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Goal:

I want to implement a Linux kernel device driver, which includes the mmap file operation. I want to write a user space program (in C++), which uses the mmap system call to read/write to the device's memory.

What I (believe to) understand:

I read this (and other chapters of this book). The mmap file operation needs a filep. The user space program needs a file, which it can access. I know how to create a file in /dev/ by register_chrdev, but I learned here, that this includes higher latency (which is what I try to avoid).

Question:

What is the correct way to create a filep for the file operation mmap, along with a file that is accessible by the mmap system call?

kiigass
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  • You can find 100s of examples online including stackoverflow. You might find it worthwhile to go through [this post](https://stackoverflow.com/questions/45972/mmap-vs-reading-blocks) and [this post](https://stackoverflow.com/questions/26259421/use-mmap-in-c-to-write-into-memory) –  Jul 15 '18 at 19:52
  • Those posts are about the system call only. I have more trouble with the file operation, respectively with the creation of the file/filep. (I think I need to do that in the driver, which is written in C, hence the C tag.) – kiigass Jul 16 '18 at 06:09

1 Answers1

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Basically, you need to expose to user space some object (file) which will be mmapable and you want to control the kernel side of things.

The easiest way to achieve this is to create a procfs, sysfs or debugfs file (it will be available somewhere under "/proc", "/sys" or wherever the debugfs is mounted on your system). All those filesystems support easy (on the kernel side) creation of files immediately visible to the userspace.

Additional options include:

  1. Registering a char or block device (all devices are mmapable, but the boilerplate may get extensive for a custom device; I don't think latency is of any special concern, device variety notwithstanding).

  2. Using shmem_file_setup and friends - the slight difficulty here is exposing the newly created shmem file to the user space.

If you just want to play a bit with mmaps, procfs feels like the simplest approach overall (a single call to proc_create will do).

oakad
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