function::printk - Linux

Overview

function::printk is a powerful debugging tool that allows developers to print kernel messages from user space. It provides a direct interface to the kernel’s logging mechanism, making it an invaluable aid for troubleshooting and monitoring system behavior.

Syntax

function::printk [<level>] <message>

Parameters:

  • <level>: Optional kernel message log level (default: DEFAULT).
  • <message>: The message to be printed to the kernel log.

Supported Log Levels:

  • DEFAULT
  • EMERGENCY
  • ALERT
  • CRITICAL
  • ERROR
  • WARNING
  • NOTICE
  • INFO
  • DEBUG

Options/Flags

  • -c: Clears the kernel log before printing the message.
  • -n: Suppresses the inclusion of a newline character at the end of the message.
  • -i: Prints the message as an integer.
  • -s: Prints the message as a string.
  • -u: Prints the message as a user-defined format.

Examples

Simple Usage:

function::printk ERROR "Error encountered in line 10"

Using a Custom Log Level:

function::printk CRITICAL "Critical error detected"

Clearing the Kernel Log:

function::printk -c INFO "Kernel log has been cleared"

Suppressing the Newline Character:

function::printk -n WARNING "Warning: Out of memory"

Common Issues

  • Message not appearing in kernel log: Ensure that the log level specified is supported and corresponds to the current kernel logging configuration.
  • Incorrect formatting: Check the format specifier used when printing custom messages (-u) to match the intended message type.
  • Access denied: Verify that the user has sufficient privileges to print messages to the kernel log.

Integration

Capturing Kernel Output in a File:

function::printk -c INFO "Kernel messages" | tee kernel-output.log

Monitoring Kernel Logs Continuously:

tail -f /var/log/kern.log | function::printk -n ERROR

Related Commands

  • dmesg: Displays kernel messages in user space.
  • klogd: The kernel log daemon that manages kernel message logging.
  • /var/log/kern.log: The system kernel log file.