select with reglar file descriptor

最新推荐文章于 2021-02-09 15:23:14 发布

原创最新推荐文章于 2021-02-09 15:23:14 发布 · 499 阅读

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本文介绍如何使用C语言通过`select()`函数实时监测文件读取状态，并在文件有新数据时输出提示信息。

Disk files are always ready to read (but the read might return 0 bytes if you're already at the end of the file), so you can't use select() on a disk file to find out when new data is added to the file.

POSIX says:

File descriptors associated with regular files shall always select true for ready to read, ready to write, and error conditions.
int
main(void)
{
    fd_set rfds;
    struct timeval tv;
    int retval;

    int fd_file = open("/home/myfile.txt", O_RDONLY);

   /* Watch stdin (fd 0) to see when it has input. */
    FD_ZERO(&rfds);
    FD_SET(0, &rfds);
    FD_SET(fd_file, &rfds);

   /* Wait up to five seconds. */
    tv.tv_sec = 5;
    tv.tv_usec = 0;

   while (1)
   {
     retval = select(fd_file+1, &rfds, NULL, NULL, &tv);
     /* Don't rely on the value of tv now! */

     if (retval == -1)
        perror("select()");
     else if (retval)
        printf("Data is available now.\n");
        /* FD_ISSET(0, &rfds) will be true. */
     else
        printf("No data within five seconds.\n");
   }

   exit(EXIT_SUCCESS);
}
所以，只要文件/home/myfile.txt存在，程序都将在while(1)中不停的输出：

Data is available now.

Data is available now.

Data is available now.

Data is available now.

……