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
1 #define KERN_SOH " 01" /* ASCII Start Of Header */ 2 #define KERN_SOH_ASCII ' 01' 3 4 #define KERN_EMERG KERN_SOH "0" /* system is unusable */ 5 #define KERN_ALERT KERN_SOH "1" /* action must be taken immediately */ 6 #define KERN_CRIT KERN_SOH "2" /* critical conditions */ 7 #define KERN_ERR KERN_SOH "3" /* error conditions */ 8 #define KERN_WARNING KERN_SOH "4" /* warning conditions */ 9 #define KERN_NOTICE KERN_SOH "5" /* normal but significant condition */ 10 #define KERN_INFO KERN_SOH "6" /* informational */ 11 #define KERN_DEBUG KERN_SOH "7" /* debug-level messages */ 12 13 #define KERN_DEFAULT KERN_SOH "d" /* the default kernel loglevel */ 14 15 /* 16 * Annotation for a "continued" line of log printout (only done after a 17 * line that had no enclosing ). Only to be used by core/arch code 18 * during early bootup (a continued line is not SMP-safe otherwise). 19 */ 20 #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
1 /* printk's without a loglevel use this.. */ 2 #define DEFAULT_MESSAGE_LOGLEVEL CONFIG_DEFAULT_MESSAGE_LOGLEVEL 3 4 /* We show everything that is MORE important than this.. */ 5 #define MINIMUM_CONSOLE_LOGLEVEL 1 /* Minimum loglevel we let people use */ 6 #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
1 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
1 #define KERN_EMERG "<0>" /* system is unusable */ 2 #define KERN_ALERT "<1>" /* action must be taken immediately */ 3 #define KERN_CRIT "<2>" /* critical conditions */ 4 #define KERN_ERR "<3>" /* error conditions */ 5 #define KERN_WARNING "<4>" /* warning conditions */ 6 #define KERN_NOTICE "<5>" /* normal but significant condition */ 7 #define KERN_INFO "<6>" /* informational */ 8 #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