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" |
|
KERN_ALERT |
"1" |
|
KERN_CRIT |
"2" |
|
KERN_ERR |
"3" |
|
KERN_WARNING |
"4" |
|
KERN_NOTICE |
"5" |
|
KERN_INFO |
"6" |
|
KERN_DEBUG |
"7" |
|
KERN_DEFAULT |
"" |
|
KERN_CONT |
"c" |
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¶
-
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.
See also: printf(3)
See the vsnprintf()
documentation for format string extensions over C99.
-
pr_emerg¶
pr_emerg (fmt, ...)
Print an emergency-level message
Parameters
fmt
format string
...
arguments for the format string
Description
This macro expands to a printk with KERN_EMERG loglevel. It uses pr_fmt()
to
generate the format string.
-
pr_alert¶
pr_alert (fmt, ...)
Print an alert-level message
Parameters
fmt
format string
...
arguments for the format string
Description
This macro expands to a printk with KERN_ALERT loglevel. It uses pr_fmt()
to
generate the format string.
-
pr_crit¶
pr_crit (fmt, ...)
Print a critical-level message
Parameters
fmt
format string
...
arguments for the format string
Description
This macro expands to a printk with KERN_CRIT loglevel. It uses pr_fmt()
to
generate the format string.
-
pr_err¶
pr_err (fmt, ...)
Print an error-level message
Parameters
fmt
format string
...
arguments for the format string
Description
This macro expands to a printk with KERN_ERR loglevel. It uses pr_fmt()
to
generate the format string.
-
pr_warn¶
pr_warn (fmt, ...)
Print a warning-level message
Parameters
fmt
format string
...
arguments for the format string
Description
This macro expands to a printk with KERN_WARNING loglevel. It uses pr_fmt()
to generate the format string.
-
pr_notice¶
pr_notice (fmt, ...)
Print a notice-level message
Parameters
fmt
format string
...
arguments for the format string
Description
This macro expands to a printk with KERN_NOTICE loglevel. It uses pr_fmt()
to
generate the format string.
-
pr_info¶
pr_info (fmt, ...)
Print an info-level message
Parameters
fmt
format string
...
arguments for the format string
Description
This macro expands to a printk with KERN_INFO loglevel. It uses pr_fmt()
to
generate the format string.
-
pr_cont¶
pr_cont (fmt, ...)
Continues a previous log message in the same line.
Parameters
fmt
format string
...
arguments for the format string
Description
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.
-
pr_devel¶
pr_devel (fmt, ...)
Print a debug-level message conditionally
Parameters
fmt
format string
...
arguments for the format string
Description
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.
-
pr_debug¶
pr_debug (fmt, ...)
Print a debug-level message conditionally
Parameters
fmt
format string
...
arguments for the format string
Description
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).