GNU C Library 里的一句话理解不了,请高手指教

whao

在select()函数中,解释select的机制时,有这么一句话:
A file descriptor is considered ready for reading if it is at end of file.
file descriptor就是select的参数,我的理解应该就是一个句柄.里面先说过select来监查代进来的file descriptor有没有activity,如果有的话就开始处理数据,但是在讲如何判有没有activity的时候,就是上面的一句话.请问哪位高手知道如何理解?file descriptor 怎么可能在文件里嘛?!

whao

上下文如下:


Next, here is the description of the select function itself.

Function: int select (int nfds, fd_set *read-fds, fd_set *write-fds, fd_set *except-fds, struct timeval *timeout)
    The select function blocks the calling process until there is activity on any of the specified sets of file descriptors, or until the timeout period has expired.

    The file descriptors specified by the read-fds argument are checked to see if they are ready for reading; the write-fds file descriptors are checked to see if they are ready for writing; and the except-fds file descriptors are checked for exceptional conditions. You can pass a null pointer for any of these arguments if you are not interested in checking for that kind of condition.

    A file descriptor is considered ready for reading if it is at end of file. A server socket is considered ready for reading if there is a pending connection which can be accepted with accept; see section Accepting Connections. A client socket is ready for writing when its connection is fully established; see section Making a Connection.

whao

嗯,好像是这个意思,多谢了!不过你怎么想到at end of file.就是socket被关闭或者shutdown的意思的呢?

思一克

select用来判断普通文件的读写时

stevenyi

相当于:
A file descriptor is considered ready for reading if the file position pointer is at end of file.

蓝色键盘

这个问题的核心在于操作系统的对于描述符的处理方式。

一般的，对于socket描述符操作系统是通过一个特殊的文件系统来管理的，这个文件系统在系统boot的过程中mount上的，每个进程的PCB块中有相应的数据结构对应于这个描述符。类似的机制有管道，和随进程持续的IPC对象。

对于文件系统的描述符，能不能处理取决与这个描述符有没有在PCB中做相应的activity处理的函数指针，如果这个指针为空，那么可能是操作系统的开发者预留的功能，只是当时候由于某些原因没有添加。如果不为空，那么这个描述符可以用select处理。

更详细的描述，可参看操作系统相关资料。

whao

when i test theses functions in the code below, it report an error like :
Transport endpoint is not connected
who can tell me why ? and how to correct it? thanks!

1. 
   
2. #define PATH "/tmp/DC/data"
   
3. #ifdef HAVE_CONFIG_H
   
4. #include <config.h>;
   
5. #endif
   
6. 
   
7. #include <stdio.h>;
   
8. #include <stdlib.h>;
   
9. #include <stddef.h>;
   
10. #include <errno.h>;
    
11. #include <stdlib.h>;
    
12. #include <sys/socket.h>;
    
13. #include <sys/un.h>;
    
14. 
    
15. void  write_to_server (int filedes)
    
16. {
    
17.   int nbytes;
    
18.   char *message = "Hey Guy!";
    
19.   nbytes = write (filedes,message , 2);
    
20.   if (nbytes < 0)
    
21.     {
    
22.       perror ("write:::::");
    
23.       unlink(PATH);
    
24.       exit (EXIT_FAILURE);
    
25.     }
    
26. }
    
27. 
    
28. 
    
29. 
    
30. int make_named_socket (const char *filename)
    
31. {
    
32.   struct sockaddr_un name;
    
33.   int sock;
    
34.   size_t size;
    
35. 
    
36.   /* Create the socket. */
    
37. 
    
38.   sock = socket (PF_UNIX, SOCK_DGRAM, 0);
    
39.   if (sock < 0)
    
40.     {
    
41.       perror ("socket");
    
42.       unlink(PATH);
    
43.       exit (EXIT_FAILURE);
    
44.     }
    
45. 
    
46.   /* Bind a name to the socket. */
    
47. //  unlink(PATH);
    
48.   name.sun_family = AF_FILE;
    
49.   strcpy (name.sun_path, filename);
    
50. 
    
51.   /* The size of the address is
    
52.      the offset of the start of the filename,
    
53.      plus its length,
    
54.      plus one for the terminating null byte. */
    
55.   size = (offsetof (struct sockaddr_un, sun_path)
    
56.           + strlen (name.sun_path) + 1);
    
57. 
    
58.   if (bind (sock, (struct sockaddr *) &name, size) < 0)
    
59.     {
    
60.       perror ("bind");
    
61.       unlink(PATH);      
    
62.       exit (EXIT_FAILURE);
    
63.     }
    
64. 
    
65.   return sock;
    
66. }
    
67. 
    
68. 
    
69. 
    
70. int main(int argc, char *argv[])
    
71. {
    
72.   int sock_fd;
    
73. 
    
74.   static char *path = PATH;
    
75. 
    
76.   sock_fd = make_named_socket(path);
    
77.   write_to_server (sock_fd);
    
78. 
    
79.   return EXIT_SUCCESS;
    
80. }
    
81. 
    

复制代码

gadfly

你这个代码有点问题。make_named_socket只是个监听端口的socket，只能算半连接。write_to_server的参数必须是建立了连接的socket，即accept客户端connect后返回的描述字。
unix domain socket和tcp/ip的socket的交互过程是一样的，例程很多。参考一下吧。