What is api and how to call it in a program?

API is a predefined interface for programmers to call. Is a set of predefined functions used to control various components of the system. The operation mode is as follows:

1. Before C++ calls the system API, you need to declare the corresponding header file. The system function MessageBoxA is used here, corresponding to the header file:? ”windows.h“。

2. Using angle brackets, the compiler will search the header file in the include directory first; If it is not found, it will be searched in the directory where the source code is located. 2, using double quotation marks is the opposite, will search in the source code directory first; Usually used to contain the header files written by the program author himself.

3. As you can see in the figure in the third part, the MessageBoxA function contains four parameters. There are parameters in the parentheses of the function, and each parameter is separated by an English comma. When the mouse points at the function, the function description information will appear.

4. NULL in the parameter is a constant of , which means no setting or default setting. You can also write directly without using constants.

5. the MessageBoxA function runs as shown in the figure.