12/8/2019 Accessing Kernel Headers
Written by Date: 2010-12-08 10:50:30 00:00 Since a long time not using it I tried to use VMware again today with my Debian Squeeze, and as the kernel have been upgraded, I had to reconfigure it by running /usr/bin/vmware-config.pl What is the location of the directory of C header files that match your running kernel? /usr/src/Linux/include Install C header Linux Kernel on Debian or Ubuntu To install the Linux Kernel headers on Ubuntu or Debian Linux run this command to install the sources for your kernel specific version.
Sudo apt-get install linux-headers-$(uname -r) And this command to install the generic package, and keep your sources up to date. Every time you run sudo apt-get update && sudo apt-get upgrade Your kernel and kernel headers will upgrade if an upgrade is available. Aptitude install linux-headers-2.6-686 Replace the '-686' for '-(xen vserver openvz)-686' or '-amd64' or '-686-bigmem', if you need to. That depends on your architecture. Use uname -a to check yours.
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TL;DR: The running kernel version was older than the kernel source that VirtualBox found when trying to install. There is no need to build the source yourself; final resolution was to actually remove the newer version of kernel source, and install the matching version found in uname -r, all through yum.
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History: When trying to install VirtualBox-4.2 on a new CentOS server I was getting the following: Your kernel headers for kernel 2.6.32-358.6.2.el6.x8664 cannot be found at /lib/modules/2.6.32-358.6.2.el6.x8664/build or /lib/modules/2.6.32-358.6.2.el6.x8664/source. So in following the instructions on CentOS I tried getting the source and I now have /rpmbuild/SOURCES/kernel-2.6.32-358.6.2.el6 with 2 folders: linux-x and vanilla-x. No thanks to the down-voters, I was able to figure this out and make the process much easier, no need to manually build the source.
The problem (for me) was that the newer versions of the kernel/headers were installed and I had to remove them before I could install the older version I needed to match my running kernel: First remove 18.1 (I found that the 'headers' are the only ones you need to remove, the 'devel' can have the newer and older version side by side): sudo yum remove kernel-headers-2.6.32-358.18.1.el6.x8664 Now install 6.2: sudo yum install kernel-headers-2.6.32-358.6.2.el6.x8664 sudo yum install kernel-devel-2.6.32-358.6.2.el6.x8664 Now when installing VirtualBox everything is good.
Page Contents. Introduction In this series of articles I describe how you can write a Linux kernel module for an embedded Linux device. I begin with a straightforward “Hello World!” loadable kernel module (LKM) and work towards developing a module that can control GPIOs on an embedded Linux device (such as the BeagleBone) through the use of IRQs. I will add further follow-up articles as I identify suitable applications.
This is a complex topic that will take time to work through. Therefore, I have broken the discussion up over a number of articles, each providing a practical example and outcome.
There are entire books written on this topic, so it will be difficult to cover absolutely every aspect. There are also other articles available on writing kernel modules; however, the examples presented here are built and tested under the Linux kernel 3.8.X+, ensuring that the material is up to date and relevant, and I have focused on interfacing to hardware on embedded systems. I have also aligned the tasks performed against my book, albeit the articles are self-contained and do not require that you own a copy of the book. Figure 1: GPIO performance in kernel space This article is focused on the system configuration, tools and code required to build and deploy a “Hello World!” kernel module.
The second article in this series examines the topic of writing character device drivers and how to write C/C programs in user space that can communicate with kernel space modules. The third article examines the use of the kernel space GPIO library code — it combines the content of the first two articles to develop interrupt-driven code that can be controlled from Linux user space. For example, Figure 1 illustrates an oscilloscope capture of an interrupt-driven kernel module that triggers an LED to light when a button is pressed (click for a larger version). Under regular embedded Linux (i.e., not a real-time variant), this code demonstrates a response time of approximately 20 microseconds (±5μs), with negligible CPU overhead. What is a Kernel Module? A loadable kernel module (LKM) is a mechanism for adding code to, or removing code from, the Linux kernel at run time.
They are ideal for device drivers, enabling the kernel to communicate with the hardware without it having to know how the hardware works. The alternative to LKMs would be to build the code for each and every driver into the Linux kernel. Figure 2: Linux user space and kernel space Without this modular capability, the Linux kernel would be very large, as it would have to support every driver that would ever be needed on the BBB. You would also have to rebuild the kernel every time you wanted to add new hardware or update a device driver.
The downside of LKMs is that driver files have to be maintained for each device. LKMs are loaded at run time, but they do not execute in user space — they are essentially part of the kernel. Kernel modules run in kernel space and applications run in user space, as illustrated in Figure 2. Both kernel space and user space have their own unique memory address spaces that do not overlap. This approach ensures that applications running in user space have a consistent view of the hardware, regardless of the hardware platform. The kernel services are then made available to the user space in a controlled way through the use of system calls.
The kernel also prevents individual user-space applications from conflicting with each other or from accessing restricted resources through the use of protection levels (e.g., superuser versus regular user permissions). Why Write a Kernel Module? When interfacing to electronics circuits under embedded Linux you are exposed to sysfs and the use of low-level file operations for interfacing to electronics circuits. This approach can appear to be inefficient (especially if you have experience of traditional embedded systems); however, these file entries are memory mapped and the performance is sufficient for many applications. I have demonstrated in my book that it is possible to achieve response times of about one third of a millisecond, with negligible CPU overhead, from within Linux user space by using pthreads, callback functions and sys/poll.h. An alternative approach is to use kernel code, which has support for interrupts.
However, kernel code is difficult to write and debug. My advice is that you should always to try to accomplish your task in Linux user space, unless you are certain that there is no other possible way! Molloyd@beaglebone:$ git clone The directory is the most important resource for this article.
The auto-generated Doxygen documentation for these code examples is available in. 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 64-bit Debian you can use: molloyd@DebianJessieVM:$ sudo apt-get update molloyd@DebianJessieVM:$ apt-cache search linux-headers-$(uname -r) linux-headers-3.16.0-4-amd64 - Header files for Linux 3.16.0-4-amd64 molloyd@DebianJessieVM:$ sudo apt-get install linux-headers-3.16.0-4-amd64 molloyd@DebianJessieVM:$ cd /usr/src/linux-headers-3.16.0-4-amd64/ molloyd@DebianJessieVM:/usr/src/linux-headers-3.16.0-4-amd64$ ls arch include Makefile Module.symvers scripts You can complete the first two articles in this series using any flavor of desktop Linux. 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. For example: molloyd@beaglebone:$ uname -a Linux beaglebone 3.8.13-bone70 #1 SMP Fri Jan 23 02:15:42 UTC 2015 armv7l GNU/Linux You can download the Linux headers for the BeagleBone platform from Robert Nelson’s website. For example, at:.
Choose the exact kernel build, and download and install those Linux-headers on your BeagleBone. 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. Please back up your data and/or use an embedded system, such as the BeagleBone, which can easily be re-flashed. 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. Instruction execution begins at some entry point (typically the main point in C/C programs), statements are executed, exceptions are thrown, dynamic memory is allocated and deallocated, and the program eventually runs to completion.
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:. do not execute sequentially— a kernel module registers itself to handle requests using its initialization function, which runs and then terminates. 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. do not have automatic cleanup — any resources that are allocated to the module must be manually released when the module is unloaded, or they may be unavailable until a system reboots.
do not have printf functions — kernel code cannot access libraries of code that is written for the Linux user space. 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. can be interrupted — one conceptually difficult aspect of kernel modules is that they can be used by several different programs/processes at the same time.
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. have a higher level of execution privilege — typically, more CPU cycles are allocated to kernel modules than to user-space programs. 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. do not have floating-point support — it is kernel code that uses traps to transition from integer to floating-point mode for your user space applications. 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. When no kernel argument is provided, the code uses the printk function to display “Hello world!” in the kernel logs. If the argument “Derek” is provided, then the logs will display “Hello Derek!” The comments in Listing 1, which are written using a Doxygen style, describe the role of each statement. Further description is available after the code listing below. Listing 1: The Hello World Linux Loadable Kernel Module (LKM) Code. Moduleexit ( helloBBBexit ); In addition to the points described by the comments in Listing 1, there are some additional points:.
Line 16: The statement MODULELICENSE('GPL') provides information (via modinfo) about the licensing terms of the module that you have developed, thus allowing users of your LKM to ensure that they are using free software. Since the kernel is released under the GPL, your license choice impacts upon the way that the kernel treats your module. You can choose 'Proprietary' for non-GPL code, but the kernel will be marked as “tainted” and a warning will appear. There are non-tainted alternatives to GPL, such as 'GPL v2', 'GPL and additional rights', 'Dual BSD/GPL', 'Dual MIT/GPL', and 'Dual MPL/GPL'.
See for more information. Line 21: The name (ptr to char) is declared as static and is initialized to contain the string “hello”. You should avoid using global variables in kernel modules — it is even more important than in application programming, as global variables are shared kernel wide.
You should use the static keyword to restrict a variable’s scope to within the module. If you must use a global variable, add a prefix that is unique to the module that you are writing. Line 22: The moduleparam(name, type, permissions) macro has three parameters: name (the parameter name displayed to the user and the variable name in the module), type (the type of the parameter — i.e., one of byte, int, uint, long, ulong, short, ushort, bool, an inverse Boolean invbool, or a char pointer charp), and permissions (this is the access permissions to the the parameter when using sysfs and is covered below.
A value of 0 disables the entry, but SIRUGO allows read access for user/group/others — See the ). Line 31 and 40: The functions can have whatever names you like (e.g., helloBBBinit and helloBBBexit), however, the same names must be passed to the special macros moduleinit and moduleexit on lines 48 and 49. Line 31: The printk is very similar in usage to the printf function that you should be familiar with, and you can call it from anywhere within the kernel module code. The only significant difference is that you should specify a log level when you call the function. The log levels are defined in as one of KERNEMERG, KERNALERT, KERNCRIT, KERNERR, KERNWARNING, KERNNOTICE, KERNINFO, KERNDEBUG, and KERNDEFAULT.
Kernel Header Files
This header is included via the linux/kernel.h header file, which includes it via linux/printk.h. Essentially, when this module is loaded the helloBBBinit function will execute, and when the module is unloaded the helloBBBexit function will execute. The next step is to build this code into a kernel module. Building the Module Code A Makefile is required to build the kernel module — in fact, it is a special kbuild Makefile. The kbuild Makefile required to build the kernel module in this article can be viewed in Listing 2. Listing 2: The Makefile Required to Build the Hello World LKM. Make - C / lib / modules / $ ( shell uname - r ) / build / M = $ ( PWD ) clean The first line of this Makefile is called a goal definition and it defines the module to be built ( hello.o).
The syntax is surprisingly intricate, for example obj-m defines a loadable module goal, whereas obj-y indicates a built-in object goal. The syntax becomes more complex when a module is to be built from multiple objects, but this is sufficient to build this example LKM. The reminder of the Makefile is similar to a regular Makefile. The $(shell uname -r) is a useful call to return the current kernel build version — this ensures a degree of portability for the Makefile. The -C option switches the directory to the kernel directory before performing any make tasks. The M=$(PWD) variable assignment tells the make command where the actual project files exist. The modules target is the default target for external kernel modules.
An alternative target is modulesinstall which would install the module (the make command would have to be executed with superuser permissions and the module installation path is required). All going well, the process to build the kernel module should be straightforward, provided that you have installed the Linux headers as described earlier. The steps are as follows: molloyd@beaglebone:/exploringBB/extras/kernel/hello$ ls -l total 8 -rw-r-r- 1 molloyd molloyd 154 Mar 17 17:47 Makefile -rw-r-r- 1 molloyd molloyd 2288 Apr 4 23:26 hello.c molloyd@beaglebone:/exploringBB/extras/kernel/hello$ make make -C /lib/modules/3.8.13-bone70/build/ M=/home/molloyd/exploringBB/extras/kernel/hello modules make1: Entering directory '/usr/src/linux-headers-3.8.13-bone70' CC M /home/molloyd/exploringBB/extras/kernel/hello/hello.o Building modules, stage 2. MODPOST 1 modules CC /home/molloyd/exploringBB/extras/kernel/hello/hello.mod.o LD M /home/molloyd/exploringBB/extras/kernel/hello/hello.ko make1: Leaving directory '/usr/src/linux-headers-3.8.13-bone70' molloyd@beaglebone:/exploringBB/extras/kernel/hello$ ls Makefile Module.symvers hello.c hello.ko hello.mod.c hello.mod.o hello.o modules.order You can see that there is now a hello loadable kernel module in the build directory with the file extension.ko. Click for the HTML and PDF version of the auto-generated Doxygen code documentation Hopefully you have built your first loadable kernel module (LKM).
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Despite the simplicity of the functionality of this module there was a lot of material to cover — by the end of this article: you should have a broad idea of how loadable kernel modules work; you should have your system configured to build, load and unload such modules; and, you should be able to define custom parameters for your LKMs. The next step is to build on this work to develop a kernel space LKM that can communicate with a user space C/C program by developing a basic character driver. Then we can move on to the more interesting task of interacting with GPIOs. Derek- I figured it out — maybe this will help someone if you approve this comment.
Kernel Headers Cannot Be Found
I’m running an older 2014-04-23 debian image (from dogtag file) kernel 3.8.13-bone69 To get the kernel headers from Robert Nelsons repo I had to add this to /etc/apt/sources.list deb arch=armhf wheezy main Then apt-get update and apt-cache search kernel-headers reveals a whole bunch of headers available to install including my kernel 3.8.13-bone69 Now I’m going to try and compile the kenrel module Nathanial Lewis wrote that supports the TI eQEP encoder hardware. Thanks for all you do — makes using the beaglebone platform a pleasure! Folx download for mac. Hi, I tried to write a LKM for epaper lcd display which uses SPI,PWM and GPIOs. After loading the linux devicetree for BBB, I get an error message in dmesg. The last few lines of dmesg 269.742198 check pwm 269.742273 /ocp/spi@48030000/epd@0: could not get #pwm-cells for /ocp/epwmss@48302000/ehrpwm@48302200 269.752250 epd: Cannot get pwm -22 269.777286 Call epdthermremove 269.781123 i2c temperature probe excluded 269.799667 epd: Fail to create COG-G1 269.812813 prvdsp,g1-epd: probe of spi1.0 failed with error -22 May I get some help in device tree coding for BBB.
Regards venkat. Hey Derek, I’m using the BeagleBone black with: Linux beaglebone 3.8.13-bone47 sudo apt-get update command give this error W: There is no public key available for the following key IDs: 7638D0442B90D010 W: There is no public key available for the following key IDs: 7638D0442B90D010 W: GPG error: wheezy Release: The following signatures couldn’t be verified because the public key is not available: NOPUBKEY D284E608A4C46402 W: There is no public key available for the following key IDs: 9D6D8F6BC857C906 Also unable to install header files. Please help me with this. I think we should avoid to develop the modules for kernel.i think we should try to connect the kernel with a simple and more popular programming language.my opinion is this programming language is complex of present,so i think the performance of programmers is going to high when they get the easy of help-information and isn’t wrecking on the code.i think i can know how to create an operating system and i know how it’s make a sense from transistors,we could make it more simpler of syntax,but why we didn’t? Hi Derek, I must say really nice tutorials and you have done really well in organizing the stuffs.
I have done some character driver programming on my Linux machine on my laptop.Now I want to interface some hardware and do driver programming on BeagleBone. The problem I am facing is I am not getting the Kernel Headers for the version of Linus installed on my BeagleBone. I have 3.8.13-bone81, but kernel headers I am not able to get. I would really appreciate if you can guide me in this respect. Hey Derek, I Just want to ask about the linux headers for cross compiling Kernel module on the host machin.
Here you write to download the linux headers of the host machin kernel version and in other tutorial was writen to download the linux headers of the BBB kernel version to the host machin. I also have other linux device that based on the AM335x and i succeed to cross compile kernel module and “insmod” it on the device with the explanation of the tutorial about the linux kernel headers of the target device. I Am very confusing about the cross compiling kernel module process. Also, if i want to compile on my BBB, i need to download the linux headers for my beagle kernel that is 3.8.13-bone50 but i cant find headers for that kernel.
Thanks, orenz. Great tutorial.
We are trying external interrupt for NVidia TX2 (ARM). When I try to build the Hello project, I got this:. make -C /lib/modules/4.4.38-tegra/build/ M=/home/nvidia/Downloads/exploringBB/extras/kernel/hello modules make1: Entering directory ‘/lib/modules/4.4.38-tegra/build’ make1:. No rule to make target ‘modules’. The 'monster' image that is associated with your comment is auto-generated - it makes it easier to follow the conversation threads. If you wish to replace this image with a less (or perhaps more) monstrous version, add an image at against the e-mail address that you use to submit your comment. Your image will henceforth be used on most WordPress sites.
Please note that I will remove any messages that contain blatant advertisement or that refer to illegal software, content etc. I may tidy up some messages if they contain code dumps etc. E-mail addresses are used only to notify you of any responses, and to authenticate your future comments on this website - they are not made public nor used for any other purpose. See the for a full description. I manually approve all new posts in order to keep the website spam free, but once your post is approved, all future posts should be automatically approved. Please let me know if your messages do not appear.
I really appreciate it when you answer the questions of others on the page, as it is difficult for me to do so and continue to produce new content. Thanks for your understanding, Derek. Current ye@r. Leave this field empty.
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