In that case, the following is how to program these files and not overwrite the partition table on the device. We assume that the files exist on a SD card.
Prepare the System for Building LKMs The system must be prepared to build kernel code, and to do this you must have the Linux headers installed on your device.
On a typical Linux desktop machine you can use your package manager to locate the correct package to install. For example, under bit Debian you can use: However, in this series of articles I build the LKM on the BeagleBone itself, which simplifies the process when compared to cross-compiling.
You must install the headers for the exact version of your kernel build. Similar to the desktop installation, use uname to identify the correct installation.
Choose the exact kernel build, and download and install those Linux-headers on your BeagleBone. This step is not necessary under the bone70 build. It is very easy to crash the system when you are writing and testing LKMs. It is always possible that such a system crash could corrupt your file system — it is unlikely, but it is possible.
Performing a sudo reboot, or pressing the reset button on the BeagleBone will usually put everything back in order. No BeagleBones were corrupted in the writing of these articles despite many, many system crashes! The Module Code The run-time life cycle of a typical computer program is reasonably straightforward.
A loader allocates memory for the program, then loads the program and any required shared libraries.
On program exit, the operating system identifies any memory leaks and frees lost memory to the pool. A kernel module is not an application — for a start there is no main function!
Some of the key differences are that kernel modules: The type of requests that it can handle are defined within the module code. This is quite similar to the event-driven programming model that is commonly utilized in graphical-user interface GUI applications.
The kernel module lives and runs in kernel space, which has its own memory address space. The interface between kernel space and user space is clearly defined and controlled. We do however have a printk function that can output information, which can be viewed from within user space.
We have to carefully construct our modules so that they have a consistent and valid behavior when they are interrupted. The BeagleBone has a single-core processor for the moment but we still have to consider the impact of multiple processes accessing the module simultaneously.
This sounds like an advantage, however, you have to be very careful that your module does not adversely affect the overall performance of your system.
However, it is very difficult to perform these traps in kernel space. The alternative is to manually save and restore floating point operations — a task that is best avoided and left to your user-space code. The concepts above are a lot to digest and it is important that they are all addressed, but not all in the first article!
Listing 1 provides the code for a first example LKM. Further description is available after the code listing below.Advanced Linux Programming Contents At a Glance I Advanced UNIX Programming with Linux 1 Getting Started 3 2 Writing Good GNU/Linux Software 17 3 Processes 45 4 Threads 61 5 Interprocess Communication95 II Mastering Linux 6 Devices 7 The /proc File System 8 Linux System Calls 9 Inline Assembly Code 10 Security 11 A Sample GNU/Linux Application III .
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