select函数简单用法例程

1 #include <sys/time.h> /* For portability */
2 #include <sys/select.h>
3 int select(int nfds, fd_set *readfds, fd_set *writefds, fd_set *exceptfds,
4 struct timeval *timeout);
5 
6 Returns number of ready file descriptors, 0 on timeout, or –1 on error

1 #include <sys/select.h>
2 void FD_ZERO(fd_set *fdset);
3 void FD_SET(int fd, fd_set *fdset);
4 void FD_CLR(int fd, fd_set *fdset);
5 int FD_ISSET(int fd, fd_set *fdset);
6     Returns true (1) if fd is in fdset, or false (0) otherwise

 1 /************************************************************************* 2 *                  Copyright (C) Michael Kerrisk, 2015.                   *
 3 *                                                                         *
 4 * This program is free software. You may use, modify, and redistribute it *
 5 * under the terms of the GNU General Public License as published by the   *
 6 * Free Software Foundation, either version 3 or (at your option) any      *
 7 * later version. This program is distributed without any warranty.  See   *
 8 * the file COPYING.gpl-v3 for details.                                    *
 9 \*************************************************************************/
10 
11 /* Listing 63-1 */
12 
13 /* t_select.c
14 
15    Example of the use of the select() system call to monitor multiple
16    file descriptors.
17 
18    Usage as shown in usageError().
19 */
20 #include <sys/time.h>
21 #if ! defined(__hpux)
22 /* HP-UX 11 doesn‘t have this header file */
23 #include <sys/select.h>
24 #endif
25 #include "tlpi_hdr.h"
26 
27 static void
28 usageError(const char *progName)
29 {
30     fprintf(stderr, "Usage: %s {timeout|-} fd-num[rw]...\n", progName);
31     fprintf(stderr, "    - means infinite timeout; \n");
32     fprintf(stderr, "    r = monitor for read\n");
33     fprintf(stderr, "    w = monitor for write\n\n");
34     fprintf(stderr, "    e.g.: %s - 0rw 1w\n", progName);
35     exit(EXIT_FAILURE);
36 }
37 
38 int
39 main(int argc, char *argv[])
40 {
41     fd_set readfds, writefds;//读, 写文件描述符
42     int ready, nfds, fd, numRead, j;
43     struct timeval timeout;//超时
44     struct timeval *pto;
45     char buf[10];                       /* Large enough to hold "rw\0" */
46 
47     if (argc < 2 || strcmp(argv[1], "--help") == 0)
48         usageError(argv[0]);
49 
50     /* Timeout for select() is specified in argv[1] */
51 
52     if (strcmp(argv[1], "-") == 0) {
53         pto = NULL;                     /* Infinite timeout */
54     } else {
55         pto = &timeout;
56         timeout.tv_sec = getLong(argv[1], 0, "timeout");
57         timeout.tv_usec = 0;            /* No microseconds */
58     }
59 
60     /* Process remaining arguments to build file descriptor sets */
61     
62     nfds = 0;
63     FD_ZERO(&readfds);
64     FD_ZERO(&writefds);
65 
66     for (j = 2; j < argc; j++) {
67         numRead = sscanf(argv[j], "%d%2[rw]", &fd, buf);//将文件描述符输入fd, 将输入/输出参数写入buf[]
68         if (numRead != 2)
69             usageError(argv[0]);
70         if (fd >= FD_SETSIZE)
71             cmdLineErr("file descriptor exceeds limit (%d)\n", FD_SETSIZE);
72 
73         if (fd >= nfds)
74             nfds = fd + 1;              /* Record maximum fd + 1 */
75         if (strchr(buf, ‘r‘) != NULL)
76             FD_SET(fd, &readfds);
77         if (strchr(buf, ‘w‘) != NULL)
78             FD_SET(fd, &writefds);
79     }
80 
81     /* We‘ve built all of the arguments; now call select() */
82 
83     ready = select(nfds, &readfds, &writefds, NULL, pto);
84                                         /* Ignore exceptional events */
85     if (ready == -1)
86         errExit("select");
87 
88     /* Display results of select() */
89 
90     printf("ready = %d\n", ready);
91     for (fd = 0; fd < nfds; fd++)
92         printf("%d: %s%s\n", fd, FD_ISSET(fd, &readfds) ? "r" : "",
93                 FD_ISSET(fd, &writefds) ? "w" : "");
94 
95     if (pto != NULL)
96         printf("timeout after select(): %ld.%03ld\n",
97                (long) timeout.tv_sec, (long) timeout.tv_usec / 1000);
98     exit(EXIT_SUCCESS);
99 }