hostid.c/获取主机标识

   还是一般的套路，看完hostid.c后，了解了一个系统调用：  gethostid()  ，ok，先来自己粗燥地实现一下^_^

[root@bjxdurs235 20091028]# cat my_hostid.c
#include

int main(void)
{
        int id;
        id = gethostid();
        printf("%0x\n",id);
}
[root@bjxdurs235 20091028]# ./a.out
6aca27c7
[root@bjxdurs235 20091028]# ./a.out
6aca1ec3

    为了避免看完一个程序后，就把系统调用或者库函数给忘记了，我觉得有必要从今天开始维护一个日记页面，总结从现在到以后学习中遇到的系统调用或者库函数，为将来总结这一段时间的学习所用，简单明了即可。


下面再罗列一下代码注释：

/* print the hexadecimal identifier for the current host

   Copyright (C) 1997, 1999, 2000, 2001, 2002, 2003, 2004 Free
   Software Foundation, Inc.

   This program is free software; you can redistribute it and/or modify
   it under the terms of the GNU General Public License as published by
   the Free Software Foundation; either version 2, or (at your option)
   any later version.

   This program is distributed in the hope that it will be useful,
   but WITHOUT ANY WARRANTY; without even the implied warranty of
   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
   GNU General Public License for more details.

   You should have received a copy of the GNU General Public License
   along with this program; if not, write to the Free Software Foundation,
   Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.  */

/* Written by Jim Meyering.  */
//包含进来4个标准头文件
#include
#include
#include
#include

//包含4个本地文件夹里的头文件
#include "system.h"
#include "long-options.h"
#include "error.h"
#include "quote.h"

/* The official name of this program (e.g., no `g' prefix).  */
#define PROGRAM_NAME "hostid" //定义宏PROGRAM_NAME，其值为一个字符串

#define AUTHORS "Jim Meyering" //定义宏AUTHORS，其值也是个字符串，作者，这哥们好像写了不少程序啊

/* The name this program was run with, for error messages. */
char *program_name;//注释里说这个变量的目的是为了错误信息

void
usage (int status)//通用的帮助函数
{
  if (status != EXIT_SUCCESS)//如果函数接受到的形参status不等于EXIT_SUCCESS
    fprintf (stderr, _("Try `%s --help' for more information.\n"),
         program_name);//则向标准错误打印信息，提示用户使用帮助
  else //否则打印下面的代码段里的信息
    {
      printf (_("\
Usage: %s\n\
  or:  %s OPTION\n\
Print the numeric identifier (in hexadecimal) for the current host.\n\
\n\
"),
          program_name, program_name);//这句话表明了程序的作用，是打印当前主机的十六进制标识
      fputs (HELP_OPTION_DESCRIPTION, stdout);
      fputs (VERSION_OPTION_DESCRIPTION, stdout);
      printf (_("\nReport bugs to <%s>.\n"), PACKAGE_BUGREPORT);
    }
  exit (status);//函数的退出状态是接受到的形参status的值
}

int
main (int argc, char **argv)//标准的main函数
{
  unsigned int id;//无符号整型变量 id

  initialize_main (&argc, &argv);//初始化命令行参数
  program_name = argv[0];//将命令行参数的第一个赋值给变量program_name
  setlocale (LC_ALL, "");//以下3句设置本地环境
  bindtextdomain (PACKAGE, LOCALEDIR);
  textdomain (PACKAGE);

  atexit (close_stdout);//登记出口函数

  parse_long_options (argc, argv, PROGRAM_NAME, GNU_PACKAGE, VERSION,
              usage, AUTHORS, (char const *) NULL);//解析命令行选项
  if (getopt_long (argc, argv, "", NULL, NULL) != -1)//如果没有正常解析命令行选项
    usage (EXIT_FAILURE);//则调用usage函数打印帮助信息

  if (optind < argc)//如果调用getopt_long函数后得到的optind值小于argc，命令行的参数个数，则调用error函数
    {
      error (0, 0, _("extra operand %s"), quote (argv[optind]));
      usage (EXIT_FAILURE);//并打印帮助信息
    }

  id = gethostid ();//核心的地方，通过系统调用gethostid获得当前主机的一个32位标识符

  /* POSIX says gethostid returns a "32-bit identifier" but is silent
     whether it's sign-extended.  Turn off any sign-extension.  This
     is a no-op unless unsigned int is wider than 32 bits. 
系统调用gethostid返回的值并没有说是否会溢出sign类型的最大值，程序在这里进行了位运算以避免溢出
     */
  id &= 0xffffffff;

  printf ("%08x\n", id); //打印格式为十六进制

  exit (EXIT_SUCCESS);//程序最终退出状态为EXIT_SUCCESS
}