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奔Linux内核入门篇_chapter8.5.3_后台守护进程

2021-01-24 10:01:30 阅读：349 来源： 互联网

标签：kernel chapter8.5 log 0.000000 int 入门篇 tm buffer Linux

奔跑吧Linux内核入门篇第8章进程管理 8.5.3 实验3: 后台守护进程

文章目录

实验详述
实验解析
程序源码
编译
验证执行
kernel log buffer

实验详述

实验目的
通过本实验了解和熟悉Linux是如何创建和使用后台守护进程的.
实验步骤
(1) 写一个用户程序, 创建一个守护进程.
(2) 该守护进程每隔5秒去查看当前内核的日志中是否有Oops错误.

实验解析

本实验完全是个用户态的程序, 需要知道

如何创建一个守护进程? (跑在后台, 无控制终端, 无法对前台输出信息)
如何去读内核的日志?

问题1可以用glibc提供的daemon()函数

man 3 daemon

NAME
       daemon - run in the background

SYNOPSIS
       #include <unistd.h>

       int daemon(int nochdir, int noclose);

   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

       daemon():
           Since glibc 2.21:
               _DEFAULT_SOURCE
           In glibc 2.19 and 2.20:
               _DEFAULT_SOURCE || (_XOPEN_SOURCE && _XOPEN_SOURCE < 500)
           Up to and including glibc 2.19:
               _BSD_SOURCE || (_XOPEN_SOURCE && _XOPEN_SOURCE < 500)

问题2 可以用glibc提供的klogctl()函数

man 3 klogctl

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

SYNOPSIS
       int syslog(int type, char *bufp, int len);
                       /* No wrapper provided in glibc */

       /* The glibc interface */
       #include <sys/klog.h>

       int klogctl(int type, char *bufp, int len);

本实验相对简单,直接看代码.

程序源码

文件名: my_daemon.c
这个直接参考了笨叔的答案
(runninglinuxkernel_4.0/rlk_lab/rlk_basic/chapter_8/lab3_daemon/daemon_test1.c)


/* create a daemon which checks if there are opps errors in the kernel log
 * every 5 second
 */

#include <stdio.h>
#include <unistd.h>
#include <errno.h>
#include <stdlib.h>
#include <time.h>
#include <fcntl.h>
#include <string.h>
#include <sys/stat.h>
#include <sys/klog.h>

#define FALLBACK_KLOG_BUF_SHIFT 17  /* CONFIG_LOG_BUF_SHIFT in kernel */
#define FALLBACK_KLOG_BUF_LEN   (1 << FALLBACK_KLOG_BUF_SHIFT)

#define KLOG_CLOSE         0
#define KLOG_OPEN          1
#define KLOG_READ          2
#define KLOG_READ_ALL      3
#define KLOG_READ_CLEAR    4
#define KLOG_CLEAR         5
#define KLOG_CONSOLE_OFF   6
#define KLOG_CONSOLE_ON    7
#define KLOG_CONSOLE_LEVEL 8
#define KLOG_SIZE_UNREAD   9
#define KLOG_SIZE_BUFFER   10

/* we use 'Linux version' string instead of Oops in this lab */
//#define OOPS_LOG  "Oops"
#define OOPS_LOG  "Linux version"

int save_kernel_log(char *buffer)
{
        char path[128];
        time_t t;
        struct tm *tm;
        int fd;

        /* time(2): returns the time as the number of seconds
         * since the Epoch, 1970-01-01 00:00:00 +0000 (UTC). */
        t = time(0);
        /* transform date and time to broken-down time or ASCII */
        tm = localtime(&t);

        /* the structure tm, which is defined in <time.h> as follows:
         * struct tm {
         *      int tm_sec;    // Seconds (0-60)
         *      int tm_min;    // Minutes (0-59)
         *      int tm_hour;   // Hours (0-23)
         *      int tm_mday;   // Day of the month (1-31)
         *      int tm_mon;    // Month (0-11)
         *      int tm_year;   // Year - 1900
         *      int tm_wday;   // Day of the week (0-6, Sunday = 0)
         *      int tm_yday;   // Day in the year (0-365, 1 Jan = 0)
         *      int tm_isdst;  // Daylight saving time
         * };
         */
        snprintf(path, 128, "/mnt/%d.%d.%d.%d.%d.%d.log", tm->tm_year+1900,
                        tm->tm_mon+1, tm->tm_mday, tm->tm_hour,
                        tm->tm_min, tm->tm_sec);
        printf("%s\n", path);

        fd = open(path, O_WRONLY|O_CREAT, 0644);
        if(fd == -1) {
                printf("open error\n");
                return -1;
        }
        write(fd, buffer, strlen(buffer));
        close(fd);

        return 0;
}

int check_kernel_log()
{
        char *buffer;
        char *p;
        ssize_t klog_size;
        int ret = -1;
        int size;

        printf("start kernel log\n");

        /* klogctl(3): read and/or clear kernel message
         * ring buffer; set console_loglevel.
         * Here returns the total size of the kernel log buffer.
         * */
        klog_size = klogctl(KLOG_SIZE_BUFFER, 0, 0);
        if (klog_size <= 0) {
                klog_size = FALLBACK_KLOG_BUF_LEN;
        }

        printf("kernel log size: %d\n", klog_size);

        buffer = malloc(klog_size + 1);
        if (!buffer)
                return -1;

        /* Read all messages remaining in the ring buffer,
         * placing them in the buffer pointed to by buffer.
         * The  call reads  the  last klog_size bytes from
         * the log buffer (nondestructively) */
        size = klogctl(KLOG_READ_ALL, buffer, klog_size);
        if (size < 0) {
                printf("klogctl read error\n");
                goto done;
        }

        buffer[size] = '\0';

        /* strstr(3), finds the first occurrence of the substring */
        /* check if oops in klog */
        p = strstr(buffer,OOPS_LOG);
        if (p) {
                printf("we found '%s' on kernel log\n", OOPS_LOG);
                save_kernel_log(buffer);
                ret = 0;
        }
done:
        free(buffer);
        return ret;
}


int main(void)
{
        /* The daemon(3) function is for programs wishing to
         * detach themselves from the controlling terminal
         * and run in the background as system daemons. */
        if (daemon(0, 0) == -1) {
                printf("failed to daemon(): %s(%d)\n", strerror(errno), errno);
                return -1;
        }

        while (1) {
                check_kernel_log();
                sleep(5);
        }

        return 0;
}

编译

这里直接编译就行

arm-linux-gnueabi-gcc my_daemon.c -o my_daemon.exe --static

验证执行

/mnt #
/mnt # pwd
/mnt
/mnt # find . -name "*.log"
/mnt # ./my_daemon.exe
/mnt #
/mnt # find . -name "*.log"
./2021.1.24.1.45.50.log
./2021.1.24.1.45.55.log
./2021.1.24.1.46.0.log
/mnt #
/mnt # pgrep my_daemon
792
/mnt # kill -9 792
/mnt #
/mnt #
/mnt # head 2021.1.24.1.45.50.log
<6>[    0.000000] Booting Linux on physical CPU 0x0
<6>[    0.000000] Initializing cgroup subsys cpuset
<5>[    0.000000] Linux version 4.0.0+ (gewkiff@ukylin) (gcc version 5.5.0 20171010 (Ubuntu/Linaro 5.5.0-12ubuntu1) ) #2 SMP Sun Sep 20 06:37:58 CST 2020
<6>[    0.000000] CPU: ARMv7 Processor [410fc090] revision 0 (ARMv7), cr=10c5387d
<6>[    0.000000] CPU: PIPT / VIPT nonaliasing data cache, VIPT nonaliasing instruction cache
<6>[    0.000000] Machine model: V2P-CA9
<6>[    0.000000] Memory policy: Data cache writealloc
<7>[    0.000000] On node 0 totalpages: 25600
<7>[    0.000000] free_area_init_node: node 0, pgdat c10e9e40, node_mem_map c6332000
<7>[    0.000000]   Normal zone: 200 pages used for memmap
/mnt #
/mnt #
/mnt # head 2021.1.24.1.45.55.log
<6>[    0.000000] Booting Linux on physical CPU 0x0
<6>[    0.000000] Initializing cgroup subsys cpuset
<5>[    0.000000] Linux version 4.0.0+ (gewkiff@ukylin) (gcc version 5.5.0 20171010 (Ubuntu/Linaro 5.5.0-12ubuntu1) ) #2 SMP Sun Sep 20 06:37:58 CST 2020
<6>[    0.000000] CPU: ARMv7 Processor [410fc090] revision 0 (ARMv7), cr=10c5387d
<6>[    0.000000] CPU: PIPT / VIPT nonaliasing data cache, VIPT nonaliasing instruction cache
<6>[    0.000000] Machine model: V2P-CA9
<6>[    0.000000] Memory policy: Data cache writealloc
<7>[    0.000000] On node 0 totalpages: 25600
<7>[    0.000000] free_area_init_node: node 0, pgdat c10e9e40, node_mem_map c6332000
<7>[    0.000000]   Normal zone: 200 pages used for memmap
/mnt #

kernel log buffer

这是在man 3 klogctl 中看到的关于kernel log buffer的简短介绍

The kernel log buffer
The kernel has a cyclic buffer of length LOG_BUF_LEN in which
messages given as arguments to the kernel function printk()
are stored (regardless of their log level).
In early kernels, LOG_BUF_LEN had the value 4096;
from kernel 1.3.54, it was 8192; from kernel 2.1.113, it was 16384;
since kernel 2.4.23/2.6, the value is a kernel configuration option
(CONFIG_LOG_BUF_SHIFT, default value dependent on the architecture).
Since Linux 2.6.6, the size can be queried with command type 10 (see below).

标签：kernel,chapter8.5,log,0.000000,int,入门篇,tm,buffer,Linux
来源： https://blog.csdn.net/weixin_42992444/article/details/113068927

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