This document explains how to get started with syslogger APIs.

Component manifest dependency

Ensure that your component has the required capabilities to log by including the following in your component manifest:

  include: [

The syslog library will fallback to stderr if the LogSink connection fails.

Default configuration

The global logger is lazily instantiated on the first use of the API (more specifically, on the first call to fx_log_get_logger). The default configuration for the global logger is:

  • Use process name as the tag
  • Write logs to fuchsia.logger.LogSink
  • Min log level of FX_LOG_INFO

In C dependency


Log messages

FX_LOGF(INFO, "tag", "my msg: %d", 10);
FX_LOG(INFO, "tag", "my msg");
FX_LOGF(INFO, NULL, "my msg: %d", 10);

Using non-default configuration

#include <lib/syslog/global.h>

int main(int argc, char** argv) {
    fx_logger_config_t config = {
      .min_severity = FX_LOG_INFO,
      .tags = (const char * []) {"gtag", "gtag2"},
      .num_tags = 2,



In C++ dependency


Log messages

#include <lib/syslog/cpp/macros.h>

FX_LOGS(INFO) << "my message";
FX_LOGST(INFO, "tag") << "my message";

Set tags

By default, the process name is used as the tag, but this can be changed by calling fuchsia_logging::SetTags.

#include <lib/syslog/cpp/log_settings.h>

int main(int argc, char** argv) {
     fuchsia_logging::SetTags({"tag1", "tag2"});

Set settings

#include "<lib/syslog/cpp/log_settings.h>

int main(int argc, char** argv) {
     fuchsia_logging::LogSettings settings = {
         .min_log_level = fuchsia_logging::LOG_ERROR,
     fuchsia_logging::SetLogSettings(settings, {"tag1", "tag2"});

Set settings from command line

#include "src/lib/fxl/command_line.h"
#include "src/lib/fxl/log_settings_command_line.h"

int main(int argc, char** argv) {
    auto command_line = fxl::CommandLineFromArgcArgv(argc, argv);
    fxl::SetLogSettingsFromCommandLine(command_line, {"my_program"});

GTest main with syslog initialized from command line

No initialization is required for using the default configuration of syslog. If you would like your test suite to change the configuration based on command line arguments (e.g. --verbose), use:



C++ APIs
Command line initialization API