关于流和缓冲区的理解以及一般标准输入问题的解决方法小结

antigloss

原帖由 kernelxu 于 2005-8-3 21:39 发表
这个程序存在问题，这里使用setbuf()来“治疗”：

#include <stdio.h>;

int main(void)
{
   int i,j;
   char c;
   printf("\n do you want to cal:y/n \n");

   while (c=getchar()=='y')
   {
       printf("input number:");
       scanf("%d%d",&i,&j);
       printf("i*j=%d",i*j);
       setbuf(stdin, NULL);
       printf("\n do you want to cal:y/n \n");
   }
  /*system("pause");*/
   return 0;
}

粗粗的看了一下帖子，LZ似乎是在说使用 setbuf / setvbuf 清空输入流。不过上面举的这个例子也是不符合标准的。

From C99
Description
The setvbuf function may be used only after the stream pointed to by stream has
been associated with an open file and before any other operation (other than an
unsuccessful call to setvbuf) is performed on the stream.

Description
Except that it returns no value, the setbuf function is equivalent to the setvbuf
function invoked with the values _IOFBF for mode and BUFSIZ for size,or(if buf
is a null pointer), with the value _IONBF for mode.

显然，上例在调用 setbuf(stdin, NULL); 之前，scanf("%d%d",&i,&j); 就已经对 stdin 进行了操作，与标准的定义不符合。

MMMIX

原帖由 antigloss 于 2007-5-3 17:30 发表


粗粗的看了一下帖子，LZ似乎是在说使用 setbuf / setvbuf 清空输入流。不过上面举的这个例子也是不符合标准的。



显然，上例在调用 setbuf(stdin, NULL); 之前，scanf("%d%d",&i,&j) ...

在这里使用 setbuf/setvbuf 确实是不妥的。

关于如何清空标准输入，可以参看 comp.lang.c 的 FAQ：
Question 12.26b

Q: If fflush won't work, what can I use to flush input?

A: It depends on what you're trying to do. If you're trying to get rid of an unread newline or other unexpected input after calling scanf (see questions 12.18a-12.19), you really need to rewrite or replace the call to scanf (see question 12.20). Alternatively, you can consume the rest of a partially-read line with a simple code fragment like

        while((c = getchar()) != '\n' && c != EOF)
                /* discard */ ;

(You may also be able to use the curses flushinp function.)

There is no standard way to discard unread characters from a stdio input stream. Some vendors do implement fflush so that fflush(stdin) discards unread characters, although portable programs cannot depend on this. (Some versions of the stdio library implement fpurge or fabort calls which do the same thing, but these aren't standard, either.) Note, too, that flushing stdio input buffers is not necessarily sufficient: unread characters can also accumulate in other, OS-level input buffers. If you're trying to actively discard input (perhaps in anticipation of issuing an unexpected prompt to confirm a destructive action, for which an accidentally-typed ``y'' could be disastrous), you'll have to use a system-specific technique to detect the presence of typed-ahead input; see questions 19.1 and 19.2. Keep in mind that users can become frustrated if you discard input that happened to be typed too quickly.

References: ISO Sec. 7.9.5.2
H&S Sec. 15.2

militala

呵呵.....太好了, 真是值得收藏,

xi2008wang

今天正好看了一遍流..

我认为流就是引用打开文件的一种方式,就像文件描述符其实就是已打开文件的号码一样.
以这种方式打开文件后,就以进行以后的标准I/O读写了..

xi2008wang

以fopen方式打开文件,内核一样会分配这个文件一个文件描述符,通过引用这个文件描述符照样可以通过write来写这个文件

1. 
   
2. #include    <stdio.h>
   
3. #include    <fcntl.h>
   
4. 
   
5. int
   
6. main(void)
   
7. {
   
8.     int fd;
   
9.     char buf[]="0123456789";
   
10. 
    
11.     if ((fd = open("t1.c", O_RDONLY)) < 0) {
    
12.         printf("open t1.c error\n");
    
13.         exit(1);
    
14.     }
    
15.     else   
    
16.         printf("fd1 = %d,", fd);
    
17.     if (fopen("t2.c", "w") == NULL) {
    
18.         printf("open t2.c error\n");
    
19.         exit(2);
    
20.     }
    
21.     if ((fd = open("t3.c", O_RDONLY)) < 0) {
    
22.         printf("open t3.c error\n");
    
23.         exit(3);
    
24.     }
    
25.     else   
    
26.         printf("fd2 = %d\n", fd);
    
27.     if (write(4, buf, 10) != 10) {
    
28.         printf("write error\n");   
    
29.         exit(5);
    
30.     }
    
31.     exit(0);
    
32. }
    
33. [root@mylinux ap]# ./a.out
    
34. fd1 = 3,fd2 = 5
    

复制代码

[ 本帖最后由 xi2008wang 于 2007-12-17 19:44 编辑 ]

fantimond

精华，绝对的精华。

dlv_201

真是篇难得一见的好文章

青春就像卫生纸

谢谢，确实不错，应该顶起