setbuf、setvbuf

setbuf, setbuffer, setlinebuf, setvbuf - stream buffering operations

用于更改buffering
这些调用必须在fopen流之后，操作这些stream流之前调用。

#include <stdio.h>

void setbuf(FILE *stream, char *buf);
int setvbuf(FILE *stream, char *buf, int mode, size_t size);
//Returns: 0 if OK, nonzero on error

void setbuffer(FILE *stream, char *buf, size_t size);
void setlinebuf(FILE *stream);   

setbuf

开启buffering，``buf所代指的buffer大小必须为BUFSIZ（定义在stdio.h），一般是fully buffered，一些系统在与终端设备关联的时候，采用的是行缓冲
关闭buffering，buf为NULL即可

setvbuf

使用mode来指定想要的类型

mode
_IOFBF	fully buffered
_IOLBF	line buffered
_IONBF	unbuffered

If we specify an unbuffered stream, the buf and size arguments are ignored.

If we specify fully buffered or line buffered, buf and size can optionally specify a buffer and its size.

If the stream is buffered andbuf is NULL, the standard I/O library will automatically allocate its own buffer of the appropriate size for the stream. By appropriate size, we mean the value specified by the constant BUFSIZ.

一些C库如GNU C库，使用stat structure的st_blksize来决定合适的标准IObuffer尺寸。