Day 15 : 22 June 2022 : Add Settings from Environment Variables

My 100 Daze of Code

https://github.com/davidjwalling/100-days-of-code

#15 : Adding Settings from Environment Variables

So far, we've shown how program arguments can be read and applied to configure Ava settings. Now we'll look at applying setting from Environment variables. This really just makes use of the getenv function in the C library. We'll define setting names and identifiers in our namespace enumerations and const strings.

Environment variable settings will be applied before program arguments so that program arguments, which are specific to each execution of the program, can override Environment variables, which are specific to the operating context or user.

Here are the environment variable names added in the "driver" namespace. Since the Environment is not specific to the program, we prefix the settings with the program name.


Here is a new method in Driver, called GetEnvVars. It simply reads each variable and calls the appropriate method on the singleton Log instance using public functions to set appropriate values.

Note we continue to use char* for val here because getenv can return a null pointer which would throw an exception if we tried to assign that to a std::string.

In our Configure method, we add the call to GetEnvVars just before the call to GetArgVars. If a setting is defined as both an Environment variable and specific in the program arguments, the program argument value will overwrite the value from the Environment.

We'll test by adding the AVA_LOGPATH environment variable in our system.

Remember to restart Visual Studio if you're using that on Windows so that changes to the Environment will be active in your debugging session.

Now we can single-step debug to see our environment variable read and applied.

Our logging now reports the variable applied from the Environment.

And we can confirm our logs are written to the folder specific in our environment variable.








Comments

Post a Comment

Popular posts from this blog

Day 12 : 19 June 2022 : Adding Windows Service Logic

Image
My 100 Daze of Code https://github.com/davidjwalling/100-days-of-code #12 : Adding Windows Service Logic Today, we'll add some Win32 API logic in Ava to install, uninstall and run as a service on Windows. We'll add logic in Driver::Service that check if the program was started with the "service" argument as the first program argument. If so, the Service Control Manager will be invoked to start the program as a service. We'll also check for "install" and "uninstall" program arguments and handle these with logic that installs and uninstalls the program, respectively. In a later post, we'll do the equivalent on Linux and macOS and show how to enable Ava to run as a daemon configured in sysctl. Nowadays there are shortcut methods to install and run an executable as a service on Windows. But I prefer to illustrate the APIs that make this work. Some Code Refactoring Up to now we've been using a pointer-to-implementation (pimpl) method for exp...
Read more

Day 5 : 12 June 2022 : CMake on Windows

Image
 My 100 Daze of Code https://github.com/davidjwalling/100-days-of-code Day 5 : 12 June 2022 : CMake on Windows So far, we've seen using CMake on Linux and MacOS. Also, we've looked at native compilation on Windows using Visual Studio (VS). Today, we'll look at using CMake on Windows. I decided to devote an entire day (hour) to this because there are several ways to invoke CMake on Windows, using different "Generators". A generator will write the inputs to the native build tools. But Wait, There's More ... Before heading to CMake, I wanted to first point out that VS' native tools can build just fine without using CMake, of course. To build using "msbuild," open an x64 Native Tools Command Prompt for Visual Studio - this is provided when VS is installed. Invoke msbuild and pass the solution name and the desired platform and configuration. msbuild ava.sln /p:Platform=x64 /p:Configuration=Debug Repeat for Debug and Release configurations for Win32 an...
Read more

Day 11: 18 June 2022 : Handling Program Arguments

Image
My 100 Daze of Code https://github.com/davidjwalling/100-days-of-code #11 : Handling Program Arguments Up to now we've been carrying around Ava's program argument count (argc) and value array (argv) into a Configure method. Now it's time to start doing something with those values. We'll define four program settings that can be set by passing program arguments: the logging level, path, prefix and suffix, and show examples of using them. Logging Parameter Storage The singleton theLog is instantiated inside the Ava library (libava). Default values for several logging parameters are set by the Log::Init method, which is called both by the Log class constructor and the destructor, by way of the Reset method, so that runtime values are not left in deallocated storage on operating systems that do not automatically clear memory upon deallocation. Logging Parameter Symbols We use namespaces and enums to define symbolic names for the logging parameters so that we have a single lo...
Read more