pw_log_tokenized/docs.rst - pigweed/pigweed - Git at Google

 .. _module-pw_log_tokenized:

 ----------------
 pw_log_tokenized
 ----------------
 The ``pw_log_tokenized`` module contains utilities for tokenized logging. It
 connects ``pw_log`` to ``pw_tokenizer`` and supports
 :ref:`module-pw_log-tokenized-args`.

 C++ backend
 ===========
 ``pw_log_tokenized`` provides a backend for ``pw_log`` that tokenizes log
 messages with the ``pw_tokenizer`` module. The log level, 16-bit tokenized
 module name, and flags bits are passed through the payload argument. The macro
 eventually passes logs to the :c:func:`pw_log_tokenized_HandleLog` function,
 which must be implemented by the application.

 .. doxygenfunction:: pw_log_tokenized_HandleLog

 Example implementation:

 .. code-block:: cpp

    extern "C" void pw_log_tokenized_HandleLog(
        uint32_t payload, const uint8_t message[], size_t size) {
      // The metadata object provides the log level, module token, and flags.
      // These values can be recorded and used for runtime filtering.
      pw::log_tokenized::Metadata metadata(payload);

      if (metadata.level() < current_log_level) {
        return;
      }

      if (metadata.flags() & HIGH_PRIORITY_LOG != 0) {
        EmitHighPriorityLog(metadata.module(), message, size);
      } else {
        EmitLowPriorityLog(metadata.module(), message, size);
      }
    }

 See the documentation for :ref:`module-pw_tokenizer` for further details.

 Metadata in the format string
 -----------------------------
 With tokenized logging, the log format string is converted to a 32-bit token.
 Regardless of how long the format string is, it's always represented by a 32-bit
 token. Because of this, metadata can be packed into the tokenized string with
 no cost.

 ``pw_log_tokenized`` uses a simple key-value format to encode metadata in a
 format string. Each field starts with the ``■`` (U+25A0 "Black Square")
 character, followed by the key name, the ``♦`` (U+2666 "Black Diamond Suit")
 character, and then the value. The string is encoded as UTF-8. Key names are
 comprised of alphanumeric ASCII characters and underscore and start with a
 letter.

 .. code-block::

    "■key1♦contents1■key2♦contents2■key3♦contents3"

 This format makes the message easily machine parseable and human readable. It is
 extremely unlikely to conflict with log message contents due to the characters
 used.

 ``pw_log_tokenized`` uses three fields: ``msg``, ``module``, and ``file``.
 Implementations may add other fields, but they will be ignored by the
 ``pw_log_tokenized`` tooling.

 .. code-block::

    "■msg♦Hyperdrive %d set to %f■module♦engine■file♦propulsion/hyper.cc"

 Using key-value pairs allows placing the fields in any order.
 ``pw_log_tokenized`` places the message first. This is prefered when tokenizing
 C code because the tokenizer only hashes a fixed number of characters. If the
 file were first, the long path might take most of the hashed characters,
 increasing the odds of a collision with other strings in that file. In C++, all
 characters in the string are hashed, so the order is not important.

 Metadata in the tokenizer payload argument
 -------------------------------------------
 ``pw_log_tokenized`` packs runtime-accessible metadata into a 32-bit integer
 which is passed as the "payload" argument for ``pw_log_tokenizer``'s global
 handler with payload facade. Packing this metadata into a single word rather
 than separate arguments reduces the code size significantly.

 Four items are packed into the payload argument:

 - Log level -- Used for runtime log filtering by level.
 - Line number -- Used to track where a log message originated.
 - Log flags -- Implementation-defined log flags.
 - Tokenized :c:macro:`PW_LOG_MODULE_NAME` -- Used for runtime log filtering by
   module.

 Configuring metadata bit fields
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 The number of bits to use for each metadata field is configurable through macros
 in ``pw_log/config.h``. The field widths must sum to 32 bits. A field with zero
 bits allocated is excluded from the log metadata.

 .. c:macro:: PW_LOG_TOKENIZED_LEVEL_BITS

   Bits to allocate for the log level. Defaults to :c:macro:`PW_LOG_LEVEL_BITS`
   (3).

 .. c:macro:: PW_LOG_TOKENIZED_LINE_BITS

   Bits to allocate for the line number. Defaults to 11 (up to line 2047). If the
   line number is too large to be represented by this field, line is reported as
   0.

   Including the line number can slightly increase code size. Without the line
   number, the log metadata argument is the same for all logs with the same level
   and flags. With the line number, each metadata value is unique and must be
   encoded as a separate word in the binary. Systems with extreme space
   constraints may exclude line numbers by setting this macro to 0.

   It is possible to include line numbers in tokenized log format strings, but
   that is discouraged because line numbers change whenever a file is edited.
   Passing the line number with the metadata is a lightweight way to include it.

 .. c:macro:: PW_LOG_TOKENIZED_FLAG_BITS

   Bits to use for implementation-defined flags. Defaults to 2.

 .. c:macro:: PW_LOG_TOKENIZED_MODULE_BITS

   Bits to use for the tokenized version of :c:macro:`PW_LOG_MODULE_NAME`.
   Defaults to 16, which gives a ~1% probability of a collision with 37 module
   names.

 Creating and reading Metadata payloads
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 ``pw_log_tokenized`` provides a C++ class to facilitate the creation and
 interpretation of packed log metadata payloads.

 .. doxygenclass:: pw::log_tokenized::GenericMetadata
 .. doxygentypedef:: pw::log_tokenized::Metadata

 The following example shows that a ``Metadata`` object can be created from a
 ``uint32_t`` log metadata payload.

 .. code-block:: cpp

    extern "C" void pw_log_tokenized_HandleLog(
        uint32_t payload,
        const uint8_t message[],
        size_t size_bytes) {
      pw::log_tokenized::Metadata metadata = payload;
      // Check the log level to see if this log is a crash.
      if (metadata.level() == PW_LOG_LEVEL_FATAL) {
        HandleCrash(metadata, pw::ConstByteSpan(
            reinterpret_cast<const std::byte*>(message), size_bytes));
        PW_UNREACHABLE;
      }
      // ...
    }

 It's also possible to get a ``uint32_t`` representation of a ``Metadata``
 object:

 .. code-block:: cpp

    // Logs an explicitly created string token.
    void LogToken(uint32_t token, int level, int line_number, int module) {
      const uint32_t payload =
          log_tokenized::Metadata(
              level, module, PW_LOG_FLAGS, line_number)
              .value();
      std::array<std::byte, sizeof(token)> token_buffer =
          pw::bytes::CopyInOrder(endian::little, token);

      pw_log_tokenized_HandleLog(
          payload,
          reinterpret_cast<const uint8_t*>(token_buffer.data()),
          token_buffer.size());
    }

 The binary tokenized message may be encoded in the :ref:`prefixed Base64 format
 <module-pw_tokenizer-base64-format>` with the following function:

 .. doxygenfunction:: PrefixedBase64Encode(span<const std::byte>)

 Build targets
 -------------
 The GN build for ``pw_log_tokenized`` has two targets: ``pw_log_tokenized`` and
 ``log_backend``. The ``pw_log_tokenized`` target provides the
 ``pw_log_tokenized/log_tokenized.h`` header. The ``log_backend`` target
 implements the backend for the ``pw_log`` facade. ``pw_log_tokenized`` invokes
 the ``pw_log_tokenized:handler`` facade, which must be implemented by the user
 of ``pw_log_tokenized``.

 GCC has a bug resulting in section attributes of templated functions being
 ignored. This in turn means that log tokenization cannot work for templated
 functions, because the token database entries are lost at build time.
 For more information see https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70435.
 If you are using GCC, the ``gcc_partially_tokenized`` target can be used as a
 backend for the ``pw_log`` facade instead which tokenizes as much as possible
 and uses the ``pw_log_string:handler`` for the rest using string logging.

 Python package
 ==============
 ``pw_log_tokenized`` includes a Python package for decoding tokenized logs.

 pw_log_tokenized
 ----------------
 .. automodule:: pw_log_tokenized
   :members:
   :undoc-members:
	.. _module-pw_log_tokenized:

	----------------
	pw_log_tokenized
	----------------
	The ``pw_log_tokenized`` module contains utilities for tokenized logging. It
	connects ``pw_log`` to ``pw_tokenizer`` and supports
	:ref:`module-pw_log-tokenized-args`.

	C++ backend
	===========
	``pw_log_tokenized`` provides a backend for ``pw_log`` that tokenizes log
	messages with the ``pw_tokenizer`` module. The log level, 16-bit tokenized
	module name, and flags bits are passed through the payload argument. The macro
	eventually passes logs to the :c:func:`pw_log_tokenized_HandleLog` function,
	which must be implemented by the application.

	.. doxygenfunction:: pw_log_tokenized_HandleLog

	Example implementation:

	.. code-block:: cpp

	extern "C" void pw_log_tokenized_HandleLog(
	uint32_t payload, const uint8_t message[], size_t size) {
	// The metadata object provides the log level, module token, and flags.
	// These values can be recorded and used for runtime filtering.
	pw::log_tokenized::Metadata metadata(payload);

	if (metadata.level() < current_log_level) {
	return;
	}

	if (metadata.flags() & HIGH_PRIORITY_LOG != 0) {
	EmitHighPriorityLog(metadata.module(), message, size);
	} else {
	EmitLowPriorityLog(metadata.module(), message, size);
	}
	}

	See the documentation for :ref:`module-pw_tokenizer` for further details.

	Metadata in the format string
	-----------------------------
	With tokenized logging, the log format string is converted to a 32-bit token.
	Regardless of how long the format string is, it's always represented by a 32-bit
	token. Because of this, metadata can be packed into the tokenized string with
	no cost.

	``pw_log_tokenized`` uses a simple key-value format to encode metadata in a
	format string. Each field starts with the ``■`` (U+25A0 "Black Square")
	character, followed by the key name, the ``♦`` (U+2666 "Black Diamond Suit")
	character, and then the value. The string is encoded as UTF-8. Key names are
	comprised of alphanumeric ASCII characters and underscore and start with a
	letter.

	.. code-block::

	"■key1♦contents1■key2♦contents2■key3♦contents3"

	This format makes the message easily machine parseable and human readable. It is
	extremely unlikely to conflict with log message contents due to the characters
	used.

	``pw_log_tokenized`` uses three fields: ``msg``, ``module``, and ``file``.
	Implementations may add other fields, but they will be ignored by the
	``pw_log_tokenized`` tooling.

	.. code-block::

	"■msg♦Hyperdrive %d set to %f■module♦engine■file♦propulsion/hyper.cc"

	Using key-value pairs allows placing the fields in any order.
	``pw_log_tokenized`` places the message first. This is prefered when tokenizing
	C code because the tokenizer only hashes a fixed number of characters. If the
	file were first, the long path might take most of the hashed characters,
	increasing the odds of a collision with other strings in that file. In C++, all
	characters in the string are hashed, so the order is not important.

	Metadata in the tokenizer payload argument
	-------------------------------------------
	``pw_log_tokenized`` packs runtime-accessible metadata into a 32-bit integer
	which is passed as the "payload" argument for ``pw_log_tokenizer``'s global
	handler with payload facade. Packing this metadata into a single word rather
	than separate arguments reduces the code size significantly.

	Four items are packed into the payload argument:

	- Log level -- Used for runtime log filtering by level.
	- Line number -- Used to track where a log message originated.
	- Log flags -- Implementation-defined log flags.
	- Tokenized :c:macro:`PW_LOG_MODULE_NAME` -- Used for runtime log filtering by
	module.

	Configuring metadata bit fields
	^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
	The number of bits to use for each metadata field is configurable through macros
	in ``pw_log/config.h``. The field widths must sum to 32 bits. A field with zero
	bits allocated is excluded from the log metadata.

	.. c:macro:: PW_LOG_TOKENIZED_LEVEL_BITS

	Bits to allocate for the log level. Defaults to :c:macro:`PW_LOG_LEVEL_BITS`
	(3).

	.. c:macro:: PW_LOG_TOKENIZED_LINE_BITS

	Bits to allocate for the line number. Defaults to 11 (up to line 2047). If the
	line number is too large to be represented by this field, line is reported as
	0.

	Including the line number can slightly increase code size. Without the line
	number, the log metadata argument is the same for all logs with the same level
	and flags. With the line number, each metadata value is unique and must be
	encoded as a separate word in the binary. Systems with extreme space
	constraints may exclude line numbers by setting this macro to 0.

	It is possible to include line numbers in tokenized log format strings, but
	that is discouraged because line numbers change whenever a file is edited.
	Passing the line number with the metadata is a lightweight way to include it.

	.. c:macro:: PW_LOG_TOKENIZED_FLAG_BITS

	Bits to use for implementation-defined flags. Defaults to 2.

	.. c:macro:: PW_LOG_TOKENIZED_MODULE_BITS

	Bits to use for the tokenized version of :c:macro:`PW_LOG_MODULE_NAME`.
	Defaults to 16, which gives a ~1% probability of a collision with 37 module
	names.

	Creating and reading Metadata payloads
	^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
	``pw_log_tokenized`` provides a C++ class to facilitate the creation and
	interpretation of packed log metadata payloads.

	.. doxygenclass:: pw::log_tokenized::GenericMetadata
	.. doxygentypedef:: pw::log_tokenized::Metadata

	The following example shows that a ``Metadata`` object can be created from a
	``uint32_t`` log metadata payload.

	.. code-block:: cpp

	extern "C" void pw_log_tokenized_HandleLog(
	uint32_t payload,
	const uint8_t message[],
	size_t size_bytes) {
	pw::log_tokenized::Metadata metadata = payload;
	// Check the log level to see if this log is a crash.
	if (metadata.level() == PW_LOG_LEVEL_FATAL) {
	HandleCrash(metadata, pw::ConstByteSpan(
	reinterpret_cast<const std::byte*>(message), size_bytes));
	PW_UNREACHABLE;
	}
	// ...
	}

	It's also possible to get a ``uint32_t`` representation of a ``Metadata``
	object:

	.. code-block:: cpp

	// Logs an explicitly created string token.
	void LogToken(uint32_t token, int level, int line_number, int module) {
	const uint32_t payload =
	log_tokenized::Metadata(
	level, module, PW_LOG_FLAGS, line_number)
	.value();
	std::array<std::byte, sizeof(token)> token_buffer =
	pw::bytes::CopyInOrder(endian::little, token);

	pw_log_tokenized_HandleLog(
	payload,
	reinterpret_cast<const uint8_t*>(token_buffer.data()),
	token_buffer.size());
	}

	The binary tokenized message may be encoded in the :ref:`prefixed Base64 format
	<module-pw_tokenizer-base64-format>` with the following function:

	.. doxygenfunction:: PrefixedBase64Encode(span<const std::byte>)

	Build targets
	-------------
	The GN build for ``pw_log_tokenized`` has two targets: ``pw_log_tokenized`` and
	``log_backend``. The ``pw_log_tokenized`` target provides the
	``pw_log_tokenized/log_tokenized.h`` header. The ``log_backend`` target
	implements the backend for the ``pw_log`` facade. ``pw_log_tokenized`` invokes
	the ``pw_log_tokenized:handler`` facade, which must be implemented by the user
	of ``pw_log_tokenized``.

	GCC has a bug resulting in section attributes of templated functions being
	ignored. This in turn means that log tokenization cannot work for templated
	functions, because the token database entries are lost at build time.
	For more information see https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70435.
	If you are using GCC, the ``gcc_partially_tokenized`` target can be used as a
	backend for the ``pw_log`` facade instead which tokenizes as much as possible
	and uses the ``pw_log_string:handler`` for the rest using string logging.

	Python package
	==============
	``pw_log_tokenized`` includes a Python package for decoding tokenized logs.

	pw_log_tokenized
	----------------
	.. automodule:: pw_log_tokenized
	:members:
	:undoc-members: