Creating message character device - /dev/msg

-
In this post, let's create a simple character device - /dev/msg, which will store a user message.

Theory

We use copy_to_user and copy_from_user to copy data to/fro in between user space and kernel space.

unsigned long copy_to_user ( void __user *  to, const void *  from, unsigned long  n); -- Copies data from kernel space to user space.

unsigned long copy_from_user (void *to, const void __user *from,unsigned long n); -- Copies data from user space to kernel space.

Both on success returns 0 else returns number of bytes not copied.

Why copy_to_user and copy_from_user is needed instead of memcpy?
  • It verifies that the memory passed is in a region of memory that the process have access to and the address is not pointing to kernel space by using access_ok macro
  • User pages being addressed might not be currently present in memory, if the kernel directly tries to access the pages which are not in memory, it will result in page fault and kernel will panic on page fault. But if there is page fault during copy_from_user kernel does not panic, but returns -EFAULT.
  • It also handles if it encounters an invalid address during copy.

Code:



Sample Test Application:




Output:

$ ./testapp 
write : 12
Read:hello world
Read:



Comments

Post a Comment

Popular posts from this blog

bb.utils.contains yocto

-
Image
bb.utils.contains  is most commonly used function in Yocto. grep in poky directory returned with 696 count. $ grep -nr 'bb.utils.contains' poky/ | wc -l 696 It's definition is present in 'poky/bitbake/lib/bb/utils.py' file This function returns third argument if the second argument is subset of the first argument else returns fourth argument Let's take an example to understand this. You can see from the above screenshot, when the second argument is a subset of first argument, "present" was returned, else "notpresent" was returned We can also use bb.utils.contains inside if loop. This we can use inside a recipe. if ${@bb.utils.contains('SOMEFLAG','NEWVALUE3','true','false',d)}; then     FOO = 'HELLO' fi
Read more

Difference between RDEPENDS and DEPENDS in Yocto

-
We normally observe these two (RDEPENDS and DEPENDS) whenever we open any recipe. What should we choose when we write our own recipe. DEPENDS -> Build Time Dependency RDEPENDS -> Run Time Dependency DEPENDS: When a recipe 'A' is DEPENDS on recipe 'B'. In this case, Bitbake first builds recipe 'B' and then recipe 'A' E.g. You need 'dbus' to be built before 'wpa_supplicant'. RDEPENDS: When a recipe 'A' is RDEPENDS on recipe 'B'. In this case, Bitbake deploys 'B' on the target system when it deploys 'A'.  E.g. 'perf' RDEPENDS on 'bash' In other words, DEPENDS are those set of packages that should be available while building package, whereas RDEPENDS are set of packages that should be available during execution of the program.
Read more

make config vs oldconfig vs defconfig vs menuconfig vs savedefconfig

-
Image
Building the Linux Kernel is a two-step process: Configuring the Kernel options Building the Kernel with those options. There are many methods available for configuring the kernel. make config: Text-based Configuration.  Options are prompted one after another. All options need to be answered Access to former options is not possible make menuconfig: Menu-driven user interface Allows to navigate forwards and backward directly between features Allows to load and save files with filenames different from ".config" Provides search feature It uses ncurses library for GUI. If the ncurses library is not installed, make menuconfig option fails. To install ncurses library on Ubuntu: sudo apt-get install libncurses5-dev make defconfig: Creates a ".config" file with default options from the ARCH supplied defconfig Configurations are generally stored in the directory: arch/$(ARCH)/configs $ ls arch/x86/configs/ i386_defconfi
Read more