dmesg and logs

Learn kernel logs with dmesg.

dmesg and kernel logs

The dmesg command reads the kernel message buffer and prints it. Hardware discovery, driver loading and error messages from boot onward accumulate here.

The kernel writes logs with printk; each message is tagged with a priority level. KERN_EMERG is highest, KERN_DEBUG is lowest. User space keeps similar records through syslog and /var/log files. Logs are critical for both debugging and auditing; dmesg -T shows human-readable timestamps.

Priority levels and filtering

Kernel logs have eight levels: KERN_EMERG (0) is highest, KERN_DEBUG (7) is lowest. The buffer is bounded; new messages push out old ones.

You can filter with dmesg --level=err to see only errors. In printk a prefix sets the level: printk(KERN_ERR "error\n"). Drivers choose levels carefully; noisy logs overflow the buffer. console_loglevel controls which levels reach the console.

Exercise Print log levels

Define void print_level(const char *name, int level) with body printf("[%d] %s\n", level, name);. In main call it with ("KERN_EMERG", 0), ("KERN_ALERT", 1), ("KERN_ERR", 3), ("KERN_INFO", 6).

Show example solution

#include <stdio.h>

void print_level(const char *name, int level) {
    printf("[%d] %s\n", level, name);
}

int main(void) {
    print_level("KERN_EMERG", 0);
    print_level("KERN_ALERT", 1);
    print_level("KERN_ERR", 3);
    print_level("KERN_INFO", 6);
    return 0;
}

Test yourself

Which command shows kernel logs?
- dmesg
- lsmod
- ps
- top
Show answer

dmesg — dmesg reads the kernel message buffer.
Match log levels

KERN_EMERG

System crash
KERN_ALERT

Act immediately
KERN_ERR

Error condition
KERN_INFO

Informational

Complete the printk level prefix

printk(KERN_____ "driver loaded
");

Show answer

printk(KERN_INFO "driver loaded
");

Next lesson: Summary - Kernel basics

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