I am going to implement a file system in C and i'm wondering how can i test it without installing it in the kernel nor using FUSE API. Ideally what i'd like to do is to use dd command to create a virtual hard drive and interact with it using linux system calls like write and read (the idea is to not write drivers). Is that posible?
(I'm sorry if i misspelled words, but eanglish isn't my first language. Also i'm sorry if this is off-topic, it's my first question)
Thanks.
If you are really implementing a file system, you can test it in virtual machine.
Otherwise, you can implement a file system in a file which exist in real file system, and implement some functions like read/write/etc...
Virtual hard drive and virtual filesystem are a bit different things, - you write different functions and handle different requests when implementing them. Given that you implement a filesystem, your best bet on linux is to expose your filesystem via FUSE for testing. Then write different tests that will access your FUSE-based filesystem to perform various tasks.
Unfortunately testing a filesystem is hard and requires writing many tests. Manual testing with different software (file managers) is also required.
Related
Is there a way to limit a hard drive from reading a certain file? Ex. It's given to Program A the order to open a .txt file. Program B overloads the .txt file opening hundreds times a second. Program A is unable to open the txt file.
So I'm trying to stress test a game engine that relies on extracting all used textures from a single file at once. I think that this extraction method is causing some core problems to the game developing experience of the engine overall. My theory is that the problem is caused by the slow reading time of some hard drives. But I'm not sure if I'm right on this, and I needed I way to test this out.
Most operating systems support file locking and file sharing so that you can establish rules for processes that share access to a file.
.NET, for example (which runs on Windows, Linux, and MacOS), provides the facility to open a file in a variety of sharing modes.
For very rapid access like you describe, you may want to consider a memory-mapped file. They are supported on many operating systems and via various programming languages. .NET also provides support.
I'm working on a LKM which needs to retrieve and write a certain set of information to files. I looked up common ways to do so, but could not find a working one for Linux 4.x. I also found out that it is possible to retrieve system calls from memory and effectively call them.
As I found currently no better way I'd be interested if it'd be feasible to find the system call table, and call open, read/write and close this way.
This is strongly discouraged in most situations.
https://www.linuxjournal.com/article/8110 was a really good read for me the first time I thought I had to do this as well.
From within the Linux kernel, however, reading data out of a file for configuration information is considered to be forbidden. This is due to a vast array of different problems that could result if a developer tries to do this.
Indeed this is possible to do using system calls from within the kernel, but the practice of calling system calls from within the kernel is also generally discouraged. They're designed as interfaces for userspace applications to ask things of the kernel, not for the kernel to get itself to do work.
What kind of files do you want to manipulate from within the kernel? If the kind of file you'd like to manipulate is provided by the proc filesystem or the sysfs filesystem or the dev filesystem, you can modify the contents of the file from within the kernel (since the kernel provides these to userspace itself) -- this should be done NOT with file manipulation calls. If it's a normal userspace file, almost never do you want the kernel to be able to modify it.
If you provide more specifics I'd be interested to hear them, but this is usually a bad idea.
I want to get information about the battery in C on linux. I don't want to read or parse any file! Is there any low-level interface to acpi/the kernel or any other module to get the information I want to have?
I already searched the web, but every question results in the answer "parse /proc/foo/bar". I really don't want to do this because I think, low-level interfaces won't change as fast as Files do.
best regards.
The /proc filesystem does not exist on a disk. Instead, the kernel creates it in memory. They are generated on-demand by the kernel when accessed. As such, your concerns are invalid -- the /proc files will change as quickly as the kernel becomes aware of changes.
Check this for more info about /proc file system.
In any case, I don't believe there's any alternative interface.
You might be looking for UPower: http://upower.freedesktop.org/
This is a common need for both desktop environments and mobile devices, so there have been many solutions over time. For example, one of the oldest ones was acpid, which is pretty much obsolete now.
While I'd recommend using a light-weight abstraction like UPower for code clarity reasons, the files in /proc and (to some extent) /sys are considered part of the Linux kernel ABI, which means that changing them is generally frowned upon.
I am wondering if it's possible to write an application that will access a foreign filesystem, but without needing support for that filesystem from the operating system. For example, I'd like to write an app in C that runs on Mac OS X that can browse / copy files from an ext2/ext3 formatted disk. Of course, you'd have to do all the transfers through the application (not through the system using cp or the Finder), but that would be OK for my purpose. Is this possible?
There are user space libraries that allow you to access file systems.
The Linux-NTFS library (libntfs) allows you to access NTFS file systems and there are user space programs like ntfsfix to do things to the file system.
E2fsprogs does the same for ext2, ext3 and ext4 filesystems.
As Basile mentioned, Mtools is another one that provides access to FAT partitions.
There was even a program that does exactly what you're looking for on Windows. It's called ext2explore and allows you to access ext2 partitions from Windows.
It is possible. For example the GNU mtools utility are doing that (assuming a way to access the raw device or partition) for MS-DOS FAT file systems.
However, file systems inside the kernel are usually very well tested and optimized.
Yes and No. For a regular user Application is usually not possible because access to block devices is restricted to root only. Every block device should give read/write to the needed block device for that effect. This would need at best a server/client approach where a service is started on the machine and configured to give the permissions on a per block device manner.
The somewhat easier alternative would be you to use the MacFUSE implementation.
Look here:
http://code.google.com/p/macfuse/
http://groups.google.com/group/macfuse?pli=1
The MacFuse project seems no longer mantained, but can give you a starting point for your project.
The dirty and quick approach is the following as root chmod 666 /dev/diskN
You can hijack syscalls and library calls from your application and then redirect reads/writes to anything like a KV store or a distributed DB layer (using the regular calls for the "virtual devices" that you do not support).
Then, the possibilities are boundless because you don't have to reach the physical/virtual devices when someone asks for them (resolving privilege issues).
I am wondering how the OS is reading/writing to the hard drive.
I would like as an exercise to implement a simple filesystem with no directories that can read and write files.
Where do I start?
Will C/C++ do the trick or do I have to go with a more low level approach?
Is it too much for one person to handle?
Take a look at FUSE: http://fuse.sourceforge.net/
This will allow you to write a filesystem without having to actually write a device driver. From there, I'd start with a single file. Basically create a file that's (for example) 100MB in length, then write your routines to read and write from that file.
Once you're happy with the results, then you can look into writing a device driver, and making your driver run against a physical disk.
The nice thing is you can use almost any language with FUSE, not just C/C++.
I found it quite easy to understand a simple filesystem while using the fat filesystem on the avr microcontroller.
http://elm-chan.org/fsw/ff/00index_e.html
Take look at the code you will figure out how fat works.
For learning the ideas of a file system it's not really necessary to use a disk i think. Just create an array of 512 byte byte-arrays. Just imagine this a your Harddisk an start to experiment a bit.
Also you may want to hava a look at some of the standard OS textbooks like http://codex.cs.yale.edu/avi/os-book/OS8/os8c/index.html
The answer to your first question, is that besides Fuse as someone else told you, you can also use Dokan that does the same for Windows, and from there is just a question of doing Reads and Writes to a physical partition (http://msdn.microsoft.com/en-us/library/aa363858%28v=vs.85%29.aspx (read particularly the section on Physical Disks and Volumes)).
Of course that in Linux or Unix besides using something like Fuse you only have to issue, a read or write call to the wanted device in /dev/xxx (if you are root), and in these terms the Unices are more friendly or more insecure depending on your point of view.
From there try to implement a simple filesystem like Fat, or something more exoteric like an tar filesystem, or even some simple filesystem based on Unix concepts like UFS or Minux, or just something that only logs the calls that are made and their arguments to a log file (and this will help you understand, the calls that are made to the filesystem driver during the regular use of your computer).
Now your second question (that is much more simple to answer), yes C/C++ will do the trick, since they are the lingua franca of system development, also a lot of your example code will be in C/C++ so you will at least read C/C++ in your development.
Now for your third question, yes, this is doable by one person, for example the ext filesystem (widely known in Linux world by it's successors as ext2 or ext3) was made by a single developer Theodore Ts'o, so don't think that these things aren't doable by a single person.
Now the final notes, remember that a real filesystem interacts with a lot of other subsystems in a regular kernel, for example, if you have a laptop and hibernate it the filesystem has to flush all changes made to the open files, if you have a pagefile on the partition or even if the pagefile has it's own filesystem, that will affect your filesystem, particularly the block sizes, since they will tend to be equal or powers of the page block size, because it's easy to just place a block from the filesystem on memory that by coincidence is equal to the page size (because that's just one transfer).
And also, security, since you will want to control the users and what files they read/write and that usually means that before opening a file, you will have to know what user is logged on, and what permissions he has for that file. And obviously without filesystem, users can't run any program or interact with the machine. Modern filesystem layers, also interact with the network subsystem due to the fact that there are network and distributed filesystems.
So if you want to go and learn about doing kernel filesystems, those are some of the things you will have to worry about (besides knowing a VFS interface)
P.S.: If you want to make Unix permissions work on Windows, you can use something like what MS uses for NFS on the server versions of windows (http://support.microsoft.com/kb/262965)