西邮Linux兴趣小组

文章来源：http://www.linuxsa.org.au/tips/time.html

Introduction

This document explains how to set your computer's clock from Linux, how to set your timezone, and other stuff related to Linux and how it does its time-keeping.

Your computer has two timepieces; a battery-backed one that is always running (the ``hardware'', ``BIOS'', or ``CMOS'' clock), and another that is maintained by the operating system currently running on your computer (the ``system'' clock). The hardware clock is generally only used to set the system clock when your operating system boots, and then from that point until you reboot or turn off your system, the system clock is the one used to keep track of time.

On Linux systems, you have a choice of keeping the hardware clock in UTC/GMT time or local time. The preferred option is to keep it in UTC because then daylight savings can be automatically accounted for. The only disadvantage with keeping the hardware clock in UTC is that if you dual boot with an operating system (such as DOS) that expects the hardware clock to be set to local time, the time will always be wrong in that operating system.

Setting your timezone

The timezone under Linux is set by a symbolic link from /etc/localtime[1] to a file in the /usr/share/zoneinfo[2] directory that corresponds with what timezone you are in. For example, since I'm in South Australia, /etc/localtime is a symlink to /usr/share/zoneinfo/Australia/South. To set this link, type:
ln -sf ../usr/share/zoneinfo/your/zone /etc/localtime

Replace your/zone with something like Australia/NSW or Australia/Perth. Have a look in the directories under /usr/share/zoneinfo to see what timezones are available.

[1] This assumes that /usr/share/zoneinfo is linked to /etc/localtime as it is under Red Hat Linux.

[2] On older systems, you'll find that /usr/lib/zoneinfo is used instead of /usr/share/zoneinfo. See also the later section ``The time in some applications is wrong''.

Setting UTC or local time

When Linux boots, one of the initialisation scripts will run the /sbin/hwclock program to copy the current hardware clock time to the system clock. hwclock will assume the hardware clock is set to local time unless it is run with the --utc switch. Rather than editing the startup script, under Red Hat Linux you should edit the /etc/sysconfig/clock file and change the ``UTC'' line to either ``UTC=true'' or ``UTC=false'' as appropriate.

Setting the system clock

To set the system clock under Linux, use the date command. As an example, to set the current time and date to July 31, 11:16pm, type ``date 07312316'' (note that the time is given in 24 hour notation). If you wanted to change the year as well, you could type ``date 073123161998''. To set the seconds as well, type ``date 07312316.30'' or ``date 073123161998.30''. To see what Linux thinks the current local time is, run date with no arguments.

Setting the hardware clock

To set the hardware clock, my favourite way is to set the system clock first, and then set the hardware clock to the current system clock by typing ``/sbin/hwclock --systohc'' (or ``/sbin/hwclock --systohc --utc'' if you are keeping the hardware clock in UTC). To see what the hardware clock is currently set to, run hwclock with no arguments. If the hardware clock is in UTC and you want to see the local equivalent, type ``/sbin/hwclock --utc''

The time in some applications is wrong

If some applications (such as date) display the correct time, but others don't, and you are running Red Hat Linux 5.0 or 5.1, you most likely have run into a bug caused by a move of the timezone information from /usr/lib/zoneinfo to /usr/share/zoneinfo. The fix is to create a symbolic link from /usr/lib/zoneinfo to /usr/share/zoneinfo: ``ln -s ../share/zoneinfo /usr/lib/zoneinfo''.

Summary

* /etc/sysconfig/clock sets whether the hardware clock is stored as UTC or local time.

* Symlink /etc/localtime to /usr/share/zoneinfo/... to set your timezone.

* Run ``date MMDDhhmm'' to set the current system date/time.

* Type ``/sbin/hwclock --systohc [--utc]'' to set the hardware clock.

Other interesting notes

The Linux kernel always stores and calculates time as the number of seconds since midnight of the 1st of January 1970 UTC regardless of whether your hardware clock is stored as UTC or not. Conversions to your local time are done at run-time. One neat thing about this is that if someone is using your computer from a different timezone, they can set the TZ environment variable and all dates and times will appear correct for their timezone.

If the number of seconds since the 1st of January 1970 UTC is stored as an signed 32-bit integer (as it is on your Linux/Intel system), your clock will stop working sometime on the year 2038. Linux has no inherent Y2K problem, but it does have a year 2038 problem. Hopefully we'll all be running Linux on 64-bit systems by then. 64-bit integers will keep our clocks running quite well until aproximately the year 292271-million.

Other programs worth looking at

* rdate - get the current time from a remote machine; can be used to set the system time.

* xntpd - like rdate, but it's extremely accurate and you need a permanent 'net connection. xntpd runs continuously and accounts for things like network delay and clock drift, but there's also a program (ntpdate) included that just sets the current time like rdate does.

Further information

* date(1)

* hwclock(8)

* /usr/doc/HOWTO/mini/Clock

下面一段代码的作用是用mmap实现文件的拷贝，它可以给你演示mmap的使用，同时也告诉你在Linux上如何正确有效地处理错误。（代码可以在这里下载：http://wangcong.org/src/mmcpy.c）

读完这段代码，请试着回答后面的几个问题。

/*
 * Copyright (C) WANG Cong, Apr. 2007.
 * GPLv2 applies.
 */
#include <sys/mman.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <unistd.h>
#include <fcntl.h>
#include <stdio.h>
#include <string.h>
#include <stdlib.h>

int main(int argc, char *argv[]){
int infd, outfd;
void *from, *to;
struct stat infstat;
int ret = 0;
if(argc != 3){
fprintf(stderr, "Bad usage!\n");
ret = -1;
goto fail;
}
infd = open(argv[1], O_RDWR| O_CREAT, 0600);
if(infd == -1){
perror("Open failed");
ret = -1;
goto fail;
}
outfd = open(argv[2], O_RDWR| O_CREAT, 0600);
if(outfd == -1){
perror("Open failed");
ret = -1;
goto close_fail1;
}
if(fstat(infd, &infstat)){
perror("fstat failed");
ret = -1;
goto close_fail2;
}
if(lseek(outfd, infstat.st_size-1, SEEK_SET) == (off_t) -1){
perror("lseek failed");
ret = -1;
goto close_fail2;
}
if(write(outfd, "", 1) != 1){
perror("write failed");
ret = -1;
goto close_fail2;
}
if(lseek(outfd, 0, SEEK_SET) == (off_t) -1){
perror("lseek failed");
ret = -1;
goto close_fail2;
}
from = mmap(0, (size_t)infstat.st_size, PROT_READ,  MAP_PRIVATE | MAP_NORESERVE, infd, 0);
if(from == (void*) -1){
perror("mmap failed");
ret = -1;
goto close_fail2;
}
to = mmap(0, (size_t)infstat.st_size, PROT_WRITE, MAP_SHARED, outfd, 0);
if(to == (void*) -1){
perror("mmap failed");
ret = -1;
goto unmap_fail1;
}
memcpy(to, from, infstat.st_size);
if( -1 == msync(to, (size_t)infstat.st_size, MS_SYNC)){
perror("msync failed");
ret = -1;
}
if (-1 == munmap(to, infstat.st_size)){
perror("munmap failed");
ret = -1;
}

unmap_fail1:
if( -1 == munmap(from, infstat.st_size)){
perror("munmap failed");
ret = -1;
}

close_fail2:
if( -1 == close(outfd)){
perror("close failed");
ret = -1;
}

close_fail1:
if( -1 == close(infd)){
perror("close failed");
ret = -1;
}
fail:
exit(ret);
}

问题：
1. mmap利用了操作系统中的什么原理？
2. 我们知道好的程序应该避免goto的使用，这里的goto没有问题吗？
3. 为什么这种情况下goto比单纯的return要更好？
4. 这种情况下的错误处理有什么特点？

文章来源：
http://koria.blog.hexun.com/8514590_d.html

1.1. 问题 Problem

You need to deal with data that doesn't fit in the ASCII character set.

你需要处理不适合用ASCII字符集表示的数据.

1.2. 解决 Solution

Unicode strings can be encoded in plain strings in a variety of ways, according to whichever encoding you choose:

Unicode字符串可以用多种方式编码为普通字符串, 依照你所选择的编码(encoding):

1 #将Unicode转换成普通的Python字符串:"编码(encode)"
2 unicodestring = u"Hello world"
3 utf8string = unicodestring.encode("utf-8")
4 asciistring = unicodestring.encode("ascii")
5 isostring = unicodestring.encode("ISO-8859-1")
6 utf16string = unicodestring.encode("utf-16")
7
8
9 #将普通的Python字符串转换成Unicode: "解码(decode)"
10 plainstring1 = unicode(utf8string, "utf-8")
11 plainstring2 = unicode(asciistring, "ascii")
12 plainstring3 = unicode(isostring, "ISO-8859-1")
13 plainstring4 = unicode(utf16string, "utf-16")
14
15 assert plainstring1==plainstring2==plainstring3==plainstring4

1.3. 讨论 Discussion

If you find yourself dealing with text that contains non-ASCII characters, you have to learn about Unicode梬hat it is, how it works, and how Python uses it.

如果你发现自己正在处理包含非ASCII码字符的文本, 你必须学习Unicode，关于它是什么,如何工作，而且Python如何使用它。

Unicode is a big topic.Luckily, you don't need to know everything about Unicode to be able to solve real-world problems with it: a few basic bits of knowledge are enough.First, you must understand the difference between bytes and characters.In older, ASCII-centric languages and environments, bytes and characters are treated as the same thing.Since a byte can hold up to 256 values, these environments are limited to 256 characters.Unicode, on the other hand, has tens of thousands of characters.That means that each Unicode character takes more than one byte, so you need to make the distinction between characters and bytes.

Unicode是一个大的主题。幸运地，你并不需要知道关于Unicode码的每件事，就能够用它解决真实世界的问题: 一些基本知识就够了。首先，你得了解在字节和字符之间的不同。原先，在以ASCII码为中心的语言和环境中，字节和字符被当做相同的事物。由于一个字节只能有256个值，这些环境就受限为只支持 256个字符。Unicode码，另一方面，有数万个字符,那意谓着每个Unicode字符占用多个字节，因此，你需要在字符和字节之间作出区别。

Standard Python strings are really byte strings, and a Python character is really a byte.Other terms for the standard Python type are "8-bit string" and "plain string.",In this recipe we will call them byte strings, to remind you of their byte-orientedness.

标准的Python字符串确实是字节字符串，而且一个Python字符真的是一个字节。换个术语,标准的 Python字符串类型的是 "8位字符串(8-bit string)"和"普通字符串(plain string)". 在这一份配方中我们把它们称作是字节串(byte strings), 并记住它们是基于字节的。

Conversely, a Python Unicode character is an abstract object big enough to hold the character, analogous to Python's long integers.You don't have to worry about the internal representation;the representation of Unicode characters becomes an issue only when you are trying to send them to some byte-oriented function, such as the write method for files or the send method for network sockets.At that point, you must choose how to represent the characters as bytes.Converting from Unicode to a byte string is called encoding the string.Similarly, when you load Unicode strings from a file, socket, or other byte-oriented object, you need to decode the strings from bytes to characters.

相反地，一个Python Unicode码字符是一个大到足够支持(Unicode)字符的一个抽象对象, 类似于Python中的长整数。 你不必要为内在的表示担忧; 只有当你正在尝试把它们传递给给一些基于字节的函数的时候，Unicode字符的表示变成一个议题, 比如文件的write方法或网络套接字的send 方法。那时，你必须要选择该如何表示这些(Unicode)字符为字节。从Unicode码到字节串的转换被叫做编码。同样地，当你从文件，套接字或其他的基于字节的对象中装入一个Unicode字符串的时候,你需要把字节串解码为(Unicode)字符。

There are many ways of converting Unicode objects to byte strings, each of which is called an encoding.For a variety of historical, political, and technical reasons, there is no one "right" encoding.Every encoding has a case-insensitive name, and that name is passed to the decode method as a parameter. Here are a few you should know about:

将Unicode码对象转换成字节串有许多方法, 每个被称为一个编码(encoding)。由于多种历史的，政治上的，和技术上的原因,没有一个 "正确的"编码。每个编码有一个大小写无关的名字，而且那一个名字被作为一个叁数传给解码方法。这里是一些你应该知道的:

* The UTF-8 encoding can handle any Unicode character.It is also backward compatible with ASCII, so a pure ASCII file can also be considered a UTF-8 file, and a UTF-8 file that happens to use only ASCII characters is identical to an ASCII file with the same characters.This property makes UTF-8 very backward-compatible, especially with older Unix tools.UTF-8 is far and away the dominant encoding on Unix.It's primary weakness is that it is fairly inefficient for Eastern texts.
* UTF-8 编码能处理任何的Unicode字符。它也是与ASCII码向后兼容的，因此一个纯粹的ASCII码文件也能被考虑为一个UTF-8 文件，而且一个碰巧只使用ASCII码字符的 UTF-8 文件和拥有同样字符的ASCII码文件是相同的。这个特性使得UTF-8的向后兼容性非常好,尤其使用较旧的 Unix工具时。UTF-8 无疑地是在 Unix 上的占优势的编码。它主要的弱点是对东方文字是非常低效的。
* The UTF-16 encoding is favored by Microsoft operating systems and the Java environment.It is less efficient for Western languages but more efficient for Eastern ones.A variant of UTF-16 is sometimes known as UCS-2.
* UTF-16 编码在微软的操作系统和Java环境下受到偏爱。它对西方语言是比较低效,但对于东方语言是更有效率的。一个 UTF-16 的变体有时叫作UCS-2 。
* The ISO-8859 series of encodings are 256-character ASCII supersets.They cannot support all of the Unicode characters;they can support only some particular language or family of languages.ISO-8859-1, also known as Latin-1, covers most Western European and African languages, but not Arabic.ISO-8859-2, also known as Latin-2,covers many Eastern European languages such as Hungarian and Polish.
* ISO-8859编码系列是256个字符的ASCII码的超集。他们不能够支援所有的Unicode码字符; 他们只能支援一些特别的语言或语言家族。ISO-8859-1, 也既Latin-1,包括大多数的西欧和非洲语言, 但是不含阿拉伯语。ISO-8859-2,也既Latin-2,包括许多东欧的语言,像是匈牙利语和波兰语。

If you want to be able to encode all Unicode characters, you probably want to use UTF-8.You will probably need to deal with the other encodings only when you are handed data in those encodings created by some other application.

如果你想要能够编码所有的Unicode码字符,你或许想要使用UTF-8。只有当你需要处理那些由其他应用产生的其它编码的数据时，你或许才需要处理其他编码。

我们的开发项目──XYLFTP的当前进度如下：

客户端已经开始进入详细设计阶段，服务器端到了概要设计阶段。

这些阶段编写的正式文档如下：

1. 服务器端需求分析：
http://www.cocobear.cn/blog/download/2007/04/ssrsv0_4.pdf
2. 服务器端用户信息管理工具：
http://wangcong.org/src/xylftppass.py
3. 客户端需求分析：
http://wangcong.org/down/Requirements_Analysis_Client.pdf
4. 客户端概要设计：
http://wangcong.org/down/General_Design_Client.pdf

BTW：我刚在sourceforge.net给我们的项目进行了注册，主页是：https://sourceforge.net/projects/xylftp。以后会陆续更新。

希望大家继续努力！早日完成任务！

作者：西邮 王聪

小组简介：

我们这个兴趣小组主要是学习Linux系统配置，Linux应用程序开发，Linux内核编程。每周三一般都会邀请系里的老师来做一次讲座，周一周五晚上七点讨论。我们在网上有自己的新闻组进行讨论，有自己的博客发布文章。

最近在做的一个项目叫xylftp（Xi You Linux FTP），它是关于FTP的一套程序，包括一个完整的服务器端和客户端。服务器端是用C编写，而客户端是用Java编写。做这个项目的目的有两个：一是熟悉Linux上的网络编程/Java编程，二是熟悉软件工程，我们这个项目是按照软件工程的规范一步一步来的，各种文档也是按照规范写的。

我们上学期做的主要是一些小的Linux应用程序，主要是为了帮助大家熟悉Linux编程环境。其实这也是我们这学期给新手准备的内容。;-)

如果你对Linux感兴趣，并且非常想学习关于它的更多的知识，欢迎你加入我们！我们将尽力为你提供一个好的学习环境。

报名方法：

新校区：每周一，周五晚七点，周三下午4:30，到二号实验楼434，找金明洁同学。
老校区：直接联系梅延涛同学，或者发邮件到meiyantaoster@gmail.com。

相关链接：

我们的博客：http://my.opera.com/xiyoulinux/blog/
我们的新闻组：http://groups.google.com/group/xiyoulinux

致谢：

感谢金明洁同学，他能不辞辛苦地帮我们管理新来的同学。
多亏了梅延涛同学，才使得我们也能够在老校区发展。
史宇航同学虽然不是我们兴趣小组的成员，但他能帮助我们在xiyoubbs上宣传，非常感谢！

一些问题：

1. 非计算机专业的行吗？
答：没问题。其实这里有很多非计算机专业的同学，最好的例子是梅延涛，他其实是学管理的。;-p

2. 有什么要求吗？
答：一般来说没有，只要有这方面的兴趣即可。如果你的编程能力不错，那将会更好。

3. 有截止日期吗？
答：具体日期没有，但是越早加入越好，因为我们不会因为你而改变整个小组的安排。我们这样做主要是想让更多人加入，然后筛选一些能坚持下去，确实感兴趣的同学。是的，我们完全凭兴趣，所以坚持是最重要的。

4. 参加这能加学分吗？
答：不能！如果你只是想要学分的话，我劝你最好别加入。我们*每个人*绝对不会因为加入这而得到半个学分，连0.001个学分也没有。

5. 里面有什么考核吗？
答：一般不会有，但我不排除其他人安排的一些测试（可能主要是为了提高大家的积极性）。但是，请放心，那也不会成为筛选的标准。只要你肯坚持学习，我们没理由让你退出。