEcho("-echo-string", "string")

The last three lines instantiate the class three times, creating three new GDB/MI commands -echo-dict, -echo-list, and -echo-string. Each time a subclass of gdb.MICommand is instantiated, the new command is automatically registered with GDB.

Depending on how the Python code is read into GDB, you may need to import the gdb module explicitly.

The following example shows a GDB session in which the above commands have been added:

(gdb)
-echo-dict abc def ghi
^done,dict={argv=["abc","def","ghi"]}
(gdb)
-echo-list abc def ghi
^done,list=["abc","def","ghi"]
(gdb)
-echo-string abc def ghi
^done,string="abc, def, ghi"
(gdb)

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