select()函数

shina

各位有没有使用过select函数，它是应用在哪个情况下的？请教各位。

devel

1. 
   
2. /*  select()  check for files that are ready for reading or writing
   
3. #include <sys/time.h>
   
4. int select( int width,fd_set * readfds,fd_set * writefds,fd_set * exceptfds,
   
5.             struct timeval * timeout );
   
6. FD_SET( int fd, fd_set * fdset );
   
7. FD_CLR( int fd, fd_set * fdset );
   
8. FD_ISSET( int fd, fd_set * fdset );
   
9. FD_ZERO( fd_set * fdset );
   
10. FD_ZERO(&fdset)             Initializes a descriptor set fdset to the null set.
    
11. FD_SET(fd, &fdset)          Add the file descriptor fd to the set fdset.
    
12. FD_CLR(fd, &fdset)          Removes fd from fdset.
    
13. FD_ISSET(fd, &fdset)        Is nonzero if fd is a member of fdset; otherwise, zero.
    
14. This example opens a console and a serial port for read mode, and calls select()
    
15. with a 5 second timeout. It waits for data to be available on either descriptor. */
    
16. #include <unistd.h>
    
17. #include <stdlib.h>
    
18. #include <fcntl.h>
    
19. #include <sys/time.h>
    
20. int main( void )
    
21. {
    
22.     int console, serial;
    
23.     struct timeval tv;
    
24.     fd_set rfd;
    
25.     int n;
    
26.     if( ( console = open( "/dev/con1", O_RDONLY ) ) == -1
    
27.     ||    ( serial  = open( "/dev/ser1", O_RDONLY ) ) == -1 ) {
    
28.       perror( "open" );
    
29.       return EXIT_FAILURE;
    
30.     }
    
31.     FD_ZERO( &rfd );
    
32.     FD_SET( console, &rfd );
    
33.     FD_SET( serial,  &rfd );
    
34.     tv.tv_sec = 5; /* set a 5 second timeout */
    
35.     tv.tv_usec = 0;
    
36.     switch ( n = select( 1 + max( console, serial ),&rfd, 0, 0, &tv ) ) {
    
37.       case -1:
    
38.             perror( "select" );
    
39.             return EXIT_FAILURE;
    
40.       case  0:
    
41.             puts( "select timed out" );
    
42.             break;
    
43.       default:
    
44.             printf( "%d descriptors ready ...\n", n );
    
45.             if( FD_ISSET( console, &rfd ) )
    
46.                           puts( " -- console descriptor has data pending" );
    
47.             if( FD_ISSET( serial, &rfd ) )
    
48.                           puts( " -- serial descriptor has data pending" );
    
49.      }
    
50.     return EXIT_SUCCESS;
    
51. }
    
52. /*int select( int width,fd_set * readfds,fd_set * writefds,fd_set * exceptfds,
    
53.           struct timeval * timeout ); */
    

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dancingpig

首先设定检测的套接字
然后select检测是否有套接字准备完毕（比如有socket连接）同时把检测的套接字集“清空”，请特别注意哦.写错了，应该是FD_ISSET宏晴空的~
select返回的是准备完毕的套接字数目