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?
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Answer
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.