linux内核打印级别

　　1.printk()是一个内核的一个记录日志的机制，经常用来记录信息或者警告。printk可以指定输出日志的优先级，在include/linux/kern_levels.h中有相应的宏定义　

 #define KERN_SOH    "\001"      /* ASCII Start Of Header */
 #define KERN_SOH_ASCII  '\001'  

 #define KERN_EMERG  KERN_SOH "0"    /* system is unusable */
 #define KERN_ALERT  KERN_SOH "1"    /* action must be taken immediately */
 #define KERN_CRIT   KERN_SOH "2"    /* critical conditions */
 #define KERN_ERR    KERN_SOH "3"    /* error conditions */
 #define KERN_WARNING    KERN_SOH "4"    /* warning conditions */
 #define KERN_NOTICE KERN_SOH "5"    /* normal but significant condition */
 #define KERN_INFO   KERN_SOH "6"    /* informational */
 #define KERN_DEBUG  KERN_SOH "7"    /* debug-level messages */  

 #define KERN_DEFAULT    KERN_SOH "d"    /* the default kernel loglevel */  

 /*
  * Annotation for a "continued" line of log printout (only done after a
  * line that had no enclosing \n). Only to be used by core/arch code
  * during early bootup (a continued line is not SMP-safe otherwise).
  */
 #define KERN_CONT   ""  

　　如果不指定优先级，这printk就使用默认的优先级，DEFAULT_MESSAGE_LOGLEVEL 在linux-3.6.10/kernel/printk.c中有定义

 /* printk's without a loglevel use this.. */
 #define DEFAULT_MESSAGE_LOGLEVEL CONFIG_DEFAULT_MESSAGE_LOGLEVEL  

 /* We show everything that is MORE important than this.. */
 #define MINIMUM_CONSOLE_LOGLEVEL 1 /* Minimum loglevel we let people use */
 #define DEFAULT_CONSOLE_LOGLEVEL 7 /* anything MORE serious than KERN_DEBUG */

如果未对CONFIG_DEFAULT_MESSAGE_LOGLEVEL进行配置，则默认值是4

dmesg:分析核心产生的讯息

dmesg |more                          //输出所有的核心开机时的信息。

截取命令：cut grep

　　grep：分析一行讯息，若当中有我们所需要的信息，就将该行拿出来

1、查看当前控制台的打印级别
  cat /proc/sys/kernel/printk 
4    4    1    7
其中第一个“4”表示内核打印函数printk的打印级别，只有级别比他高的信息才能在控制台上打印出来，既 0－3级别的信息

2、修改打印
echo "新的打印级别  4    1    7" >/proc/sys/kernel/printk

3、不够打印级别的信息会被写到日志中可通过dmesg 命令来查看

4、printk的打印级别

 #define KERN_EMERG        "<0>" /* system is unusable */
 #define KERN_ALERT         "<1>" /* action must be taken immediately */
 #define KERN_CRIT            "<2>" /* critical conditions */
 #define KERN_ERR             "<3>" /* error conditions */
 #define KERN_WARNING   "<4>" /* warning conditions */
 #define KERN_NOTICE       "<5>" /* normal but significant condition */
 #define KERN_INFO            "<6>" /* informational */
 #define KERN_DEBUG       "<7>" /* debug-level messages */

5、printk函数的使用

      printk(打印级别  “要打印的信息”)

打印级别  既上面定义的几个宏