《APUE》：对各个标准I/O流打印缓冲状态

《Unix环境高级编程》这本书附带了许多短小精美的小程序，我在阅读此书的时候，将书上的代码按照自己的理解重写了一遍（大部分是抄书上的），加深一下自己的理解（纯看书太困了，呵呵）。此例子在Ubuntu10.04上测试通过。

//《APUE》：程序5－3//对各个标准I/O流打印缓冲状态#include <stdio.h>#include <unistd.h>#include <libio.h>#include <stdlib.h>void pr_stdio(const char *, FILE *);int main(void){FILE *fp;fputs("enter any character\n", stdout);if( getchar() == EOF ){fprintf(stderr, "Getchar error\n");exit(1);}fputs("one line to standard error\n", stderr);pr_stdio("stdin", stdin);pr_stdio("stdout", stdout);pr_stdio("stderr", stderr);//我的系统上找不到/etc/motd这个文件，所以用一个普通的txt文件代替fp = fopen("output.txt", "r");if( NULL == fp ){fprintf(stderr, "fopen error\n");exit(1);}if( EOF == getc(fp) ){fprintf(stderr, "getc error\n");exit(1);}pr_stdio("output.txt", fp);return 0;}void pr_stdio(const char *name, FILE *fp){printf("stream = %s,  ", name);if( fp->_IO_file_flags & _IO_UNBUFFERED )printf("unbuffered");else if( fp->_IO_file_flags & _IO_LINE_BUF )printf("line buffered");elseprintf("fully buffered");printf(", buffer size = %d\n", fp->_IO_buf_end - fp->_IO_buf_base);}

运行示例（加下划线的为输入）：

qch@ubuntu:~/code$gcc temp.c -o temp
qch@ubuntu:~/code$ ./temp
enter any character
（键入回车键）
one line to standard error
stream = stdin, line buffered, buffer size = 1024
stream = stdout, line buffered, buffer size = 1024
stream = stderr, unbuffered, buffer size = 1
stream = output.txt, fully buffered, buffer size = 4096
qch@ubuntu:~/code$ ./temp < output.txt > std.out 2>std.err
qch@ubuntu:~/code$ cat std.err
one line to standard error
qch@ubuntu:~/code$ cat std.out
enter any character
stream = stdin, fully buffered, buffer size = 4096
stream = stdout, fully buffered, buffer size = 4096
stream = stderr, unbuffered, buffer size = 1
stream = output.txt, fully buffered, buffer size = 4096

结论：
从此可见，该系统默认的是：当标准输入，输出连终端时，它们是行缓冲的。行缓冲的长度是1024；当将这个流重定向到文件时，它们就变成全缓冲的，其缓冲区的长度是该文件系统优先选用的I/O长度；还有，标准出错是非缓冲的，而普通文件被系统默认是全缓冲的。