函数名:setvbuf
功 能: 把缓冲区与流相关
用 法:int setvbuf(FILE *stream, char *buf, int type, unsigned size);
参数：stream ：指向流的指针 ；
buf ： 期望缓冲区的地址；
type ： 期望缓冲区的类型：
_IOFBF(满缓冲）：当缓冲区为空时，从流读入数据。或者当缓冲区满时，向流写入数 据。
_IOLBF(行缓冲）：每次从流中读入一行数据或向流中写入一行数据。
_IONBF(无缓冲）：直接从流中读入数据或直接向流中写入数据，而没有缓冲区。
size ： 缓冲区内字节的数量。
注意：This function should be called once the file associated with the stream has already been opened but before any input or output operation has taken place.
意思是这个函数应该在打开流后,立即调用,在任何对该流做输入输出前
程序例:
#include <stdio.h>
int main(void)
{
FILE *input, *output;
char bufr[512];
input = fopen("file.in", "r+b");
output = fopen("file.out", "w");
/* set up input stream for minimal disk access,
using our own character buffer */
if (setvbuf(input, bufr, _IOFBF, 512) != 0)
printf("failed to set up buffer for input file
");
else
printf("buffer set up for input file
");
/* set up output stream for line buffering using space that
will be obtained through an indirect call to malloc */
if (setvbuf(output, NULL, _IOLBF, 132) != 0)
printf("failed to set up buffer for output file
");
else
printf("buffer set up for output file
");
/* perform file I/O here */
/* close files */
fclose(input);
fclose(output);
return 0;
}