Gecko Logging

A minimal C++ logging framework is provided for use in core Gecko code. It is enabled for all builds, thread-safe, and the preferred alternative to NSPR logging.

More in-depth documentation on modules, message formats, and enabling logging can be found on the NSPR logging page.

Logging framework

Declaring a log module

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.

#include "mozilla/Logging.h"

static mozilla::LazyLogModule sFooLog("foo");

Logging interface

A basic interface is provided in the form of 2 macros and an enum class.

Mozilla Logging Macros
MOZ_LOG(module, level, message)

Outputs the given message if the module has the given log level enabled.

  • module - The log module to use.
  • level - The log level of the message.
  • message - A printf-style message to output. Must be enclosed in parentheses.
MOZ_LOG_TEST(module, level)

Checks if the module has the given level enabled.

  • module - The log module to use.
  • level - The output level of the message.
Log Level Numeric Value Purpose
Mozilla Logging Levels
Disabled 0 Indicates logging is disabled. This should not be used directly in code.
Error 1 An error occurred, generally something you would consider asserting in a debug build.
Warning 2 A warning often indicates an unexpected state.
Info 3 An informational message, often indicates the current program state.
Debug 4 A debug message, useful for debugging but too verbose to be turned on normally.
Verbose 5 A message that will be printed a lot, useful for debugging program flow and will probably impact performance.

Example Usage

Code Sample

#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!"));

Enabling Logging

The log level for a module is controlled by setting an environment variable before launching the application.

By default all logging output is disabled.

set 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.

Special module name  
append Append new logs to existing log file.
sync Print each log synchronously, this is useful to check behavior in real time or get logs immediately before crash.
timestamp Insert timestamp at start of each log line.
rotate:N 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'.

For example, if you want to specify "sync", "timestamp" and "rotate":

set MOZ_LOG="example_logger:3,timestamp,sync,rotate:10"

See NSPR log modules reference for further details.

Redirecting logging output to a file

Logging output  can be redirected to a file by passing its path via an environment variable.

By default logging output goes to stderr.

set MOZ_LOG_FILE="log.txt"

The rotate and append options described above only apply when logging to a file.

E10S Note

When a content process is in sandbox, it cannot write to stderr or any file. You may need to set preference security.sandbox.content.level to 0 to see the log.


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"

The log crate lists the available log levels:

Log Level Purpose
Rust Logging Levels
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.

Child Processes / e10s

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 to specify log settings that will only apply to child processes.

Document Tags and Contributors

 Contributors to this page: jryans, upsuper, bobowen, gsvelto, honzabambas, Masayuki, erahm, rolfedh
 Last updated by: jryans,