pw_log_tokenized#

The pw_log_tokenized module contains utilities for tokenized logging. It connects pw_log to pw_tokenizer and supports Tokenized log arguments.

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 pw_log_tokenized_HandleLog() function, which must be implemented by the application.

void pw_log_tokenized_HandleLog(uint32_t metadata, const uint8_t encoded_message[], size_t size_bytes)#

Function that is called for each log message. The metadata uint32_t can be converted to a pw::log_tokenized::Metadata . The message is passed as a pointer to a buffer and a size. The pointer is invalidated after this function returns, so the buffer must be copied.

Example implementation:

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 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.

"■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.

"■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.

The format string is created by the PW_LOG_TOKENIZED_FORMAT_STRING macro.

PW_LOG_TOKENIZED_FORMAT_STRING(module, message)#

This macro takes the pw_log module name and format string to produce a new string that will be tokenized. Any information can be packed into this string without affecting code size, since tokenization removes it from the binary. By default, pw_log_tokenized includes three fields as key-value pair: log module, message, and file path (__FILE__).

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 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.

PW_LOG_TOKENIZED_LEVEL_BITS#

Bits to allocate for the log level. Defaults to PW_LOG_LEVEL_BITS (3).

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.

PW_LOG_TOKENIZED_FLAG_BITS#

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

PW_LOG_TOKENIZED_MODULE_BITS#

Bits to use for the tokenized version of 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.

template<unsigned kLevelBits, unsigned kLineBits, unsigned kFlagBits, unsigned kModuleBits, typename T = uintptr_t>
class GenericMetadata#

GenericMetadata facilitates the creation and interpretation of packed log metadata payloads. The GenericMetadata class allows flags, log level, line number, and a module identifier to be packed into bit fields of configurable size.

Typically, the Metadata alias should be used instead.

using pw::log_tokenized::Metadata = GenericMetadata<PW_LOG_TOKENIZED_LEVEL_BITS, PW_LOG_TOKENIZED_LINE_BITS, PW_LOG_TOKENIZED_FLAG_BITS, PW_LOG_TOKENIZED_MODULE_BITS>#

The Metadata alias simplifies the bit field width templatization of GenericMetadata by pulling from this module’s configuration options. In most cases, it’s recommended to use Metadata to create or read metadata payloads.

A Metadata object can be created from a uint32_t.

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

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:

// 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 prefixed Base64 format with the following function:

inline InlineString<kBase64EncodedBufferSizeBytes> pw::log_tokenized::PrefixedBase64Encode(
span<const std::byte> binary_message,
)#

Encodes a binary tokenized log in the prefixed Base64 format. Calls pw::tokenizer::PrefixedBase64Encode() for a string sized to fit a kEncodingBufferSizeBytes tokenized log.

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#

Tools for working with tokenized logs.

class pw_log_tokenized.FormatStringWithMetadata(string: str)#

Parses metadata from a log format string with metadata fields.

__init__(string: str) → None#
property file: str#
property message: str#

Displays the msg field or the whole string if it is not present.

property module: str#
class pw_log_tokenized.Metadata(value: int, log_bits: int = 3, line_bits: int = 11, flag_bits: int = 2, module_bits: int = 16)#

Parses the metadata payload used by pw_log_tokenized.

__init__(value: int, log_bits: int = 3, line_bits: int = 11, flag_bits: int = 2, module_bits: int = 16) → None#
flag_bits: int = 2#
line_bits: int = 11#
log_bits: int = 3#
module_bits: int = 16#
value: int#
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