glog (Google Logging Library)

ObservabilityLoggingC++

What it is

glog is Google’s C++ logging library that provides application-level logging with different severity levels (INFO, WARNING, ERROR, FATAL). It is designed for robustness, simplicity, and performance, making it suitable for large-scale systems.

glog provides macros for logging at different severity levels. It also supports logging to files, conditional logging, and custom log sinks.

Installation

find_package(glog CONFIG REQUIRED)

Getting started

The smallest useful thing you can do with it, and what each part means.

Simple logging

cpp

12345678910#include <glog/logging.h>

int main(int argc, char* argv[]) {
    google::InitGoogleLogging(argv[0]);
    LOG(INFO) << "This is an info message.";
    LOG(WARNING) << "This is a warning message.";
    LOG(ERROR) << "This is an error message.";
    // LOG(FATAL) << "This is fatal and will terminate.";
    return 0;
}

Logs messages at different severity levels. `FATAL` logs terminate the program after printing a stack trace.

Advanced usage

Where the library earns its place over a simpler alternative.

Conditional logging

cpp

int x = 5;
LOG_IF(INFO, x > 0) << "x is positive";

Logs a message only if the condition is true.

Verbose logging

cpp

FLAGS_v = 2;
VLOG(1) << "Verbose logging at level 1";
VLOG(2) << "Verbose logging at level 2";

Provides verbose logging with runtime-controlled levels.

Check macros

cpp

int* ptr = nullptr;
CHECK_NOTNULL(ptr);

`CHECK` macros validate conditions and abort with logs if they fail.

Custom log sink

cpp

12345678910class MyLogSink : public google::LogSink {
  void send(google::LogSeverity severity, const char* full_filename,
            const char* base_filename, int line, const struct ::tm* tm_time,
            const char* message, size_t message_len) override {
      std::cout << "Custom log: " << message << std::endl;
  }
};

MyLogSink sink;
google::AddLogSink(&sink);

Implements a custom sink to redirect logs to a different output.

Errors and fixes

The failures you are most likely to hit, and what actually resolves them.

Logs not appearing: Ensure `InitGoogleLogging` is called and check that log files are accessible.
Fatal errors terminate unexpectedly: Use `LOG(ERROR)` instead of `LOG(FATAL)` unless termination is required.
Performance overhead in production: Use lower verbosity levels and disable unnecessary logs with runtime flags.

Best practices

Initialize glog early in the main function using `InitGoogleLogging`.
Use severity levels consistently to separate normal logs from warnings and errors.
Avoid `LOG(FATAL)` except for unrecoverable errors; it terminates the application.
Use `CHECK` macros to enforce invariants in critical code paths.
Combine glog with monitoring tools by redirecting log sinks if necessary.

Background

Why it exists, and what it was reacting to.

glog was developed at Google to provide developers with a powerful yet simple logging solution for C++ projects. Unlike syslog or printf-based approaches, glog adds structured logging with severity levels, stack traces on fatal errors, and flexible runtime configuration. It has since been open-sourced and is widely used across many C++ projects.

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glog (Google Logging Library)

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