xref: /aosp_15_r20/external/trace-cmd/Documentation/README.PythonPlugin (revision 58e6ee5f017f6a8912852c892d18457e4bafb554)

 PYTHON PLUGIN DOCUMENTATION
=============================

With the python plugin (make python-plugin) you can now
write plugins in python. The API exported by the python
plugin itself (written in C) allows you to access most
information about a record from python.

To write a python plugin, put a new .py file into a new
~/.trace-cmd/python/ directory.

The most basic python plugin is this:

--- %< ---
def register(pevent):
    pass
--- >% ---

which obviously does nothing at all.

To register a callback, use the pevent.register_event_handler
function:

--- %< ---
import tracecmd

def my_event_handler(trace_seq, event):
    pass

def register(pevent):
    pevent.register_event_handler("subsys", "event_name",
                                  my_event_handler)
--- >% ---


There are four object types that you get, described below.

 tracecmd.PEvent
-----------------

This is the class of the 'pevent' object above,
you get one of those via your register callback.
It has one method and one property:
 * register_event_handler() - example above, to register
                              an event handler function
 * file_endian              - either '<' or '>' indicating
                              which endianness the file has,
                              to be used with struct.unpack()

 tracecmd.TraceSeq
-------------------

This is the class of the 'trace_seq' parameter to your callback
function. It has only one method, puts(), to put data into the
buffer. Formatting must be done in python.

 tracecmd.Event
----------------------

This is the class of the 'event' parameter to your callback
function. Note that it doesn't just contain the format, but
also the event data. As such, you can do much with this, and
this is what you'll usually use. Each instance of this allows
access to record items via the dict protocol, and you can get
the items via its keys() methods. So for example, your
callback could be

--- %< ---
def my_callback(trace_seq, event):
    for fieldname in event.keys():
        field = event[fieldname]
--- >% ---

Each field returned from the dict protocol is an instance of
the next (and last) class:

 tracecmd.Field
----------------------

This is an instance of a field, including its data. It affords
numerous use cases and is what you'll be using most.

 * If this is an integer field, i.e. 1, 2, 4 or 8 bytes long,
   you can convert it to the number contained, according to
   the file's endianness, by simply casting it to a long:

     field = event['myint']
     value = long(field)

 * You can access the field's data, as field.data, and if the
   data is really a "__data_loc" type that will be resolved
   automatically. (If you don't know what this means, don't
   worry about it and just use field.data)


This is it. It's pretty simple. A fully-featured plugin could
look like this:

--- %< ---
def my_event_handler(trace_seq, event):
    trace_seq.puts("myev: %u", long(event['myfield']))

def register(pevent):
    pevent.register_event_handler("subsys", "event_name",
                                  my_event_handler)
--- >% ---


 Tips and tricks
-----------------

Be familiar with the struct module and use it, always
checking endianness and potentially using pevent.file_endian.


If you need access to pevent in your callbacks, simply
pass it in yourself:

--- %< ---
def my_event_handler(pevent, trace_seq, event):
    pass

def register(pevent):
    pevent.register_event_handler("subsys", "event_name",
        lambda *args: my_event_handler(pevent, *args)
    )
--- >% ---