113 lines
4.9 KiB
ReStructuredText
113 lines
4.9 KiB
ReStructuredText
.. SPDX-License-Identifier: GPL-2.0
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===========================
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Message logging with printk
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===========================
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printk() is one of the most widely known functions in the Linux kernel. It's the
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standard tool we have for printing messages and usually the most basic way of
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tracing and debugging. If you're familiar with printf(3) you can tell printk()
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is based on it, although it has some functional differences:
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- printk() messages can specify a log level.
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- the format string, while largely compatible with C99, doesn't follow the
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exact same specification. It has some extensions and a few limitations
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(no ``%n`` or floating point conversion specifiers). See :ref:`How to get
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printk format specifiers right <printk-specifiers>`.
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All printk() messages are printed to the kernel log buffer, which is a ring
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buffer exported to userspace through /dev/kmsg. The usual way to read it is
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using ``dmesg``.
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printk() is typically used like this::
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printk(KERN_INFO "Message: %s\n", arg);
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where ``KERN_INFO`` is the log level (note that it's concatenated to the format
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string, the log level is not a separate argument). The available log levels are:
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+----------------+--------+-----------------------------------------------+
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| Name | String | Alias function |
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+================+========+===============================================+
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| KERN_EMERG | "0" | pr_emerg() |
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+----------------+--------+-----------------------------------------------+
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| KERN_ALERT | "1" | pr_alert() |
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+----------------+--------+-----------------------------------------------+
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| KERN_CRIT | "2" | pr_crit() |
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+----------------+--------+-----------------------------------------------+
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| KERN_ERR | "3" | pr_err() |
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+----------------+--------+-----------------------------------------------+
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| KERN_WARNING | "4" | pr_warn() |
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+----------------+--------+-----------------------------------------------+
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| KERN_NOTICE | "5" | pr_notice() |
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+----------------+--------+-----------------------------------------------+
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| KERN_INFO | "6" | pr_info() |
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+----------------+--------+-----------------------------------------------+
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| KERN_DEBUG | "7" | pr_debug() and pr_devel() if DEBUG is defined |
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+----------------+--------+-----------------------------------------------+
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| KERN_DEFAULT | "" | |
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+----------------+--------+-----------------------------------------------+
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| KERN_CONT | "c" | pr_cont() |
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+----------------+--------+-----------------------------------------------+
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The log level specifies the importance of a message. The kernel decides whether
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to show the message immediately (printing it to the current console) depending
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on its log level and the current *console_loglevel* (a kernel variable). If the
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message priority is higher (lower log level value) than the *console_loglevel*
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the message will be printed to the console.
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If the log level is omitted, the message is printed with ``KERN_DEFAULT``
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level.
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You can check the current *console_loglevel* with::
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$ cat /proc/sys/kernel/printk
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4 4 1 7
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The result shows the *current*, *default*, *minimum* and *boot-time-default* log
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levels.
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To change the current console_loglevel simply write the desired level to
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``/proc/sys/kernel/printk``. For example, to print all messages to the console::
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# echo 8 > /proc/sys/kernel/printk
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Another way, using ``dmesg``::
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# dmesg -n 5
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sets the console_loglevel to print KERN_WARNING (4) or more severe messages to
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console. See ``dmesg(1)`` for more information.
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As an alternative to printk() you can use the ``pr_*()`` aliases for
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logging. This family of macros embed the log level in the macro names. For
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example::
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pr_info("Info message no. %d\n", msg_num);
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prints a ``KERN_INFO`` message.
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Besides being more concise than the equivalent printk() calls, they can use a
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common definition for the format string through the pr_fmt() macro. For
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instance, defining this at the top of a source file (before any ``#include``
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directive)::
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#define pr_fmt(fmt) "%s:%s: " fmt, KBUILD_MODNAME, __func__
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would prefix every pr_*() message in that file with the module and function name
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that originated the message.
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For debugging purposes there are also two conditionally-compiled macros:
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pr_debug() and pr_devel(), which are compiled-out unless ``DEBUG`` (or
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also ``CONFIG_DYNAMIC_DEBUG`` in the case of pr_debug()) is defined.
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Function reference
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==================
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.. kernel-doc:: include/linux/printk.h
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:functions: printk pr_emerg pr_alert pr_crit pr_err pr_warn pr_notice pr_info
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pr_fmt pr_debug pr_devel pr_cont
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