dmesg and logs
- C course
- Expert
- Lesson 5/47
- 5 min
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.
Practice 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).
Starter code
#include <stdio.h>
// Define print_level function
// Call it for KERN_EMERG, KERN_ALERT, KERN_ERR, KERN_INFO
int main(void) {
return 0;
}Sample 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;
}Expected output
[0] KERN_EMERG [1] KERN_ALERT [3] KERN_ERR [6] KERN_INFO
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Which command shows kernel logs?
- dmesg Correct answer
- lsmod
- ps
- top
Explanation: dmesg reads the kernel message buffer.
2 Match log levels
| Term | Meaning |
|---|---|
KERN_EMERG |
System crash |
KERN_ALERT |
Act immediately |
KERN_ERR |
Error condition |
KERN_INFO |
Informational |
3 Complete the printk level prefix
printk(KERN_____ "driver loaded
");printk(KERN_INFO "driver loaded
");Explanation: KERN_INFO tags informational messages.