Message logging with printk

printk() is one of the most widely known functions in the Linux kernel. It’s the standard tool we have for printing messages and usually the most basic way of tracing and debugging. If you’re familiar with printf(3) you can tell printk() 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 the exact same specification. It has some extensions and a few limitations (no %n or floating point conversion specifiers). See How to get printk format specifiers right.

All printk() messages are printed to the kernel log buffer, which is a ring buffer exported to userspace through /dev/kmsg. The usual way to read it is using dmesg.

printk() is typically used like this:

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

where KERN_INFO is the log level (note that it’s concatenated to the format string, 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 whether to show the message immediately (printing it to the current console) depending on its log level and the current console_loglevel (a kernel variable). If the message priority is higher (lower log level value) than the console_loglevel the message will be printed to the console.

If the log level is omitted, the message is printed with KERN_DEFAULT level.

You can check the current console_loglevel with:

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

The result shows the current, default, minimum and boot-time-default log levels.

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, using dmesg:

# dmesg -n 5

sets the console_loglevel to print KERN_WARNING (4) or more severe messages to console. See dmesg(1) for more information.

As an alternative to printk() you can use the pr_*() aliases for logging. This family of macros embed the log level in the macro names. For example:

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

prints a KERN_INFO message.

Besides being more concise than the equivalent printk() calls, they can use a common definition for the format string through the pr_fmt() macro. For instance, defining this at the top of a source file (before any #include directive):

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

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

For debugging purposes there are also two conditionally-compiled macros: pr_debug() and pr_devel(), which are compiled-out unless DEBUG (or also CONFIG_DYNAMIC_DEBUG in the case of pr_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 common string. 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 module name.

pr_emerg

pr_emerg (fmt, ...)

Print an emergency-level message

Parameters

fmt

format string

...

arguments for the format string

Description

This macro expands to a printk with KERN_EMERG loglevel. It uses pr_fmt() to generate the format string.

pr_alert

pr_alert (fmt, ...)

Print an alert-level message

Parameters

fmt

format string

...

arguments for the format string

Description

This macro expands to a printk with KERN_ALERT loglevel. It uses pr_fmt() to generate the format string.

pr_crit

pr_crit (fmt, ...)

Print a critical-level message

Parameters

fmt

format string

...

arguments for the format string

Description

This macro expands to a printk with KERN_CRIT loglevel. It uses pr_fmt() to generate the format string.

pr_err

pr_err (fmt, ...)

Print an error-level message

Parameters

fmt

format string

...

arguments for the format string

Description

This macro expands to a printk with KERN_ERR loglevel. It uses pr_fmt() to generate the format string.

pr_warn

pr_warn (fmt, ...)

Print a warning-level message

Parameters

fmt

format string

...

arguments for the format string

Description

This macro expands to a printk with KERN_WARNING loglevel. It uses pr_fmt() to generate the format string.

pr_notice

pr_notice (fmt, ...)

Print a notice-level message

Parameters

fmt

format string

...

arguments for the format string

Description

This macro expands to a printk with KERN_NOTICE loglevel. It uses pr_fmt() to generate the format string.

pr_info

pr_info (fmt, ...)

Print an info-level message

Parameters

fmt

format string

...

arguments for the format string

Description

This macro expands to a printk with KERN_INFO loglevel. It uses pr_fmt() to generate the format string.

pr_cont

pr_cont (fmt, ...)

Continues a previous log message in the same line.

Parameters

fmt

format string

...

arguments for the format string

Description

This macro expands to a printk with KERN_CONT loglevel. It should only be used when continuing a log message with no newline (‘n’) enclosed. Otherwise it defaults back to KERN_DEFAULT loglevel.

pr_devel

pr_devel (fmt, ...)

Print a debug-level message conditionally

Parameters

fmt

format string

...

arguments for the format string

Description

This macro expands to a printk with KERN_DEBUG loglevel if DEBUG is defined. Otherwise it does nothing.

It uses pr_fmt() to generate the format string.

pr_debug

pr_debug (fmt, ...)

Print a debug-level message conditionally

Parameters

fmt

format string

...

arguments for the format string

Description

This macro expands to dynamic_pr_debug() if CONFIG_DYNAMIC_DEBUG is set. Otherwise, if DEBUG is defined, it’s equivalent to a printk with KERN_DEBUG loglevel. If DEBUG is not defined it does nothing.

It uses pr_fmt() to generate the format string (dynamic_pr_debug() uses pr_fmt() internally).