Debugging with GDB: Functions In Python

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#### 23.2.2.21 Writing new convenience functions

You can implement new convenience functions (see Convenience Vars) in Python. A convenience function is an instance of a subclass of the class `gdb.Function`.

Function: Function.__init__ (name)

  The initializer for `Function` registers the new function with GDB. The argument name is the name of the function, a string. The function will be visible to the user as a convenience variable of type `internal function`, whose name is the same as the given name.

  The documentation for the new function is taken from the documentation string for the new class.

Function: Function.invoke (*args)

  When a convenience function is evaluated, its arguments are converted to instances of `gdb.Value`, and then the function’s `invoke` method is called. Note that GDB does not predetermine the arity of convenience functions. Instead, all available arguments are passed to `invoke`, following the standard Python calling convention. In particular, a convenience function can have default values for parameters without ill effect.

  The return value of this method is used as its value in the enclosing expression. If an ordinary Python value is returned, it is converted to a `gdb.Value` following the usual rules.

The following code snippet shows how a trivial convenience function can be implemented in Python:

    class Greet (gdb.Function):
      """Return string to greet someone.
    Takes a name as argument."""

      def __init__ (self):
        super (Greet, self).__init__ ("greet")

      def invoke (self, name):
        return "Hello, %s!" % name.string ()

    Greet ()

The last line instantiates the class, and is necessary to trigger the registration of the function with GDB. Depending on how the Python code is read into GDB, you may need to import the `gdb` module explicitly.

Now you can use the function in an expression:

    (gdb) print $greet("Bob")
    $1 = "Hello, Bob!"

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