The difference between #ifndef and #pragma once

When typing the code, I found #ifndef and #pragma once. By the way, record what these two are going on.
Find the answer directly on csdn, and attach the link below.

In addition, there will be spelling errors, it should be pragma not program

1. The difference between #ifndef and #program once
2. [Daily One C of Seventeen] The usage and difference between #ifndef and #pragma once