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Last week, some readers encountered an error code with printk debug levels. This issue occurs due to a number of factors. Now let’s discuss some of them. The main downside is that printk is pretty static, so we need to figure out in advance what clients want to track, and if you want to track something specific, you need to recompile your password, which can get pretty cumbersome.
Perhaps the easiest way to generate debug information from your kernel code is to print various information to printf using the kernel equivalent, the printk function and its derivatives.The k in printk can be used specifically to remind kernel programmers that the environment is different.
printk works in much the same way as it does in userspace printf, so if you’ve already debugged your main userspace program with printf, you’re ready and able to do the same with, for example, kernel code. by adding a debugger:
printk("Var1 %d var2 %dn", var1, var2);
Is printk buffered?
All printk() messages are written to the kernel log buffer, which has become a circular buffer exported to user space via /dev/kmsg. The usual way to read it is to use dmesg. If the priority of the message is literally higher (lower log level value) than the console_loglevel value, the message is sent to the console.
The command is present in one of your shells – it will print the entire human kernel logbuffer for you.
Most of the printf() routines created by theThe conversions supported by the userspace type library are concurrently available in the kernel; There are usually a few notable additions, including “%pf” which can return a symbol name instead of a numeric pointer. Please note if there is. You are welcome
Note. Always use %zu, %zd or %zx when working with printing.size_t and ssize_t values. ssize_t in conjunction with size_t are fairly common values â€‹â€‹in this kernel, soPlease use %z to avoid annoying warnings.
Useful tips from the author:
If you want to debug all whoops (e.g. caused by double help) in your driver and your company doesn’t know where whoops usually occur, just add this line
printk(KERN_ALERT "DEBUG: %s %d n elapsed",__FUNCTION__,__LINE__);
after each potentially annoying statement.Recompile (re)load and the module will also throw an error – your family log will now show you the previous line that ran successfully before the whoops happened.
Of course you should remove those “rude” adsValues â€‹â€‹before committing your module. ðŸ˜‰
Connect Your Level
How do I set debug level in Linux?
Use the kitty /proc/cmdline command to display the kernel command panel used for the previous boot. To showThat’s all, the number specified for the loglevel parameter must be greater than KERN_DEBUG. It simply means that you will need to use loglevel=8. Or just use our custom ignore_loglevel setting to show all major news campaigns.
If you look directly at the actual kernel code, you will regularly see something like:
printk(KERN_ERR "something went wrong, return code: %dn",ret);
How do I increase my dmesg log level?
This level is used to provide warnings or messages about imminent failures. Log level 5 – KERN_NOTICE. Messages that use this severity level are events worth mentioning. Log level 6 is actually KERN_INFO: this is the log level used for informational messages, almost an action performed by the kernel.
where “KERN_ERR” is one of eight different logging levels.include/linux/kern_levels.h and defines the severity of all error messages.
Note that there is actually NO comma between KERN_ERR and the format string (because the main preprocessor concatenates the two strings).
The level log area starts with KERN_SOH which indicates “Header Start” for the kernel, the fact that it isa single ASCII 1 character, specified as “