116 lines
4.9 KiB
ReStructuredText
116 lines
4.9 KiB
ReStructuredText
.. SPDX-License-Identifier: GPL-2.0
|
|
|
|
===========================
|
|
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 :ref:`How to get
|
|
printk format specifiers right <printk-specifiers>`.
|
|
|
|
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 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
|
|
==================
|
|
|
|
.. kernel-doc:: kernel/printk/printk.c
|
|
:functions: printk
|
|
|
|
.. kernel-doc:: include/linux/printk.h
|
|
:functions: pr_emerg pr_alert pr_crit pr_err pr_warn pr_notice pr_info
|
|
pr_fmt pr_debug pr_devel pr_cont
|