What is #pragma once used for?

I have found the following code when refactoring:

#ifndef TARGET_OS_LINUX
#pragma once
#endif

Can anyone help me understand #pragma once?

What, when, where, and why is it used? Do you have an example?

#pragma is just the prefix for a compiler-specific feature.

In this case, #pragma once means that this header file will only ever be included once in a specific destination file. It removes the need for include guards.

• What -- it is header guard. This file will be included only once.
• When -- at a compile process
• why -- to avoid double including.

"Header guards are little pieces of code that protect the contents of a header file from being included more than once."