This may be a stupid question, but is there a way to write to the linux console from within a driver without using printk (i.e. syslog)?
For instance, working in a linux driver, I need to output a single character as an event happens. I'd like to output 'w' as an write event starts, and a 'W' when it finishes. This happens frequently, so sending that through syslog isn't ideal.
Ideally, it would be great if I could just do the equivalent of printf("W") or putc('W') and have that simply go out the default console.
TIA
Mike
Writing to the console isn't something you want to do frequently. If printk is too expensive for you, you shouldn't try the console any way.
But if you insist:
Within printk, printing to the console is handled by call_console_drivers. This function finds the console (registered via register_console) and calls it to print the data. The actual driver depends on what console you're using. The VGA screen is one option, the serial port is another (depending on boot parameters).
You can try to use the functions in console.h to interact with the console directly. I don't know how hard it would be to make it work.
Unfortunately no, as there is no concept of "console" in the kernel (that is a userspace process). You may try other kernel debugging options
Related
Im looking for a way how to get rid of (kernel?) messages that appear in my ncurses app. I wrote the app myself, so i would prefer a API that redirects these messages to /dev/null. I mean messages like, a USB stick that is inserted.
I tried to add this, but unfortunately it doesn't work
freopen("/dev/null", "w", stderr);
I'm not running X, just ncurses direct from the console.
I mean messages like, a USB stick that is inserted.
Thanks!
UPDATE 1:
Someone votes to close this question because it would not be related to programming. But it is, i wrote the ncurses app myself, i'm looking for a way how to disable the kernel message. I updated the question.
UPDATE 2:
Let me explain what i'm doing, and whats the problem in more detail:
I'm using Tiny Core linux, thats after boots starts (self written) ncurses program. Now when you for example connect a USB drive, a message (i suspect kernel) is shown over my program. I guess the message is written straight into the framebuffer. Im using TC 5.x since i need 32 bit, im running as root and have full access to the os.
You should be able to use openvt to have your program run on a new Virtual Terminal.
I'll also note that it should be possible to embed control for the VTs yourself if you prefer to break the external dependency, but note that structures used may not be stable between kernel versions, and may require recompilation.
See the KBD project's sources, specifically openvt.c to see how it works.
Try configuring the kernel through boot parameters with the option:
loglevel=3 (or a lower value)
0 (KERN_EMERG) system is unusable
1 (KERN_ALERT) action must be taken immediately
2 (KERN_CRIT) critical conditions
3 (KERN_ERR) error conditions
4 (KERN_WARNING) warning conditions
5 (KERN_NOTICE) normal but significant condition
6 (KERN_INFO) informational
7 (KERN_DEBUG) debug-level messages
source: https://www.kernel.org/doc/Documentation/kernel-parameters.txt
See also: Change default console loglevel during boot up
It might be impossible to block some other process with sufficient access from writing to /dev/console but you may be able to redefine console as some other device, at boot time by setting console=ttyS0 (first serial port), see:
https://unix.stackexchange.com/questions/60641/linux-difference-between-dev-console-dev-tty-and-dev-tty0
Also if we know exactly which software is sending the message it may be possible to reconfigure it (possibly dynamically) but it would help to know the version and edition of Tiny Core Linux you are using?
E.g. this website has a "Core", "TinyCore" and "CorePlus" versions 1.x up to 7
http://tinycorelinux.net/downloads.html
This would help reproducing the exact same behavior and testing potential solutions.
I am trying to edit the linux kernel. I want some information to be written out to a file as a part of the debugging process. I have read about the printk function. But i would like to add text to a particular file (file other from the default files that keep debug logs).
To cut it short: I would kind of like to specify the "destination" in the printk function (or at least some work-around it)
How can I achieve this? Will using fwrite/fopen work (if yes, will it work without causing much overhead compared to printk, since they are implemented differently)?
What other options do i have?
Using fopen and fwrite will certainly not work. Working with files in kernel space is generally a bad idea.
It all really depends on what you are doing in the kernel though. In some configurations, there may not even be a hard disk for you to write to. If however, you are working at a stage where you can have certain assumptions about the running kernel, you probably actually want to write a kernel module rather than edit the kernel itself. For all you care, a kernel module is just as good as any other part of the kernel, but they are inserted when the kernel is already up and running.
You may also be thinking of doing so for debugging, or have output of a kernel-level application (e.g. an application that you are forced to run at kernel level for real-time constraints etc). In that case, kio may be of interest to you, but if you want to use it, do make sure you understand why.
kio is a library I wrote just for those "kernel-level applications", which makes a kernel module see a /proc file as if it's a user of it (rather than a provider). To make it work, you should have a user-space application also opening that virtual file and redirect it to wherever you want to write your log. Something along the lines of opening the file with kopen in write mode and in user space tell cat /proc/your_file > ~/log_file.
Note: I still recommend printk unless you really know what you are doing. Since you are thinking of fopen in kernel space, I don't think you really know what you are doing.
This question maybe not related to Emacs only, but to all development environment that use Console for its debugging process. Here is the problem. I use Eshell to run the application we are being developed. It's a J2ME application. And for debugging, we just use System.out.println(). And now, suppose I want to allow only text that started with Eko: to be displayed in the console (interactively), is it possible?
I installed Cygwin in my Windows environment, and try to grep the output like this :
run | grep Eko:. It surely filtered only output with Eko: as the beginning, but it's not interactive. The output suppressed until the application quit. Well, that's useless anyway.
Is it possible to do it? What I mean is, we don't have to touch the application code itself?
I tag to linux also, because maybe some guys in Linux know the answer.
Many thanks!
The short: Try adding --line-buffered to your grep command.
The long: I assume that your application is flushing its output stream with every System.out.println(), and that grep has the lines available to read immediately, but is choosing to buffer output until it has 'enough' output saved up to make writing make sense. (This is typically 4k or 8k of data, which could be several hundred lines, depending upon your line length.)
This buffering makes great sense when the output is another program in the pipeline; reducing needless context switches is a great way to improve program throughput.
But if your printing is slow enough that it doesn't fill the buffer quickly enough for 'real time' output, then switching to line-buffered output might fix it.
how can i print something (for debugging purpose) to console inside a linux system call?
Or is there any not very diffucult way of debugging kernel code?
thanks
The accepted way of printing inside the kernel is via printk().
You should also check the different modifiers to printk (such as KERNEL_DEBUG) which will control where/how the messages are printed, including whether they are printed to all active terminals or just the system buffer
deal all,
i am a newbie for writing Linux Kernel Module.
i used printk function in linux kernel source code (2.4.29) for debugging and display messages.
now, i have to read all the messages i added via httpd.
i tried to write the messages into a file instead of printk function, so i can read the file directly.
but it's not work very well.
so, i have a stupid question...
is it possible to write a LKM to monitor the syslog and rewrite into another file??
i mean is that possible to let a LKM the aware the messages when each time the linux kernel execute "printk"??
thanks a lot
That is the wrong way to do it, because printk already does this : it writes in the file /proc/kmsg.
What you want is klogd, a user space utility dealing with /proc/kmsg.
Another options is to use dmesg, which will output the whole content of the kernel buffers holding the printk messages, but I suggest you first read the linked article
You never, ever, ever want to try to open a file on a user space mounted block file system from within the kernel. Imagine if the FS aborted and the kernel was still trying to write to it .. kaboom (amongst MANY other reasons why its a bad idea) :) As shodanex said, for your purposes, its much better to use klogd.
Now, generally speaking, you have several ways to communicate meaningful data to userspace programs, such as:
Create a character device driver that causes userspace readers to block while waiting for data. Provide an ioctl() interface to it which lets other programs find out how many messages have been sent, etc.
Create a node in /proc/yourdriver to accomplish the same thing
Really, the most practical means is to just use printk()