Advanced Usage
==============

.. highlight:: pycon

The :doc:`user_guide` covers basic usage of drgn, but drgn also supports more
advanced use cases which are covered here.

Loading Debugging Symbols
-------------------------

drgn will automatically load debugging information based on the debugged
program (e.g., from loaded kernel modules or loaded shared libraries).
:meth:`drgn.Program.load_debug_info()` can be used to load additional debugging
information::

    >>> prog.load_debug_info(['./libfoo.so', '/usr/lib/libbar.so'])

Library
-------

In addition to the CLI, drgn is also available as a library.
:func:`drgn.program_from_core_dump()`, :func:`drgn.program_from_kernel()`, and
:func:`drgn.program_from_pid()` correspond to the ``-c``, ``-k``, and ``-p``
command line options, respectively; they return a :class:`drgn.Program` that
can be used just like the one initialized by the CLI::

    >>> import drgn
    >>> prog = drgn.program_from_kernel()

C Library
---------

The core functionality of drgn is implemented in C and is available as a C
library, ``libdrgn``. See |drgn.h|_.

.. |drgn.h| replace:: ``drgn.h``
.. _drgn.h: https://github.com/osandov/drgn/blob/master/libdrgn/drgn.h.in

Full documentation can be generated by running ``doxygen`` in the ``libdrgn``
directory of the source code. Note that the API and ABI are not yet stable.

Custom Programs
---------------

The main components of a :class:`drgn.Program` are the program memory, types,
and symbols. The CLI and equivalent library interfaces automatically determine
these. However, it is also possible to create a "blank" ``Program`` and plug in
the main components.

:meth:`drgn.Program.add_memory_segment()` defines a range of memory and how to
read that memory. The following example uses a Btrfs filesystem image as the
program "memory":

.. code-block:: python3

    import drgn
    import os
    import sys


    def btrfs_debugger(dev):
        file = open(dev, 'rb')
        size = file.seek(0, 2)

        def read_file(address, count, physical, offset):
            file.seek(offset)
            return file.read(count)

        platform = drgn.Platform(drgn.Architecture.UNKNOWN,
                                 drgn.PlatformFlags.IS_LITTLE_ENDIAN)
        prog = drgn.Program(platform)
        prog.add_memory_segment(0, size, read_file)
        prog.load_debug_info([f'/lib/modules/{os.uname().release}/kernel/fs/btrfs/btrfs.ko'])
        return prog


    prog = btrfs_debugger(sys.argv[1] if len(sys.argv) >= 2 else '/dev/sda')
    print(drgn.Object(prog, 'struct btrfs_super_block', address=65536))

:meth:`drgn.Program.add_type_finder()` and
:meth:`drgn.Program.add_symbol_finder()` are the equivalent methods for
plugging in types and symbols.

Environment Variables
---------------------

Some of drgn's behavior can be modified through environment variables:

``DRGN_MAX_DEBUG_INFO_ERRORS``
    The maximum number of individual errors to report in a
    :exc:`drgn.MissingDebugInfoError`. Any additional errors are truncated. The
    default is 5; -1 is unlimited.

``DRGN_USE_LIBKDUMPFILE_FOR_ELF``
    Whether drgn should use libkdumpfile for ELF vmcores (0 or 1). The default
    is 0. This functionality will be removed in the future.