<cruntime.h> 是什么函数 ，用法？

它不是函数
只是个头文件而已

<cruntime.h> 是用于多线程环境中的函数库头文件。window程序可调用的函数库文件。建议去MSDN查看。

-----------------------------------------
cruntime.h
Language: C
Copyright: (c) 1990-1997, Microsoft Corporation. All rights reserved.
LOC: 70

/***
*cruntime.h - definitions specific to the target operating system and hardware
*
* Copyright (c) 1990-1997, Microsoft Corporation. All rights reserved.
*
*Purpose:
* This header file contains widely used definitions specific to the
* host operating system and hardware. It is included by every C source
* and most every other header file.
*
* [Internal]
*
****/