# GLog

### Installation in Ubuntu

```bash
sudo apt install libgoogle-glog-dev
```

### Usage in CMake

Google GLog team does provide `GlogConfig.cmake` file, but it is not adopted by the default apt package available in Ubuntu. You can

* compile glog from source and enable CMake support, so that config file will be installed; or
* use default Ubuntu package by `sudo apt install libgoogle-glog-dev`, and then find a `FindGlog.cmake` file and add the path to this file to `CMAKE_MODULE_PATH` variable, such that CMake will use this file to load glog.

After the package is found, use the following variables for header files and libraries. 

```bash
find_package(Glog REQUIRED)  # GlogConfig.cmake NOT available; use FindGlog.cmake
include_directories(${GLOG_INCLUDE_DIRS})

add_executable (main src/main.cpp)
target_link_libraries (main ${GLOG_LIBRARIES})
```

#### References

* [Google Glog CMake docs](https://github.com/google/glog#cmake)
* [FindGlog.cmake in slambook2](https://github.com/gaoxiang12/slambook2/blob/master/ch13/cmake_modules/FindGlog.cmake)

### Tutorial / User Guide

Google glog defines a series of macros that simplify many common logging tasks. It provides logging APIs based on C++-style streams and various helper macros. You can log messages by severity level, control logging behavior from the command line, log based on conditionals, abort the program when expected conditions are not met, introduce your own verbose logging levels, and more.

You can log a message by simply streaming things to `LOG(<severity level>)`, e.g.,

```cpp
#include <glog/logging.h>

int main(int argc, char* argv[]) {
  // Initialize Google's logging library.
  google::InitGoogleLogging(argv[0]);

  // ...
  LOG(INFO) << "Found " << num_cookies << " cookies";
}
```

Unless otherwise specified, glog writes to the filename

* `/tmp/<program_name>.<hostname>.<username>.log.<severity_level>.<date>.<time>.<pid>`, or
* `/tmp/test.hteng-nuc.hteng.log.INFO.20210806-211849` for example.

Note that if `InitGoogleLogging` is not called for initialization, glog will turn into `logtostderr` mode and this log file will not be generated under `/tmp`. ([corresponding source code](https://github.com/google/glog/blob/master/src/logging.cc#L1791))

#### Severity Level

You can specify one of the following severity levels (in increasing order of severity): `INFO`, `WARNING`, `ERROR`, and `FATAL`.&#x20;

* Logging a `FATAL` message terminates the program (after the message is logged).
* Messages of a given severity are logged not only in the logfile for that severity, but also in all logfiles of lower severity. For example, a message of severity `FATAL` will be logged to the logfiles of severity `FATAL`, `ERROR`, `WARNING`, and `INFO`.
* The `DFATAL` severity logs a `FATAL` error in debug mode (i.e., there is no `NDEBUG` macro defined), but avoids halting the program in production by automatically reducing the severity to `ERROR`.
* By default, glog copies the log messages of severity level `ERROR` or `FATAL` to standard error (stderr) in addition to log files.

See the official [User Guide](https://github.com/google/glog#user-guide) for more tutorials.