A minimal C++ logging framework is provided for use in core Gecko code. It is enabled for all builds and is thread-safe.
This page covers enabling logging for particular logging module, configuring the logging output, and how to use the logging facilities in native code.
A sandboxed content process cannot write to stderr
or any file. The easiest way to log these processes is to disable the content sandbox by setting the preference security.sandbox.content.level
to 0
, or setting the environment variable MOZ_DISABLE_CONTENT_SANDBOX
to 1
.
On Windows, you can still see child process messages by using DOS (not the MOZ_LOG_FILE
variable defined below) to redirect output to a file. For example: MOZ_LOG="CameraChild:5" mach run >& my_log_file.txt
will include debug messages from the camera's child actor that lives in a (sandboxed) content process.
When a log statement is logged on a thread and the Firefox Profiler is profiling that thread, the log statements is recorded as a profiler marker.
This allows getting logs alongside profiler markers and lots of performance and contextual information, in a way that doesn't require disabling the sandbox, and works across all processes.
The profile can be downloaded and shared e.g. via Bugzilla or email, or uploaded, and the logging statements will be visible either in the marker chart or the marker table.
While it is possible to manually configure logging module and start the profiler with the right set of threads to profile, about:logging
makes this task a lot simpler and error-proof.
Logging is configured using a special but simple syntax: which module should be enabled, at which level, and what logging options should be enabled or disabled.
The syntax is a list of terms, separated by commas. There are two types of terms:
- A log module and its level, separated by a colon (
:
), such asexample_module:5
to enable the moduleexample_module
at logging level5
(verbose). This searchfox query returns the complete list of modules available. - A special string in the following table, to configure the logging behaviour. Some configuration switch take an integer parameter, in which case it's separated from the string by a colon (
:
). Most switches only apply in a specific output context, noted in the Context column.
Special module name | Context | Action |
---|---|---|
append | File | Append new logs to existing log file. |
sync | File | Print each log synchronously, this is useful to check behavior in real time or get logs immediately before crash. |
raw | File | Print exactly what has been specified in the format string, without the process/thread/timestamp, etc. prefix. |
timestamp | File | Insert timestamp at start of each log line. |
rotate:N | File | This limits the produced log files' size. Only most recent N megabytes of log data is saved. We rotate four log files with .0, .1, .2, .3 extensions. Note: this option disables 'append' and forces 'timestamp'. |
maxsize:N | File | Limit the log to N MB. Only work in append mode. |
prependheader | File | Prepend a simple header while distinguishing logging. Useful in append mode. |
profilerstacks | Profiler | When profiling with the Firefox Profiler and log modules are enabled, capture the call stack for each log statement. |
This syntax is used for most methods of enabling logging, with the exception of settings preferences directly, see this section <Enabling logging using preferences>
for directions.
Enabling logging can be done in a variety of ways:
- via environment variables
- via command line switches
- using
about:config
preferences - using
about:logging
The first two allow logging from the start of the application and are also useful in case of a crash (when sync
output is requested, this can also be done with about:config
as well to a certain extent). The last two allow enabling and disabling logging at runtime and don't require using the command-line.
By default all logging output is disabled.
about:logging
allows enabling logging by entering a MOZ_LOG
string in the text input, and validating.
Options allow logging to a file or using the Firefox Profiler, that can be started and stopped right from the page.
Logging presets for common scenarios are available in a drop-down. They can be associated with a profiler preset.
It is possible, via URL parameters, to select a particular logging configuration, or to override certain parameters in a preset. This is useful to ask a user to gather logs efficiently without having to fiddle with prefs and/or environment variable.
URL parameters are described in the following table:
Parameter | Description |
---|---|
preset |
|
logging-preset |
|
modules |
|
module |
|
threads |
|
thread |
|
output |
|
output-type |
|
profiler-preset |
If a preset is selected, then threads
or modules
can be used to override the profiled threads or logging modules enabled, but keeping other aspects of the preset. If no preset is selected, then a generic profiling preset is used, firefox-platform
. For example:
about:logging?output=profiler&preset=media-playback&modules=cubeb:4,AudioSinkWrapper:4:AudioSink:4
will profile the threads in the Media
profiler preset, but will only log specific log modules (instead of the long list in the media-playback
preset). In addition, it disallows logging to a file.
On UNIX, setting and environment variable can be done in a variety of ways
set MOZ_LOG="example_logger:3"
export MOZ_LOG="example_logger:3"
MOZ_LOG="example_logger:3" ./mach run
In the Windows Command Prompt (cmd.exe
), don't use quotes:
set MOZ_LOG=example_logger:3
If you want this on GeckoView example, use the following adb command to launch process:
adb shell am start -n org.mozilla.geckoview_example/.GeckoViewActivity --es env0 "MOZ_LOG=example_logger:3"
There are special module names to change logging behavior. You can specify one or more special module names without logging level.
For example, if you want to specify sync
, timestamp
and rotate
:
set MOZ_LOG="example_logger:3,timestamp,sync,rotate:10"
Enabling logging usually outputs the logging statements to the terminal. To have the logs written to a file instead (one file per process), the environment variable MOZ_LOG_FILE
can be used. Logs will be written at this path (either relative or absolute), suffixed by a process type and its PID. MOZ_LOG
files are text files and have the extension .moz_log
.
For example, setting:
set MOZ_LOG_FILE="firefox-logs"
can create a number of files like so:
firefox-log-main.96353.moz_log
firefox-log-child.96354.moz_log
respectively for a parent process of PID 96353 and a child process of PID 96354.
The MOZ_LOG
syntax can be used with the command line switch on the same name, and specifying a file with MOZ_LOG_FILE
works in the same way:
./mach run -MOZ_LOG=timestamp,rotate:200,example_module:5 -MOZ_LOG_FILE=%TEMP%\firefox-logs
will enable verbose (5
) logging for the module example_module
, with timestamp prepended to each line, rotate the logs with 4 files of each 50MB (for a total of 200MB), and write the output to the temporary directory on Windows, with name starting with firefox-logs
.
To adjust the logging after Firefox has started, you can set prefs under the logging. prefix. For example, setting logging.foo to 3 will set the log module foo to start logging at level 3. A number of special prefs can be set, described in the table below:
Preference name | Preference | Preference value | Description |
---|---|---|---|
logging.config.clear_on_startup |
|
-- | Whether to clear all prefs under logging. |
logging.config.LOG_FILE |
|
A path (relative or absolute) | The path to which the log files will be written. |
logging.config.add_timestamp |
|
-- | Whether to prefix all lines by a timestamp. |
logging.config.sync |
|
-- | Whether to flush the stream after each log statements. |
logging.config.profilerstacks |
|
-- | When logging to the Firefox Profiler, whether to include the call stack in each logging statement. |
We're gradually adding more Rust code to Gecko, and Rust crates typically use a different approach to logging. Many Rust libraries use the log crate to log messages, which works together with env_logger at the application level to control what's actually printed via RUST_LOG.
You can set an overall logging level, though it could be quite verbose:
set RUST_LOG="debug"
You can also target individual modules by path:
set RUST_LOG="style::style_resolver=debug"
Note
For Linux/MacOS users, you need to use export rather than set.
Note
Sometimes it can be useful to only log child processes and ignore the parent process. In Firefox 57 and later, you can use RUST_LOG_CHILD instead of RUST_LOG to specify log settings that will only apply to child processes.
The log crate lists the available log levels:
Log Level | Purpose |
---|---|
error | Designates very serious errors. |
warn | Designates hazardous situations. |
info | Designates useful information. |
debug | Designates lower priority information. |
trace | Designates very low priority, often extremely verbose, information. |
It is common for debug and trace to be disabled at compile time in release builds, so you may need a debug build if you want logs from those levels.
Check the env_logger docs for more details on logging options.
Additionally, a mapping from RUST_LOG is available. When using the MOZ_LOG syntax, it is possible to enable logging in rust crate using a similar syntax:
MOZ_LOG=rust_crate_name::*:4
will enable debug logging for all log statements in the crate rust_crate_name
.
* can be replaced by a series of modules if more specificity is needed:
MOZ_LOG=rust_crate_name::module::submodule:4
will enable debug logging for all log statements in the sub-module submodule
of the module module
of the crate rust_crate_name
.
A table mapping Rust log levels to MOZ_LOG log level is available below:
Rust log level | MOZ_LOG level | Numerical value |
---|---|---|
|
|
|
|
|
|
|
|
|
|
|
|
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|
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While logging to the Firefox Profiler works it's sometimes useful to have system logs (adb logcat
) interleaved with application logging. With a device (or emulator) that adb devices
sees, it's possible to set environment variables like so, for e.g. GeckoView_example
:
adb shell am start -n org.mozilla.geckoview_example/.GeckoViewActivity --es env0 MOZ_LOG=MediaDemuxer:4
It is then possible to see the logging statements like so, to display all logs, including MOZ_LOG
:
adb logcat
and to only see MOZ_LOG
like so:
adb logcat Gecko:V '*:S'
This expression means: print log module Gecko
from log level Verbose
(lowest level, this means that all levels are printed), and filter out (S
for silence) all other logging (*
, be careful to quote it or escape it appropriately, it so that it's not expanded by the shell).
While interactive with e.g. GeckoView
code, it can be useful to specify more logging tags like so:
adb logcat GeckoViewActivity:V Gecko:V '*:S'
Set the logging modules using about:config (this requires a Nightly build) using the instructions outlined above, and start the profile using an appropriate profiling preset to profile the correct threads using the instructions written in Firefox Profiler documentation's dedicated page.
Bug 1803607 tracks improving the logging experience on mobile.
LazyLogModule
defers the creation the backing LogModule
in a thread-safe manner and is the preferred method to declare a log module. Multiple LazyLogModules
with the same name can be declared, all will share the same backing LogModule
. This makes it much simpler to share a log module across multiple translation units. LazyLogLodule
provides a conversion operator to LogModule*
and is suitable for passing into the logging macros detailed below.
Note: Log module names can only contain specific characters. The first character must be a lowercase or uppercase ASCII char, underscore, dash, or dot. Subsequent characters may be any of those, or an ASCII digit.
#include "mozilla/Logging.h"
static mozilla::LazyLogModule sFooLog("foo");
A basic interface is provided in the form of 2 macros and an enum class.
MOZ_LOG(module, level, message) | Outputs the given message if the module has the given log level enabled:
|
MOZ_LOG_TEST(module, level) | Checks if the module has the given level enabled:
|
Log Level | Numeric Value | Purpose |
---|---|---|
Disabled |
|
Indicates logging is disabled. This should not be used directly in code. |
Error |
|
An error occurred, generally something you would consider asserting in a debug build. |
Warning |
|
A warning often indicates an unexpected state. |
Info |
|
An informational message, often indicates the current program state. |
Debug |
|
A debug message, useful for debugging but too verbose to be turned on normally. |
Verbose |
|
A message that will be printed a lot, useful for debugging program flow and will probably impact performance. |
#include "mozilla/Logging.h"
using mozilla::LogLevel;
static mozilla::LazyLogModule sLogger("example_logger");
static void DoStuff()
{
MOZ_LOG(sLogger, LogLevel::Info, ("Doing stuff."));
int i = 0;
int start = Time::NowMS();
MOZ_LOG(sLogger, LogLevel::Debug, ("Starting loop."));
while (i++ < 10) {
MOZ_LOG(sLogger, LogLevel::Verbose, ("i = %d", i));
}
// Only calculate the elapsed time if the Warning level is enabled.
if (MOZ_LOG_TEST(sLogger, LogLevel::Warning)) {
int elapsed = Time::NowMS() - start;
if (elapsed > 1000) {
MOZ_LOG(sLogger, LogLevel::Warning, ("Loop took %dms!", elapsed));
}
}
if (i != 10) {
MOZ_LOG(sLogger, LogLevel::Error, ("i should be 10!"));
}
}