全缓冲/行缓冲/不带缓冲

使用缓冲区的目的：尽量减少使用read/write的调用
分类：
1. Fully buffered means that I/O takes place only when the buffer is fully, the process explicitly calls fflush, or the process terminates by calling exit. A common size for the standard I/O buffer is 8192 bytes;
 
2. Line buffered means that I/O takes place when a newline is encountered, when the process calls fflush, or when the process terminates by calling exit.
 
3. Unbuffered means that I/O take place each time a standard I/O output function is called.
 
 
Most unix implementations of the standard I/O libarary use the following rules.
 
1. Standard error is always unbuffered. 能够快速显示错误
 
2. Standard input and standard output are fully buffered, unless they refer to a terminal device, in which case, they are line buffered.
 
3. All other streams are fully buffered unless they refer to a terminal device,
in which case, they are line buffered.

使用setbuf()和setvbuf()可以更改缓冲的类型：
setbuf(FILE *fp, char *buf)   buf用malloc()开辟，当buf为NULL时，缓冲区为不带缓冲
setvbuf(FILE *fp,char *buf, int buf_mode, int size)
        buf_mde有三种：_IOFBF 0 全缓冲， _IOLBF 1 行缓冲， _IONBF 2 不带缓冲

在任何时刻，可以使用fflush强制刷新一个数据流，此函数使该流所有未写的数据都被传递至内核。
由于fflush(stdin)的移植性不好，gcc不支持，故可以用以下代码清空stdin缓冲区：
        while ((c = getchar()) != '\n' && c != EOF);