printk函數(shù)

        We used the printk function in earlier chapters with the simplifying assumption that it works like printf. Now it's time to introduce some of the differences.

        我們在前面章節(jié)中簡單地把printk當(dāng)作printf函數(shù)來使用。現(xiàn)在是時候來介紹它的一些不同之處了,。

        One of the differences is that printk lets you classify messages according to their severity by associating different loglevels, or priorities, with the messages. You usually indicate the loglevel with a macro. For example, KERN_INFO, which we saw prepended to some of the earlier print statements, is one of the possible loglevels of the message. The loglevel macro expands to a string, which is concatenated to the message text at compile time; that's why there is no comma between the priority and the format string in the following examples. Here are two examples of printk commands, a debug message and a critical message:

        其中一個不同點是,，printk允許你按照相關(guān)的記錄級或優(yōu)先級將消息嚴(yán)格分類,。通常你需要一個宏來指定記錄等級。例如,，KERN_INFO,我們在早先的的例子中看到過這個宏,，它就是消息記錄等級的一種。記錄等級宏的作用是擴展為一個字符串,，這個字符串會在編譯期間與相應(yīng)的消息文本相連接,；這就解釋了下面例子中為什么在優(yōu)先級和格式化字符串之間沒有逗號了。下面是兩個printk函數(shù)的例子,，一個是調(diào)試消息,，一個是臨界消息：

printk(KERN_DEBUG "Here I am: %s:%i\n", _ _FILE_ _, _ _LINE_ _);

 

 

printk(KERN_CRIT "I'm trashed; giving up on %p\n", ptr);

 

 

        There are eight possible loglevel strings, defined in the header   ; we list them in order of decreasing severity:
       
        在頭文件<linux/kernel.h>中共定義了八個可用的記錄級；我們下面按其嚴(yán)重性倒序列出：

Used for emergency messages, usually those that precede a crash.

用于突發(fā)性事件的消息,，通常在系統(tǒng)崩潰之前報告此類消息,。

A situation requiring immediate action.

在需要立即操作的情況下使用此消息。

Critical conditions, often related to serious hardware or software failures.

用于臨界條件下,，通常遇到嚴(yán)重的硬軟件錯誤時使用此消息,。

Used to report error conditions; device drivers often use KERN_ERR to report hardware difficulties.

用于報告錯誤條件；設(shè)備驅(qū)動經(jīng)常使用KERN_ERR報告硬件難題,。

Warnings about problematic situations that do not, in themselves, create serious problems with the system.

是關(guān)于問題狀況的警告,，一般這些狀況不會引起系統(tǒng)的嚴(yán)重問題。

KERN_NOTICE

Situations that are normal, but still worthy of note. A number of security-related conditions are reported at this level.

該級別較為普通,，但仍然值得注意,。許多與安全性相關(guān)的情況會在這個級別被報告。

Informational messages. Many drivers print information about the hardware they find at startup time at this level.

信息消息,。許多驅(qū)動程序在啟動時刻用它來輸出獲得的硬件信息,。

KERN_DEBUG

Used for debugging messages.

用于輸出調(diào)試信息

        Each string (in the macro expansion) represents an integer in angle brackets. Integers range from 0 to 7, with smaller values representing higher priorities.

        每一個字符串（由宏擴展而成）表示了尖括號內(nèi)的一個整數(shù)。數(shù)值范圍從0到7,，數(shù)值越小,，優(yōu)先級越高。

        A printk statement with no specified priority defaults to DEFAULT_MESSAGE_LOGLEVEL, specified in kernel/printk.c as an integer. In the 2.6.10 kernel, DEFAULT_MESSAGE_LOGLEVEL is KERN_WARNING, but that has been known to change in the past.

         一個printk的缺省優(yōu)先級是DEFAULT_MESSAGE_LOGLEVEL,它是一個在kernel/printk.c文件中指定的整數(shù),。在 2.6.10內(nèi)核中,，DEFAULT_MESSAGE_LOGLEVEL相當(dāng)于KERN_WARNING，但據(jù)說在早期版本中這是兩個不同的優(yōu)先級。

        Based on the loglevel, the kernel may print the message to the current console, be it a text-mode terminal, a serial port, or a parallel printer. If the priority is less than the integer variable console_loglevel, the message is delivered to the console one line at a time (nothing is sent unless a trailing newline is provided). If both klogd and syslogd are running on the system, kernel messages are appended to /var/log/messages (or otherwise treated depending on your syslogd configuration), independent of console_loglevel. If klogd is not running, the message won't reach user space unless you read /proc/kmsg (which is often most easily done with the dmesg command). When using klogd, you should remember that it doesn't save consecutive identical lines; it only saves the first such line and, at a later time, the number of repetitions it received.

        基于這些記錄級,，內(nèi)核可以把消息輸出到當(dāng)前的控制臺,，也可以是一個文本模式的終端，一個串口,，或是一個并口打印機,。如果優(yōu)先級小于整形變量 console_loglevel，那么一次將會只發(fā)送一行消息到控制臺中（除非遇到一個換行符,，否則將什么都不會發(fā)送）,。如果系統(tǒng)中運行了klogd和syslogd進(jìn)程，那么內(nèi)核消息就會被完整地添加到/var/log/messages文件中（或者根據(jù)你的syslogd進(jìn)程的配置狀況進(jìn)行發(fā)送）而忽略console_loglevel,，如果klogd沒有運行,，那么消息將不會到達(dá)用戶空間，除非你對/proc/kmsg文件讀?。▽嶋H上這項工作已被較早的dmesg命令完成）,。當(dāng)使用klogd時，你應(yīng)該記住它不會保留重復(fù)的消息行,；對于它接收到的重復(fù)消息，它只會保留第一條,。

        The variable console_loglevel is initialized to DEFAULT_CONSOLE_LOGLEVEL and can be modified through the sys_syslog system call. One way to change it is by specifying the -c switch when invoking klogd, as specified in the klogd manpage. Note that to change the current value, you must first kill klogd and then restart it with the -c option. Alternatively, you can write a program to change the console loglevel. You'll find a version of such a program in misc-progs/setlevel.c in the source files provided on O'Reilly's FTP site. The new level is specified as an integer value between 1 and 8, inclusive. If it is set to 1, only messages of level 0 (KERN_EMERG) reach the console; if it is set to 8, all messages, including debugging ones, are displayed.

        被初始化為DEFAULT_CONSOLE_LOGLEVEL的console_loglevel變量可以通過sys_syslog系統(tǒng)調(diào)用修改,。改變它內(nèi)容的一個方法就是在調(diào)用klogd時使用-c選項，具體參考klogd的man幫助,。請注意,，為了改變當(dāng)前的數(shù)值，你必須首先結(jié)束klogd進(jìn)程,，并且用-c選項重新啟動它,。另一種方法是，你可以寫一個應(yīng)用程序來改變控制臺的記錄等級,。你可以在O'Reilly的FTP站點提供的源代碼文件中找到/misc-progs/setlevel.c文件,，其中就有一個這樣的程序。新記錄級為一個1到8的整數(shù),。如果設(shè)置為1,，那么只有優(yōu)先級為0(KERN_EMERG)的消息才可以到達(dá)控制臺；如果等級設(shè)置為8,，那么包括調(diào)試信息在內(nèi)的所有消息都會被顯示,。

        It is also possible to read and modify the console loglevel using the text file /proc/sys/kernel/printk. The file hosts four integer values: the current loglevel, the default level for messages that lack an explicit loglevel, the minimum allowed loglevel, and the boot-time default loglevel. Writing a single value to this file changes the current loglevel to that value; thus, for example, you can cause all kernel messages to appear at the console by simply entering:

        也可以通過文本文件/proc/sys/kernel/printk來獲取和更改控制臺的記錄等級。這個文件中存儲著四個整型數(shù)值：當(dāng)前記錄級,，缺省記錄級,，最低記錄級和啟動時刻的缺省記錄級。可以向該文件寫入一個單一數(shù)值來改變當(dāng)前記錄級,；例如,，如果你可以想所有的內(nèi)核消息都可以在控制臺中顯示，可以使用以下命令：

# echo 8 > /proc/sys/kernel/printk

        It should now be apparent why the hello.c sample had the KERN_ALERT; markers; they are there to make sure that the messages appear on the console.

        現(xiàn)在你應(yīng)該明白在hello.c示例代碼中為什么會有KERN_ALERT;標(biāo)識了吧,；這樣做可以保證消息順利地輸出到控制臺中
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