printk
Printk
printk()
is one of the most widely known functions in the Linux kernelstandard tool we have for printing messages and usually the most basic way of tracing and debugging
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 usingdmesg
.
Message logging
printk()
can specify a log level :printk(KERN_INFO "Message: %s\n", arg);
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”
To check the current console_loglevel : (shows current, default, minimum and boot-time-default log levels)
This can be changed using
# echo 8 > /proc/sys/kernel/printk
or# dmesg -n 5
.pr_info
,pr_debug
are macros that help to print logs at different levels
Specifiers
There are many print specifiers present, it’s better to check these specifiers before using printk. These are available at : Printk Formats - Kernel Docs.
The common types are :
Integer Types (signed char, unsigned char, int, unsigned short int, etc.)
Pointer Types
Plain Pointers
Error Pointers
Symbol/Function Pointers (Eg :
%pS versatile_init+0x0/0x110
)Probe pointers from BPF/tracing
Kernel Pointers
Unmodified address
Pointer Differences
Struct Resources
Physical addresses (
%pa[p] 0x01234567 or 0x0123456789abcdef
)DMA addresses (
%pad 0x01234567 or 0x0123456789abcdef
)Raw buffer
MAC addresses (
%pM 00:01:02:03:04:05
)IPv4/v6 Addresses (
%pI4 1.2.3.4
)UID/GUID (
%pUb 00010203-0405-0607-0809-0a0b0c0d0e0f
)Dentrys (
%pd{,2,3,4}
)And many more …
Printk Index
The kernel messages are evolving together with the code. As a result, particular kernel messages are not
KABI
and never will be! (See : Kernel ABI doc for KABI)
The printk index helps to find changes in the message formats. Also it helps to track the strings back to the kernel sources and the related commit.
The index of printk formats are split in into separate files.
The content is inspired by the dynamic debug interface and looks like:
Subsystem specific prefix
The macro
pr_fmt()
macro allows to define a prefix that is printed before the string generated by the relatedprintk()
calls.
This results in the following printk
index entry:
References
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