Pigweed’s tracing module provides facilities for applications to trace information about the execution of their application. The module is split into two components:

  1. The facade, provided elsewhere, which is only a macro interface layer

  2. The backend (this module), is one implemention of the low level tracing.


This module is currently in development, and is therefore still undergoing significant changes.

Future work will:

  1. Add a more complete API for how to retrieve data from ring_buffer.

  2. Add a Python library to decode the trace data.

  3. Add examples with sample output (especially for filtering and triggering).

  4. Add tools to retrieve trace data.

  5. Add more sinks, such as RTT.

  6. Add support to more platforms.

  7. Improve the locking behaviour and provide default trace locking implementions.


The tokenized trace backend aims to be a reasonable tradeoff of trace features and event size for most applications. It works by encoding all compile time data for a trace event into a tokenized number. This provides a good amount of compression, while maintaining the full trace feature set.

In addition the tokenized trace backend adds flexibility through callbacks, which allows the application to do things such as filtering trace_events and triggering tracing to turn on and off. This flexibility can help maximize the effectiveness of a limited trace buffer as well as be a valuable tool while debugging.


Most of this module is compatible with C and C++, the only exception to this is the RegisterCallbackWhenCreated helper class.


pw_assert pw_log pw_preprocessor pw_status pw_tokenizer pw_trace:facade pw_varint

Macro API#

All code should use the trace API facade directly. This backend fully implements all features of the tracing facade.

Event Callbacks & Data Sinks#

The tokenized trace module adds both event callbacks and data sinks which provide hooks into tracing.

The event callbacks are called when trace events occur, with the trace event data, before the event is encoded or sent to the sinks. The callbacks may modify the run-time fields of the trace event, i.e. trace_id, data_buffer and data_size. Using the return flags, these callbacks can be used to adjust the trace behaviour at runtime in response to specific events.

If requested (using called_on_every_event) the callback will be called on every trace event regardless if tracing is currently enabled or not. Using this, the application can trigger tracing on or off when specific traces or patterns of traces are observed, or can selectively filter traces to preserve the trace buffer.

The event callback is called in the context of the traced task. It must be ISR-safe to support tracing within ISRs. It must be lightweight to prevent performance issues in the trace tasks.

The return flags pw_trace_TraceEventReturnFlags support the following behaviors:

  • PW_TRACE_EVENT_RETURN_FLAGS_SKIP_EVENT can be set true to skip this sample.

  • PW_TRACE_EVENT_RETURN_FLAGS_DISABLE_AFTER_PROCESSING can be set true to disable tracing after this sample.

pw_trace_TraceEventReturnFlags pw_trace_EventCallback(void *user_data, pw_trace_tokenized_TraceEvent *event)#
pw_Status pw_trace_RegisterEventCallback(
pw_trace_EventCallback callback,
pw_trace_EventCallbackFlags flags,
void *user_data,
pw_trace_EventCallbackHandle *handle,
pw_Status pw_trace_UnregisterEventCallback(pw_trace_EventCallbackHandle handle)#

The data sinks are called only for trace events which get processed (tracing is enabled, and the sample not skipped). The sink callback is called with the encoded bytes of the trace event, which can be used by the application to connect different data sinks. The callback is broken into three callbacks pw_trace_SinkStartBlock, pw_trace_SinkAddBytes, and pw_trace_SinkEndBlock. Start is called with the size of the block, before any bytes are emitted and can be used if needed to allocate space. AddBytes is then called multiple times with chunks of bytes. Finally End is called to allow any cleanup to be done by the sink if neccessary. Not all callbacks are required, it is acceptible to provide nullptr for any callbacks which you don’t require.

void pw_trace_SinkStartBlock(void *user_data, size_t size)#
void pw_trace_SinkAddBytes(void *user_data, const void *bytes, size_t size)#
void pw_trace_SinkEndBlock(void *user_data)#
pw_Status pw_trace_RegisterSink(
pw_trace_SinkStartBlock start,
pw_trace_SinkAddBytes add_bytes,
pw_trace_SinkEndBlock end_block,
void *user_data,
pw_trace_SinkHandle *handle,
pw_Status pw_trace_UnregisterSink(pw_trace_SinkHandle handle)#

Trace Reference#

Some use-cases might involve referencing a specific trace event, for example to use it as a trigger or filtering. Since the trace events are tokenized, a macro is provided to generate the token to use as a reference. All the fields must match exactly to generate the correct trace reference. If the trace does not have a group, use PW_TRACE_GROUP_LABEL_DEFAULT.

PW_TRACE_REF(event_type, module, label, flags, group)#
PW_TRACE_REF_DATA(event_type, module, label, flags, group, type)#

Time source#

Tracing requires the platform to provide the time source for tracing, this can be done in one of a few ways.

  1. Create a file with the default time functions, and provide as build variable pw_trace_tokenized_time, which will get pulled in as a dependency.

  2. Provide time functions elsewhere in project, and ensure they are included.

  3. Provide definitions of the following trace time functions.

PW_TRACE_TIME_TYPE pw_trace_GetTraceTime()#
size_t pw_trace_GetTraceTimeTicksPerSecond()#


The optional trace buffer adds a ring buffer which contains the encoded trace data. This is still a work in progress, in particular better methods for retrieving the data still need to be added. Currently there is an accessor for the underlying ring buffer object, but this is a short term solution.

void ClearBuffer()#
pw::ring_buffer::PrefixedEntryRingBuffer *GetBuffer()#

The buffer has two configurable options:

  1. PW_TRACE_BUFFER_SIZE_BYTES: The total size of the ring buffer in bytes.

  2. PW_TRACE_BUFFER_MAX_BLOCK_SIZE_BYTES: The maximum single trace object size. Including the token, time, and any attached data. Any trace object larger then this will be dropped.

ConstByteSpan DeringAndViewRawBuffer()#

The DeringAndViewRawBuffer function can be used to get bulk access of the full deringed prefixed-ring-buffer data. This might be neccessary for large zero-copy bulk transfers. It is the caller’s responsibility to disable tracing during access to the buffer. The data in the block is defined by the prefixed-ring-buffer format without any user-preamble.

Added dependencies#

pw_ring_buffer pw_varint


The optional trace buffer logging adds support to dump trace buffers to the log. Buffers are converted to base64-encoding then split across log lines. Trace logs are surrounded by ‘begin’ and ‘end’ tags.

Ex. Invoking PW_TRACE_INSTANT with ‘test1’ and ‘test2’, then calling this function would produce this in the output logs:

[TRACE] begin
[TRACE] end

Added dependencies#

pw_base64 pw_log pw_ring_buffer pw_string pw_tokenizer pw_varint

Python decoder#

The python decoder can be used to convert the binary trace data into json data which can be viewed in chrome://tracing. can be used for retrieveing trace data from devices which are using the trace_rpc_server. can be used to decode a binary file of trace data.


The examples all use pw_trace sample app to provide the trace data. Details for how to build, run, and decode the traces are included at the top of each example. This is early work, and is provided as an example of how different tracing concepts can look.


The basic example turns on tracing and dumps all trace output to a file provided on the command line.


The trigger example demonstrates how a trace event can be used as a trigger to start and stop capturing a trace. The examples makes use of PW_TRACE_REF and PW_TRACE_REF_DATA to specify a start and stop event for the capture. This can be useful if the trace buffer is small and you wish to capture a specific series of events.


The filter example demonstrates how a callback can be used to filter which trace events get processed and saved. In this example all events from the processing task which don’t have traceId equal to 3 are removed. Both the other task traces are not removed. This can be a useful feature while debugging as it limits the amount of events which get stored to the buffer, and only saves the events of interest.

Snapshot integration#

Tokenized trace buffers can be captured to a pw.snapshot.Snapshot or pw.trace.SnapshotTraceInfo proto in the trace_data field. The expected format is a de-ringed raw tokenized trace buffer, which can be retrieved via pw::trace::DeringAndViewRawBuffer().

pw_trace_tokenized does not yet have Python tooling integration for interpretation of serialized snapshots with a populated trace_data field.