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docs: Add chrome_os_logging.md.

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Add a document describing where Chrome log messages are
written on Chrome OS.

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Change-Id: I2cda9c7397513a75e1c69021ee31c0ad61bc0022
Reviewed-on: https://chromium-review.googlesource.com/837587
Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Commit-Queue: Dan Erat <derat@chromium.org>
Cr-Commit-Position: refs/heads/master@{#525510}
This commit is contained in:
Daniel Erat
2017-12-20 23:27:24 +00:00
committed by Commit Bot
parent 3becc5767f
commit ace8d362f8
2 changed files with 44 additions and 1 deletions

5
docs/README.md

@ -255,6 +255,9 @@ used when committed.
strings using Chrome for iOS.
* [Running iOS test suites locally](ios/testing.md)
### Misc Chrome-OS-Specific Docs
* [Chrome Logging on Chrome OS](chrome_os_logging.md)
### Media
* [Audio Focus Handling](media/audio_focus.md) - How multiple MediaSession
audio streams interact
@ -310,7 +313,7 @@ used when committed.
directories are determined on all platforms.
### Probably Obsolete
* [Old ChromeOS build instructions](old_chromeos_build_instructions.md)
* [Old Chrome OS build instructions](old_chromeos_build_instructions.md)
* [TPM Quick Reference](tpm_quick_ref.md) - Trusted Platform Module notes.
* [System Hardening Features](system_hardening_features.md) - A list of
current and planned Chrome OS security features.

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docs/chrome_os_logging.md Normal file

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# Chrome Logging on Chrome OS
## Locations
Messages written via the logging macros in [base/logging.h] end up in different
locations depending on Chrome's state:
* `/var/log/ui/ui.LATEST` contains data written to stdout and stderr by Chrome
(and technically also [session_manager]). This generally comprises messages
that are written very early in Chrome's startup process, before logging has
been initialized.
* `/var/log/chrome/chrome` contains messages that are written before a user has
logged in.
* `/home/chronos/user/log/chrome` contains messages that are written while a
user is logged in. Note that this path is within the user's encrypted home
directory and is only accessible while the user is logged in.
All of the above files are actually symlinks. Older log files can be found
alongside them in the same directories.
Also notable is `/var/log/messages`. This file contains general syslog messages,
but it also includes messages written by `session_manager` that may be useful in
determining when or why Chrome started or stopped.
## Severity
By default, only messages logged at severity `WARNING` or higher are written to
disk. More concretely, `LOG(INFO)` messages are discarded.
## Verbose Logging
When actively debugging issues, Chrome's `--vmodule` flag is sometimes used to
temporarily log messages at lower severities for particular modules. See the
[Passing Chrome flags from session_manager] document for more details, and
specifically the `/etc/chrome_dev.conf` mechanism that can be used to
change flags on development devices.
[base/logging.h]: ../base/logging.h
[session_manager]: https://chromium.googlesource.com/chromiumos/platform2/+/master/login_manager/
[Passing Chrome flags from session_manager]: https://chromium.googlesource.com/chromiumos/platform2/+/master/login_manager/docs/flags.md