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| -rw-r--r-- | Documentation/core-api/index.rst | 1 | ||||
| -rw-r--r-- | Documentation/core-api/printk-basics.rst | 115 | ||||
| -rw-r--r-- | Documentation/core-api/printk-formats.rst | 2 | ||||
| -rw-r--r-- | include/linux/printk.h | 112 |
4 files changed, 218 insertions, 12 deletions
diff --git a/Documentation/core-api/index.rst b/Documentation/core-api/index.rst index 0897ad12c119..49e3da910d9e 100644 --- a/Documentation/core-api/index.rst +++ b/Documentation/core-api/index.rst @@ -18,6 +18,7 @@ it. kernel-api workqueue + printk-basics printk-formats symbol-namespaces diff --git a/Documentation/core-api/printk-basics.rst b/Documentation/core-api/printk-basics.rst new file mode 100644 index 000000000000..563a9ce5fe1d --- /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 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 diff --git a/Documentation/core-api/printk-formats.rst b/Documentation/core-api/printk-formats.rst index 8ebe46b1af39..1e3838652348 100644 --- a/Documentation/core-api/printk-formats.rst +++ b/Documentation/core-api/printk-formats.rst @@ -2,6 +2,8 @@ How to get printk format specifiers right ========================================= +.. _printk-specifiers: + :Author: Randy Dunlap <rdunlap@infradead.org> :Author: Andrew Murray <amurray@mpc-data.co.uk> diff --git a/include/linux/printk.h b/include/linux/printk.h index e061635e0409..ccea5d4a509e 100644 --- a/include/linux/printk.h +++ b/include/linux/printk.h @@ -279,39 +279,116 @@ static inline void printk_safe_flush_on_panic(void) extern int kptr_restrict; +/** + * pr_fmt - used by the pr_*() macros to generate the printk format string + * @fmt: format string passed from a pr_*() macro + * + * 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. + */ #ifndef pr_fmt #define pr_fmt(fmt) fmt #endif -/* - * These can be used to print at the various log levels. - * All of these will print unconditionally, although note that pr_debug() - * and other debug macros are compiled out unless either DEBUG is defined - * or CONFIG_DYNAMIC_DEBUG is set. +/** + * pr_emerg - Print an emergency-level message + * @fmt: format string + * @...: arguments for the format string + * + * This macro expands to a printk with KERN_EMERG loglevel. It uses pr_fmt() to + * generate the format string. */ #define pr_emerg(fmt, ...) \ printk(KERN_EMERG pr_fmt(fmt), ##__VA_ARGS__) +/** + * pr_alert - Print an alert-level message + * @fmt: format string + * @...: arguments for the format string + * + * This macro expands to a printk with KERN_ALERT loglevel. It uses pr_fmt() to + * generate the format string. + */ #define pr_alert(fmt, ...) \ printk(KERN_ALERT pr_fmt(fmt), ##__VA_ARGS__) +/** + * pr_crit - Print a critical-level message + * @fmt: format string + * @...: arguments for the format string + * + * This macro expands to a printk with KERN_CRIT loglevel. It uses pr_fmt() to + * generate the format string. + */ #define pr_crit(fmt, ...) \ printk(KERN_CRIT pr_fmt(fmt), ##__VA_ARGS__) +/** + * pr_err - Print an error-level message + * @fmt: format string + * @...: arguments for the format string + * + * This macro expands to a printk with KERN_ERR loglevel. It uses pr_fmt() to + * generate the format string. + */ #define pr_err(fmt, ...) \ printk(KERN_ERR pr_fmt(fmt), ##__VA_ARGS__) +/** + * pr_warn - Print a warning-level message + * @fmt: format string + * @...: arguments for the format string + * + * This macro expands to a printk with KERN_WARNING loglevel. It uses pr_fmt() + * to generate the format string. + */ #define pr_warn(fmt, ...) \ printk(KERN_WARNING pr_fmt(fmt), ##__VA_ARGS__) +/** + * pr_notice - Print a notice-level message + * @fmt: format string + * @...: arguments for the format string + * + * This macro expands to a printk with KERN_NOTICE loglevel. It uses pr_fmt() to + * generate the format string. + */ #define pr_notice(fmt, ...) \ printk(KERN_NOTICE pr_fmt(fmt), ##__VA_ARGS__) +/** + * pr_info - Print an info-level message + * @fmt: format string + * @...: arguments for the format string + * + * This macro expands to a printk with KERN_INFO loglevel. It uses pr_fmt() to + * generate the format string. + */ #define pr_info(fmt, ...) \ printk(KERN_INFO pr_fmt(fmt), ##__VA_ARGS__) -/* - * Like KERN_CONT, pr_cont() should only be used when continuing - * a line with no newline ('\n') enclosed. Otherwise it defaults - * back to KERN_DEFAULT. + +/** + * pr_cont - Continues a previous log message in the same line. + * @fmt: format string + * @...: arguments for the format string + * + * 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. */ #define pr_cont(fmt, ...) \ printk(KERN_CONT fmt, ##__VA_ARGS__) -/* pr_devel() should produce zero code unless DEBUG is defined */ +/** + * pr_devel - Print a debug-level message conditionally + * @fmt: format string + * @...: arguments for the format string + * + * 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. + */ #ifdef DEBUG #define pr_devel(fmt, ...) \ printk(KERN_DEBUG pr_fmt(fmt), ##__VA_ARGS__) @@ -325,8 +402,19 @@ extern int kptr_restrict; #if defined(CONFIG_DYNAMIC_DEBUG) #include <linux/dynamic_debug.h> -/* dynamic_pr_debug() uses pr_fmt() internally so we don't need it here */ -#define pr_debug(fmt, ...) \ +/** + * pr_debug - Print a debug-level message conditionally + * @fmt: format string + * @...: arguments for the format string + * + * 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). + */ +#define pr_debug(fmt, ...) \ dynamic_pr_debug(fmt, ##__VA_ARGS__) #elif defined(DEBUG) #define pr_debug(fmt, ...) \ |