setvbuf（）函数建立了一个供标准 I/O 函数使用的替换缓冲区，新手表示无法，求大神解释解释

片言碎字，好像都没说清楚关键

我想知道setvbuf（）这个函数是在什么情况下使用

The setvbuf function allows the program to control both buffering and buffer size for stream. stream must refer to an open file that has not undergone an I/O operation since it was opened. The array pointed to by buffer is used as the buffer, unless it is NULL, in which case setvbuf uses an automatically allocated buffer of length size/2 * 2 bytes

感谢指点