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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.

Avoiding lockups from excessive printk() use¶

Note

This section is relevant only for legacy console drivers (those not using the nbcon API) and !PREEMPT_RT kernels. Once all console drivers are updated to nbcon, this documentation can be removed.

Using printk() in hot paths (such as interrupt handlers, timer callbacks, or high-frequency network receive routines) with legacy consoles (e.g., console=ttyS0) may cause lockups. Legacy consoles synchronously acquire console_sem and block while flushing messages, potentially disabling interrupts long enough to trigger hard or soft lockup detectors.

To avoid this:

Use rate-limited variants (e.g., pr_*_ratelimited()) or one-time macros (e.g., pr_*_once()) to reduce message frequency.
Assign lower log levels (e.g., KERN_DEBUG) to non-essential messages and filter console output via console_loglevel.
Use printk_deferred() to log messages immediately to the ringbuffer and defer console printing. This is a workaround for legacy consoles.
Port legacy console drivers to the non-blocking nbcon API (indicated by CON_NBCON). This is the preferred solution, as nbcon consoles offload message printing to a dedicated kernel thread.

For temporary debugging, trace_printk() can be used, but it must not appear in mainline code. See Documentation/trace/debugging.rst for more information.

If more permanent output is needed in a hot path, trace events can be used. See Documentation/trace/events.rst and samples/trace_events/trace-events-sample.[ch].

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.

printk¶

printk (fmt, ...)

print a kernel message

Parameters

fmt: format string
...: variable arguments

Description

This is printk(). 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 the output and call the console drivers. If we fail to get the semaphore, we place the output into the log buffer and return. The current holder of the console_sem will notice the new output in console_unlock(); and will send it to the consoles before releasing the lock.

One effect of this deferred printing is that code which calls printk() and then changes console_loglevel may break. This is because console_loglevel is inspected when the actual printing occurs.