I lost my internet connection at the weekend and was at a bit of a loss as to what I could do so I decided to take a pop at writing a simple module for the Linux kernel. I have a copy of
Beginning Linux Programming
ISBN: 1861002971
Authors: Richard Stevens and Neil Matthew
Ed: 2nd
so I turned to the back of it and started my foray into the Kernel. Now you need to remember that I am not a C programmer by trade and turning to the back of this book was a keen reminder of just how rusty my C is getting, not that it was ever rust free.
Luckily for me I have another book that is considered the C bible ie K&R and it deserves its reputation, it is a classic and I would recommend any programmer regardless of language choice to have a flick through it. When I was looking at some odd construct that those pointy hats had invented I had a flick through K&R and soon sorted it out.
Anyway back to the kernel. I was quick to discover that writing a module for the 2.6 kernel is not quite as straight forward as copying from the book and trying to understand what was going on. Things have been changing and I was getting all sorts of weird (or at least to me) and wonderful errors when trying to compile the kernel.
I eventually started to have a read at the recent modules in the source for 2.6.5 which I am running on this box. I also have the source for a 2.4 kernel on here so I opened 2 character drivers and compared notes between them. This is where I started to notice things that had changed. I made the changes I thought where necessary and I managed to get most of the “Hello World” module compiling but I was still getting errors.
I had a hunt around and I found a reference to some new build procedures for 2.6.5 so off I went in search of kbuild documentation and found some more stuff that had changed in the kernel. Namely the build procedure. This part was actually harder than the C that I had been struggling with.
After much swearing (I hate Makefiles and adding some more sugar is a pain in the ass) I managed to get the module compiling and I was on my way.
After a days work I now had a module that, on load would say
“Hello World”
and on removal
“Goodbye World”
time well spent or not? I haven’t decided yet. I wonder how often changes like this take place in the kernel and how much porting takes place because of it.
Where to go from here. I asked a few friends who know more about this stuff than I do and I got mixed advice about continuing. Some of them think the kernel is a mess because they are always changing the driver API among other things. I cannot comment because my knowledge of the Linux kernel is limited to spelling it and I sometimes get that wrong.
I did get some useful pointers though. The following is the best book I have found so far for someone like me who is just starting out in the kernel.
Linux Device Drivers, 2nd Edition
It is written for the 2.4 kernel but has a wealth of information that is still valid today. I have started porting the scull drivers from this to the 2.6 kernel I am running and it is proving very interesting. I printed off chapter 2 and 3 yesterday and have have almost finished them (40 mins from Luton to London on the train each way helps). So far it seems to be moving along at a fair old pace, I am just hoping I can keep up.
I could have done with the following at the weekend. This tells me what I needed to know about moving from 2.4 to 2.6. I can see myself using this a lot in the next few weeks.
Driver Porting