应用程序打印kernel log

需求：

有时候需要监控应用程序关键动作，因此需要将应用程序的log写入到kernel log的缓冲区去。另外，有时候为了查看应用程序和kernel的时序，需要结合应用程序打印的log和kernel的log来查看前后关系。关于打印输出，https://elinux.org/Debugging_by_printing 做了详细的介绍。

kernel log输出介绍

1、dmesg命令官方说明的用法是：

dmesg is used to examine or control the kernel ring buffer. The default action is to read all messages from kernel ring buffer.

-n参数可以设置loglevel，详见：

-n, --console-level level
Set  the  level  at  which  logging of messages is done to the console. 
The level is a level number or abbreviation of the level name. 
For all supported levels see dmesg --help output.
For example, -n 1 or -n alert prevents all messages, except emergency (panic) messages,
from appearing on the console.  All levels  of  messages  are  still  written  to /proc/kmsg, 
so  syslogd(8) can still be used to control exactly where kernel messages appear.
When the -n option is used, dmesg will not print or clear the kernel ring buffer.

在include/linux/kern_levels.h文件中定义了内核loglevel：

#ifndef __KERN_LEVELS_H__
#define __KERN_LEVELS_H__ 

#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   KERN_SOH "c"
   
/* integer equivalents of KERN_<LEVEL> */
#define LOGLEVEL_SCHED      -2  /* Deferred messages from sched code
                     * are set to this special level */
#define LOGLEVEL_DEFAULT    -1  /* default (or last) loglevel */
#define LOGLEVEL_EMERG      0   /* system is unusable */
#define LOGLEVEL_ALERT      1   /* action must be taken immediately */
#define LOGLEVEL_CRIT       2   /* critical conditions */
#define LOGLEVEL_ERR        3   /* error conditions */
#define LOGLEVEL_WARNING    4   /* warning conditions */
#define LOGLEVEL_NOTICE     5   /* normal but significant condition */
#define LOGLEVEL_INFO       6   /* informational */
#define LOGLEVEL_DEBUG      7   /* debug-level messages */

#endif

在终端中可以查看当前设置的loglevel：

1
2
3

# cat /proc/sys/kernel/printk
	7       4       1       7
current	default	minimum	boot-time-default

关于dmesg的具体实现，可以在androidxref.com中查找或者查看busybox、toybox中关于dmesg命令的实现，最终都是调用klogctl()函数获取到kernel message。
查看klogctl函数的帮助文档man klogctl得到关于这个函数的介绍和用法：

syslog, klogctl - read and/or clear kernel message ring buffer; set console_loglevel

2、cat /proc/kmsg打印kernel log

The kernel log buffer is accessible for reading from userspace by /proc/kmsg. 
/proc/kmsg behaves more or less like a FIFO and blocks until new messages appear.
Please note - reading from /proc/kmsg consumes the messages 
in the ring buffer so they may not be available for other programs. 
It is usually a good idea to let klogd or syslog do this job and read the content of the buffer via dmesg.

3、/dev/kmsg设备节点
应用程序可以通过/dev/kmsg节点将打印信息写入到kernel log中。关于这个节点详见官方的介绍：https://www.kernel.org/doc/Documentation/ABI/testing/dev-kmsg
因此，下面的程序就是将log输出到kernel log中。

#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#include <fcntl.h>
#include <unistd.h>
#include <sys/mman.h>
#include <string.h>
#include <errno.h>
#include <stdarg.h>

#define PRINTFK_LOG_BUF_MAX 1024

enum LogLevel
{
    LOGLEVEL_EMERG = 0,     /* system is unusable */
    LOGLEVEL_ALERT,     /* action must be taken immediately */
    LOGLEVEL_CRIT,      /* critical conditions */
    LOGLEVEL_ERR,       /* error conditions */
    LOGLEVEL_WARNING,   /* warning conditions */
    LOGLEVEL_NOTICE,    /* normal but significant condition */
    LOGLEVEL_INFO,      /* informational */
    LOGLEVEL_DEBUG,     /* debug-level messages */
};

void printfk(enum LogLevel level, const char *fmt, ...)
{
    va_list args;
    int log_fd;
    int ret;
    char buf[PRINTFK_LOG_BUF_MAX] = {0};

    if ((level < LOGLEVEL_EMERG) || (level > LOGLEVEL_DEBUG) || (NULL == fmt)) {
        return;
    }
    
    buf[0] = '<';
    buf[1] = '0' + level;
    buf[2] = '>';
    
    va_start(args, fmt);
    vsnprintf(&buf[3], (PRINTFK_LOG_BUF_MAX - 3), fmt, args);
    log_fd = open("/dev/kmsg", O_WRONLY | O_CLOEXEC);
    if (-1 != log_fd) {
        ret = write(log_fd, buf, sizeof(buf));
        close(log_fd);
    }
    va_end(args);
}

int main(void)
{
    int a = 10;
    char *s = "abcdefgh";
    printfk(LOGLEVEL_DEBUG, "[xxx] bbbbbbbbb");
    printfk(LOGLEVEL_DEBUG, "%d, %s", a, s);
}

参考资料

https://github.com/karelzak/util-linux/blob/master/sys-utils/dmesg.c
https://github.com/brgl/busybox/blob/master/util-linux/dmesg.c