Message logging with printk¶

printk() is one of the most widely known functions in the Linux kernel. It’s thestandard tool we have for printing messages and usually the most basic way oftracing and debugging. If you’re familiar with printf(3) you can tellprintk()is based on it, although it has some functional differences:

printk() messages can specify a log level.
the format string, while largely compatible with C99, doesn’t follow theexact same specification. It has some extensions and a few limitations(no%n or floating point conversion specifiers). SeeHow to getprintk format specifiers right.

Allprintk() messages are printed to the kernel log buffer, which is a ringbuffer exported to userspace through /dev/kmsg. The usual way to read it isusingdmesg.

printk() is typically used like this:

printk(KERN_INFO "Message: %s\n", arg);

whereKERN_INFO is the log level (note that it’s concatenated to the formatstring, the log level is not a separate argument). The available log levels are:

Name	String	Alias function
KERN_EMERG	“0”	`pr_emerg()`
KERN_ALERT	“1”	`pr_alert()`
KERN_CRIT	“2”	`pr_crit()`
KERN_ERR	“3”	`pr_err()`
KERN_WARNING	“4”	`pr_warn()`
KERN_NOTICE	“5”	`pr_notice()`
KERN_INFO	“6”	`pr_info()`
KERN_DEBUG	“7”	`pr_debug()` and`pr_devel()` if DEBUG is defined
KERN_DEFAULT	“”
KERN_CONT	“c”	`pr_cont()`

The log level specifies the importance of a message. The kernel decides whetherto show the message immediately (printing it to the current console) dependingon its log level and the currentconsole_loglevel (a kernel variable). If themessage priority is higher (lower log level value) than theconsole_loglevelthe message will be printed to the console.

If the log level is omitted, the message is printed withKERN_DEFAULTlevel.

You can check the currentconsole_loglevel with:

$ cat /proc/sys/kernel/printk4        4        1        7

The result shows thecurrent,default,minimum andboot-time-default loglevels.

To change the current console_loglevel simply write the desired level to/proc/sys/kernel/printk. For example, to print all messages to the console:

# echo 8 > /proc/sys/kernel/printk

Another way, usingdmesg:

# dmesg -n 5

sets the console_loglevel to print KERN_WARNING (4) or more severe messages toconsole. Seedmesg(1) for more information.

As an alternative toprintk() you can use thepr_*() aliases forlogging. This family of macros embed the log level in the macro names. Forexample:

pr_info("Info message no. %d\n", msg_num);

prints aKERN_INFO message.

Besides being more concise than the equivalentprintk() calls, they can use acommon definition for the format string through thepr_fmt() macro. Forinstance, defining this at the top of a source file (before any#includedirective):

#define pr_fmt(fmt) "%s:%s: " fmt, KBUILD_MODNAME, __func__

would prefix every pr_*() message in that file with the module and function namethat originated the message.

For debugging purposes there are also two conditionally-compiled macros:pr_debug() andpr_devel(), which are compiled-out unlessDEBUG (oralsoCONFIG_DYNAMIC_DEBUG in the case ofpr_debug()) is defined.

Function reference¶

pr_fmt¶

pr_fmt(fmt)

used by the pr_*() macros to generate the printk format string

Parameters

fmt: format string passed from a pr_*() macro

Description

This macro can be used to generate a unified format string for pr_*()macros. A common use is to prefix all pr_*() messages in a file with a commonstring. For example, defining this at the top of a source file:

#define pr_fmt(fmt) KBUILD_MODNAME “: “ fmt

would prefix all pr_info, pr_emerg... messages in the file with the modulename.

printk¶

printk(fmt,...)

print a kernel message

Parameters

fmt: format string
...: variable arguments

Description

This isprintk(). It can be called from any context. We want it to work.

If printk indexing is enabled,_printk() is called from printk_index_wrap.Otherwise, printk is simply #defined to _printk.

We try to grab the console_lock. If we succeed, it’s easy - we log theoutput and call the console drivers. If we fail to get the semaphore, weplace the output into the log buffer and return. The current holder ofthe console_sem will notice the new output inconsole_unlock(); and willsend it to the consoles before releasing the lock.

One effect of this deferred printing is that code which callsprintk() andthen changes console_loglevel may break. This is because console_loglevelis inspected when the actual printing occurs.

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Message logging with printk¶

Function reference¶