TL; DR: how hard it is to port Python to new OS?
I want to use python to write applications for Verifone's VX 680. They are 32-bit ARM based devices with 128+MB of RAM. http://www.verifone.com/media/4300697/vx680_ds_ltr.pdf
My idea is to write a C application which calls Python interpreter. My application will be a bunch of python modules. The app needs to show graph rich UI, send HTTPS messages, access peripherals (e.g. WiFi radio, PinPad, thermal printer). Despite of my investigation, I'm completely lost still.
What is the list of things I need to address in order to be able to write python applications in this device?
I have personally ported CPython for my own operating system; the real problem was the lack of cross-compiling support really - I found patches for 2.5.1 to make it cross-compile cleanly.
After it was compiling cleanly, I just needed to provide quite a minimal set of system calls to work. For anything serious at least a read-only filesystem is a must. In any case, if your libc is POSIXish, you shouldn't have too many problems to get started.
The set of system calls I had in the beginning was exit, open, close, read (for console and files), write (to file descriptors 1 and 2 only), stat, fstat and sbrk (for changing the heap size). I used the newlibc C library with libgloss - everything that wasn't mapped to these, returned just the error values or defaults.
i've got a script that uses the resource-module from python (see http://docs.python.org/library/resource.html for information). Now i want to port this script to windows. is there any alternative version of this (the python-docs are labeling it as "unix only").
if there isn't, is there any other workaround?
I'm using the following method/constant:
resource.getrusage(resource.RUSAGE_CHILDREN)
resource.RLIMIT_CPU
Thank you
PS: I'm using python 2.7 / 3.2
There's no good way of doing this generically for all "Resources"" -- hence why it's a Unix only command. For CPU speed only you can either use registry keys to set the process id limit:
http://technet.microsoft.com/en-us/library/ff384148%28WS.10%29.aspx
As done here:
http://code.activestate.com/recipes/286159/
IMPORTANT: Backup your registry before trying anything with registry
Or you could set the thread priority:
http://msdn.microsoft.com/en-us/library/ms685100%28VS.85%29.aspx
As done here:
http://nullege.com/codes/search/win32process.SetThreadPriority
For other resources you'll have to scrap together similar DLL access APIs to achieve the desired effect. You should first ask yourself if you need this behavior. Oftentimes you can limit CPU time by sleeping the thread in operation at convenient times to allow the OS to swap processes and memory controls can be done problematically to check data structure sizes.
Sorry wasn't sure how to best word this question.
My scenario is that I have some python code (on a linux machine) that uses an xml file to acquire its arguements to perform a task, on completion of the task it disposes of the xml file and waits for another xml file to arrive to do it all over again.
I'm trying to find out the best way to be alerted an xml file has arrived in a specified folder.
On way would be to continually monitor the folder in the Python code, but that would mean a lot of excess resourses used while waiting for something to turn up (which may be as little as a few times a day). Another way, would be to set up a cronjob, but it's efficiency would't be any better than monitoring from within the code. An option I was hoping was possible would be to set up some sort of interrupt that would alert the code when an xml file appeared.
Any thoughts?
Thanks.
If you're looking for something "easy" to just run a specific script when new files arrive, the incron daemon provides a very handy combination of inotify(7) and cron(8)-like support for executing programs on demand.
If you want something a little better integrated into your application, or if you can't afford the constant fork(2) and execve(2) of the incron approach, then you should probably use the inotify(7) interface directly in your script. The pyinotify module can integrate with the underlying inotify(7) interfaces.
This answer, stating that the naming of classes in Python is not done because of special privileges, here confuses me.
How can I access lower rings in Python?
Is the low-level io for accessing lower level rings?
If it is, which rings I can access with that?
Is the statement "This function is intended for low-level I/O." referring to lower level rings or to something else?
C tends to be prominent language in os -programming. When there is the OS -class in Python, does it mean that I can access C -code through that class?
Suppose I am playing with bizarre machine-language code and I want to somehow understand what it means. Are there some tools in Python which I can use to analyze such things? If there is not, is there some way that I could still use Python to control some tool which controls the bizarre machine language? [ctypes suggested in comments]
If Python has nothing to do with the low-level privileged stuff, do it still offers some wrappers to control the privileged?
Windows and Linux both use ring 0 for kernel code and ring 3 for user processes. The advantage of this is that user processes can be isolated from one another, so the system continues to run even if a process crashes. By contrast, a bug in ring 0 code can potentially crash the entire machine.
One of the reasons ring 0 code is so critical is that it can access hardware directly. By contrast, when a user-mode (ring 3) process needs to read some data from a disk:
the process executes a special instruction telling the CPU it wants to make a system call
CPU switches to ring 0 and starts executing kernel code
kernel checks that the process is allowed to perform the operation
if permitted, the operation is carried out
kernel tells the CPU it has finished
CPU switches back to ring 3 and returns control to the process
Processes belonging to "privileged" users (e.g. root/Administrator) run in ring 3 just like any other user-mode code; the only difference is that the check at step 3 always succeeds. This is a good thing because:
root-owned processes can crash without taking the entire system down
many user-mode features are unavailable in the kernel, e.g. swappable memory, private address space
As for running Python code in lower rings - kernel-mode is a very different environment, and the Python interpreter simply isn't designed to run in it, e.g. the procedure for allocating memory is completely different.
In the other question you reference, both os.open() and open() end up making the open() system call, which checks whether the process is allowed to open the corresponding file and performs the actual operation.
I think SimonJ's answer is very good, but I'm going to post my own because from your comments it appears you're not quite understanding things.
Firstly, when you boot an operating system, what you're doing is loading the kernel into memory and saying "start executing at address X". The kernel, that code, is essentially just a program, but of course nothing else is loaded, so if it wants to do anything it has to know the exact commands for the specific hardware it has attached to it.
You don't have to run a kernel. If you know how to control all the attached hardware, you don't need one, in fact. However, it was rapidly realised way back when that there are many types of hardware one might face and having an identical interface across systems to program against would make code portable and generally help get things done faster.
So the function of the kernel, then, is to control all the hardware attached to the system and present it in a common interface, called an API (application programming interface). Code for programs that run on the system don't talk directly to hardware. They talk to the kernel. So user land programs don't need to know how to ask a specific hard disk to read sector 0x213E or whatever, but the kernel does.
Now, the description of ring 3 provided in SimonJ's answer is how userland is implemented - with isolated, unprivileged processes with virtual private address spaces that cannot interfere with each other, for the benefits he describes.
There's also another level of complexity in here, namely the concept of permissions. Most operating systems have some form of access control, whereby "administrators" have total control of the system and "users" have a restricted subset of options. So a kernel request to open a file belonging to an administrator should fail under this sort of approach. The user who runs the program forms part of the program's context, if you like, and what the program can do is constrained by what that user can do.
Most of what you could ever want to achieve (unless your intention is to write a kernel) can be done in userland as the root/administrator user, where the kernel does not deny any API requests made to it. It's still a userland program. It's still a ring 3 program. But for most (nearly all) uses it is sufficient. A lot can be achieved as a non-root/administrative user.
That applies to the python interpreter and by extension all python code running on that interpreter.
Let's deal with some uncertainties:
The naming of os and sys I think is because these are "systems" tasks (as opposed to say urllib2). They give you ways to manipulate and open files, for example. However, these go through the python interpreter which in turn makes a call to the kernel.
I do not know of any kernel-mode python implementations. Therefore to my knowledge there is no way to write code in python that will run in the kernel (linux/windows).
There are two types of privileged: privileged in terms of hardware access and privileged in terms of the access control system provided by the kernel. Python can be run as root/an administrator (indeed on Linux many of the administration gui tools are written in python), so in a sense it can access privileged code.
Writing a C extension or controlling a C application to Python would ostensibly mean you are either using code added to the interpreter (userland) or controlling another userland application. However, if you wrote a kernel module in C (Linux) or a Driver in C (Windows) it would be possible to load that driver and interact with it via the kernel APIs from python. An example might be creating a /proc entry in C and then having your python application pass messages via read/write to that /proc entry (which the kernel module would have to handle via a write/read handler. Essentially, you write the code you want to run in kernel space and basically add/extend the kernel API in one of many ways so that your program can interact with that code.
"Low-level" IO means having more control over the type of IO that takes place and how you get that data from the operating system. It is low level compared to higher level functions still in Python that give you easier ways to read files (convenience at the cost of control). It is comparable to the difference between read() calls and fread() or fscanf() in C.
Health warning: Writing kernel modules, if you get it wrong, will at best result in that module not being properly loaded; at worst your system will panic/bluescreen and you'll have to reboot.
The final point about machine instructions I cannot answer here. It's a totally separate question and it depends. There are many tools capable of analysing code like that I'm sure, but I'm not a reverse engineer. However, I do know that many of these tools (gdb, valgrind) e.g. tools that hook into binary code do not need kernel modules to do their work.
You can use inpout library http://logix4u.net/parallel-port/index.php
import ctypes
#Example of strobing data out with nStrobe pin (note - inverted)
#Get 50kbaud without the read, 30kbaud with
read = []
for n in range(4):
ctypes.windll.inpout32.Out32(0x37a, 1)
ctypes.windll.inpout32.Out32(0x378, n)
read.append(ctypes.windll.inpout32.Inp32(0x378)) #Dummy read to see what is going on
ctypes.windll.inpout32.Out32(0x37a, 0)
print read
[note: I was wrong. usermode code can no longer access ring 0 on modern unix systems. -- jc 2019-01-17]
I've forgotten what little I ever knew about Windows privileges. In all Unix systems with which I'm familiar, the root user can access all ring0 privileges. But I can't think of any mapping of Python modules with privilege rings.
That is, the 'os' and 'sys' modules don't give you any special privileges. You have them, or not, due to your login credentials.
How can I access lower rings in Python?
ctypes
Is the low-level io for accessing lower level rings?
No.
Is the statement "This function is intended for low-level I/O." referring to lower level rings or to something else?
Something else.
C tends to be prominent language in os -programming. When there is the OS -class in Python, does it mean that I can access C -code through that class?
All of CPython is implemented in C.
The os module (it's not a class, it's a module) is for accessing OS API's. C has nothing to do with access to OS API's. Python accesses the API's "directly".
Suppose I am playing with bizarre machine-language code and I want to somehow understand what it means. Are there some tools in Python which I can use to analyze such things?
"playing with"?
"understand what it means"? is your problem. You read the code, you understand it. Whether or not Python can help is impossible to say. What don't you understand?
If there is not, is there some way that I could still use Python to control some tool which controls the bizarre machine language? [ctypes suggested in comments]
ctypes
If Python has nothing to do with the low-level privileged stuff, do it still offers some wrappers to control the privileged?
You don't "wrap" things to control privileges.
Most OS's work like this.
You grant privileges to a user account.
The OS API's check the privileges granted to the user making the OS API request.
If the user has the privileges, the OS API works.
If the user lacks the privileges, the OS API raises an exception.
That's all there is to it.
Is there any way to monitor /proc files, such as
/proc/acpi/battery/BAT0/state
/proc/acpi/ac_adapter/ADP0/state
in a non-polling fashion, similar to inotify on a normal filesystem?
I want to do this in a PyGTK app, so I tried using PyGObject's gio.FileMonitor, but no dice. A Python solution that plays well with gtk.main() would be ideal.
Probably you can get the information you want by listening to the ACPI events. Preferably not directly (/proc/acpi/event), but via acpid or other high-level interface.
Update: the other, higher level interface is the DBus interface provided by DeviceKit-power / UPower.
Files in /proc are not regular files, rather a simple interface to kernel state, so many facilities for regular files won't work there.