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llvm-project/lldb/docs/structured_data/DarwinLog.md

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Summary

This document describes the DarwinLog logging feature.

StructuredDataDarwinLog feature

The DarwinLog feature supports logging os_log*() and NSLog() messages to the command-line lldb console, as well as making those messages available to LLDB clients via the event system. Starting with fall 2016 OSes, Apple platforms introduce a new fire-hose, stream-style logging system where the bulk of the log processing happens on the log consumer side. This reduces logging impact on the system when there are no consumers, making it cheaper to include logging at all times. However, it also increases the work needed on the consumer end when log messages are desired.

The debugserver binary has been modified to support collection of os_log*()/NSLog() messages, selection of which messages appear in the stream, and fine-grained filtering of what gets passed on to the LLDB client. DarwinLog also tracks the activity chain (i.e. os_activity() hierarchy) in effect at the time the log messages were issued. The user is able to configure a number of aspects related to the formatting of the log message header fields.

The DarwinLog support is written in a way which should support the lldb client side on non-Apple clients talking to an Apple device or macOS system; hence, the plugin support is built into all LLDB clients, not just those built on an Apple platform.

StructuredDataDarwinLog implements the 'DarwinLog' feature type, and the plugin name for it shows up as darwin-log.

The user interface to the darwin-log support is via the following:

  • plugin structured-data darwin-log enable command

    This is the main entry point for enabling the command. It can be set before launching a process or while the process is running. If the user wants to squelch seeing info-level or debug-level messages, which is the default behavior, then the enable command must be made prior to launching the process; otherwise, the info-level and debug-level messages will always show up. Also, there is a similar "echo os_log()/NSLog() messages to target process stderr" mechanism which is properly disabled when enabling the DarwinLog support prior to launch. This cannot be squelched if enabling DarwinLog after launch.

    See the help for this command. There are a number of options to shrink or expand the number of messages that are processed on the remote side and sent over to the client, and other options to control the formatting of messages displayed.

    This command is sticky. Once enabled, it will stay enabled for future process launches.

  • plugin structured-data darwin-log disable command

    Executing this command disables os_log() capture in the currently running process and signals LLDB to stop attempting to launch new processes with DarwinLog support enabled.

  • settings set plugin.structured-data.darwin-log.enable-on-startup true

    and

    settings set plugin.structured-data.darwin-log.auto-enable-options -- {options}

    When enable-on-startup is set to true, then LLDB will automatically enable DarwinLog on startup of relevant processes. It will use the content provided in the auto-enable-options settings as the options to pass to the enable command.

    Note the -- required after auto-enable-command. That is necessary for raw commands like settings set. The -- will not become part of the options for the enable command.

Message flow and related performance considerations

os_log()-style collection is not free. The more data that must be processed, the slower it will be. There are several knobs available to the developer to limit how much data goes through the pipe, and how much data ultimately goes over the wire to the LLDB client. The user's goal should be to ensure he or she only collects as many log messages are needed, but no more.

The flow of data looks like the following:

  1. Data comes into debugserver from the low-level OS facility that receives log messages. The data that comes through this pipe can be limited or expanded by the --debug, --info and --all-processes options of the plugin structured-data darwin-log enable command options. Exclude as many categories as possible here (also the default). The knobs here are very coarse - for example, whether to include os_log_info()-level or os_log_debug()-level info, or to include callstacks in the log message event data.

  2. The debugserver process filters the messages that arrive through a message log filter that may be fully customized by the user. It works similar to a rules-based packet filter: a set of rules are matched against the log message, each rule tried in sequential order. The first rule that matches then either accepts or rejects the message. If the log message does not match any rule, then the message gets the no-match (i.e. fall-through) action. The no-match action defaults to accepting but may be set to reject.

    Filters can be added via the enable command's '--filter {filter-spec}' option. Filters are added in order, and multiple --filter entries can be provided to the enable command.

    Filters take the following form:

   {action} {attribute} {op}

   {action} :=
       accept |
       reject

   {attribute} :=
       category       |   // The log message category
       subsystem      |   // The log message subsystem
       activity       |   // The child-most activity in force
                          // at the time the message was logged.
       activity-chain |   // The complete activity chain, specified
                          // as {parent-activity}:{child-activity}:
                          // {grandchild-activity}
       message        |   // The fully expanded message contents.
                          // Note this one is expensive because it
                          // requires expanding the message.  Avoid
                          // this if possible, or add it further
                          // down the filter chain.

   {op} :=
              match {exact-match-text} |
              regex {search-regex}        // uses C++ std::regex
                                          // ECMAScript variant.

e.g. --filter "accept subsystem match com.example.mycompany.myproduct" --filter "accept subsystem regex com.example.+" --filter "reject category regex spammy-system-[[:digit:]]+"

  1. Messages that are accepted by the log message filter get sent to the lldb client, where they are mapped to the StructuredDataDarwinLog plugin. By default, command-line lldb will issue a Process-level event containing the log message content, and will request the plugin to print the message if the plugin is enabled to do so.

Log message display

Several settings control aspects of displaying log messages in command-line LLDB. See the enable command's help for a description of these.