What is #pragma used for?

Question

Can anyone help me understand #pragma?

ifndef TARGET_OS_LINUX
#pragma once
endif

What,when, where, why, an example?

The above is in

Answer 1

• 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."