Section 2.7. Kernel Speak: Listening to Kernel Messages

2.7. Kernel Speak: Listening to Kernel Messages

When your Linux system is up and running, the kernel itself logs messages and provides information about its status throughout its operation. This section gives a few of the most common ways the Linux kernel speaks to an end user.

2.7.1. printk()

One of the most basic kernel messaging systems is the printk() function. The kernel uses printk() as opposed to printf() because the standard C library is not linked to the kernel. printk() uses the same interface as printf() does and displays up to 1,024 characters to the console. The printk() function operates by trying to grab the console semaphore, place the output into the console's log buffer, and then call the console driver to flush the buffer. If printk() cannot grab the console semaphore, it places the output into the log buffer and relies on the process that has the console semaphore to flush the buffer. The log-buffer lock is taken before printk() places any data into the log buffer, so concurrent calls to printk() do not trample each other. If the console semaphore is being held, numerous calls to printk() can occur before the log buffer is flushed. So, do not rely on printk() statements to indicate any program timing.

2.7.2. dmesg

The Linux kernel stores its logs, or messages, in a variety of ways. sysklogd() is a combination of syslogd() and klogd(). (More in-depth information can be found in the man page of these commands, but we can quickly summarize the system.) The Linux kernel sends its messages through klogd(), which tags them with appropriate warning levels, and all levels of messages are placed in /proc/kmsg. dmesg is a command-line tool to display the buffer stored in /proc/kmsg and, optionally, filter the buffer based on the message level.

2.7.3. /var/log/messages

This location on a Linux system is where a majority of logged system messages reside. The syslogd() program reads information in /etc/syslogd.conf for specific locations on where to store received messages. Depending on the entries in syslogd.conf, which can vary among Linux distributions, log messages can be stored in numerous files. However, /var/log/messages is usually the standard location.