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diff --git a/Documentation/core-api/printk-basics.rst b/Documentation/core-api/printk-basics.rst new file mode 100644 index 000000000..965e4281e --- /dev/null +++ b/Documentation/core-api/printk-basics.rst @@ -0,0 +1,115 @@ +.. 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 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 |