【setvbuf】
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:
1 #include <stdio.h> 2 int main(void) 3 { 4 FILE *input, *output; 5 char bufr[512]; 6 input = fopen("file.in", "r+b"); 7 output = fopen("file.out", "w"); 8 /* set up input stream for minimal disk access, 9 using our own character buffer */ 10 if (setvbuf(input, bufr, _IOFBF, 512) != 0) 11 printf("failed to set up buffer for input file "); 12 else 13 printf("buffer set up for input file "); 14 /* set up output stream for line buffering using space that 15 will be obtained through an indirect call to malloc */ 16 if (setvbuf(output, NULL, _IOLBF, 132) != 0) 17 printf("failed to set up buffer for output file "); 18 else 19 printf("buffer set up for output file "); 20 /* perform file I/O here */ 21 /* close files */ 22 fclose(input); 23 fclose(output); 24 return 0; 25 }