I'm looking to embed the Erlang VM into C code... Im familiar with ports, linkedin drivers, etc.
I want the C program to start the Erlang VM, and then pass messages to Erlang processes, and have those pass messages back to C code. I dont want the erlang VM to be the one that starts first and then invokes C code occasionally. I have my reasons.
I know that this negatively affects stability guarantees given by Erlang, meaning that when the master C code crashes it will taken down the Erlang VM also, since they're running in the same process. Im willing to live with those occurrences...
is this even possible?
The easiest way would be to just launch it as a separate process, and then use stdin and stdout to communicate. Just reading the docs of whatever platform you're targeting will let you know how to do that.
The only sane way to do this is to load the C code from the Erlang VM, not vice versa.
It's not possible out of the box, but since you have access to the Erlang source it's clearly possible to do whatever you want if you're willing to spend a lot of time modifying the code. It's not a good use of your time to go down this path.
Related
So if you want to parse command line options when starting the program you use getopt(). But how are you doing this if the program is already running in the background? I couldn't find info. let's say for example that you have a server running, but you want to change something in the way it works. How to do it? I want to do this in Linux.
There is no platform-independent way of doing this; the C programming language doesn't specify (or require the existance of) a mechanism to talk to a running program.
You're going to have to look for either platform-specific code, or some existing library which abstracts the platforms into something portable of its own.
In Linux, a Unix domain socket is one way of implementing this. Another is shared memory.
If you're on Un*x you have many options.
A FIFO pipe looks reasonable and easy to implement :)
There are a couple of ways you can do this, but they all have a common theme - Interprocess communication.
My preferred way to do this is via some sort of sockets (typically these days I use ZMQ for these purposes, but if you're starting out, read up on sockets in general before you get caught up using ZMQ). Depending whether you're on Windows or some sort of Unix will dictate what sort of sockets you have available to you.
There are other ways to do this also - such as shared memory, but sockets would be your best bet especially since you mentioned "server". I suggest you study the "client server model".
A simplest solution with server I have used - is to make file and ask server to read file once a 10 second. Put command there. That is cross platform).
Second more or less cross platform solution is to use some standard library for concurrency (pthread, for example), or to use new standard of C++ (thread and mutex libs). Make thread that will wait for some commands, while other will execute something.
You could use a configuration file and have the program listen to changes to that file.
If you are programming in Linux you can use inotify (#include <linux/inotify.h>).
In MacOS/iOS use FSEvents.
In Windows use FindFirstChangeNotification.
A little background, I'm a CMPE Student currently in an Operating Systems class. I have some basic knowledge of C coding but am more comfortable with C++ (taken about 3 semesters of that). Other than that, never had any other formal training in coding. Also, I've got a basic understanding of the linux environment.
I am working on a project that requires me and my team to code a linux kernel module that can do the following:
echoes data passed from user-level processes by printing the data received to the kernel log
is able to pass data from one user process to another.
must be possible to use the kernel module as an inter-process communication abstraction. module should provide for situations where a sender posts data to it but no receiver is waiting.module must cover the situation where a receiver asks for data but there is no data available.
module must cover the situation where a receiver asks for data but there is no data available.
must be a limit in the buffer capacity in your module.
Now I don't know how difficult this seems to those with a background in programming, but this seems like an impossibly complicated task for someone in my position.
Here's what I've done so far:
Coded, Compiled, Inserted, and Removed the basic "hello world" linux kernel module successfully
Read through about the first 4 or 5 chapters of The Linux Kernel Module Programming Guide
Read through a few stackoverflow posts, none of which seem to be able to direct me to where I need to go.
So finally here's my question: Can someone please point me in the direction that I need to go with this? I don't even know where to being to find commands to use for reading in user-level process data and I need somewhere to start me off. TLPD was great for insight on the topic but isn't helping me get to the point where I will have a workable project to turn in. In the past, I would learn off of reading source code and reverse engineering, is there anywhere I can find something like that? Any and all help is appreciated.
-Will
I've found that the Linux Kernel Module Programming Guide is a pretty good resource. From the sounds of it, something like a character device might work best for your purposes, but I'm not sure if you have other constraints.
Another direction I might consider (though this could be a bad path) is to look at examples in the Linux kernel for a kernel module that has similar functionality. I don't have a good example offhand, but perhaps look through /drivers/char/.
What you describe is pretty much the same as a pipe.
Read chapter three of Linux Device Drivers.
(But don't just copy the scull pipe example …)
I am working on an embedded project, where I need a program without external dependencies that works like screen or tmux. These two programs are not good because they need other libraries.
Since I only need to detach a program, being able to log-out and getting it back when I log-in again, I was wondering whether I can write a small program for that.
Do you know which calls (in C) I need to do to detach the program and to have it back?
If i understand your requirements correctly, you could theoretically use termios struct and ioctl to achieve this.
ioctl(0, TIOCNOTTY, NULL);
to detach and
ioctl(0, TIOCSCTTY, 1);
to attach back to the terminal. However, it doesn't actually perform the job.
The following solution describes a not so nice but practical work around
tty demulsified
The primary intention there is to attach program to another terminal but i believe that is the way you can achieve your goal too.
Regarding your mention of embedded platform, you might be able to find some help from busybox
It compiles for embedded linux with a pretty small binary and contains most of commonly used linux utilities such as getty, stty etc.
So, some background: I'm using a memory scanner called cheat engine to get real-time values for game stats (e.g. hp, mana, exp) in a non open-source video game that I'm trying to make a bot for.
For my bot to effectively use this information, I need to get it from cheat engine (or any memory scanner with similar functionality) to my bot code in a timely manner.
Now, one obvious way to do that would be to save all of the information to a file and then load the file in my bot code, but since this data needs updating about every half second or so, that isn't a real solution.
What I really need is either a terribly convenient memory scanner that allows you to use the information you uncover as a set of variables in some programming language (preferably java, c, or matlab), or a way to access the memory addresses found in one of the above languages.
This latter option should hopefully be doable, as cheat engine gives the memory address, controlling process ID, and data type.
This question doesn't have an easy answer. As far as I can tell you are very new to this area, so what you really need is a proper introduction to the subject, and for that I recommend reading Exploiting Online Games: Cheating Massively Distributed Systems.
It's an awesome book and it shows in a detailed manner how game hacks work, and it dedicates an entire chapter on how to build bots.
If you want to write an application to read/write data to those memory addresses you need to investigate functions like ReadProcessMemory() and WriteProcessMemory(). Whatever language you to choose to implement your bot needs to provide access to the Windows API. This is needed because you have to manipulate another process' memory space.
There are lots of tutorials out there that shows how to do this using C and C++, since they are the preferred languages to do this kind of stuff. Another option is to use a macro tool if you want something simple to play the game for you.
Modern computer games implement their own anti-cheat mechanisms to make it a little more difficult for people like you (and me). And since this book presents attack and defense techniques I recommend it to anyone interested on how to exploit computer games. The book is fully loaded with code examples.
I'm sorry for not providing more information but I was criticized once in the past for helping people with your curiosity and I would also never do a better job than the authors of the book explaining how to do this stuff.
Try using the Lua interface to get what you need.
Here's an example (I have not tried it, but I'm assuming it works....)
http://forum.cheatengine.org/viewtopic.php?t=530047
You can probably use COM with a script in Lua (with LuaCOM) on one side and Matlab or C on the other
You would need to use a debugging library to do that. You would set a watchpoint on your variable's location, and when it triggers, you would get its value.
scanmem does that for Linux.
Unfortunately, many closed-source games go to great lengths to avoid the use of debuggers, so this won't probably work on your game.
Have you tried Visual VM?
http://visualvm.java.net/download.html
Cheat Engine is open source, so what you do is look in the source code of Cheat Engine, and look how the memory dump thing works.
However, it is highly non-trivial to monitor a live process that you are not controlling, so unless you are a black hat code wizard, level 11, I suspect that it won't work.
Even if You say You want to look at a few integers that you can guess looking at memory dump, it is a lot harder to find that area programmatically, consistently, while possibly retracking every so often as the data may be copied or moved when the state of the program changes.
Also read this encouraging citation from Cheat Engines FAQ:
Q:Will Cheat Engine work on online games?
A:Most of the time, no
But anyway, try it - it sounds fun and I am sure you will learn something, and there is always a chance that you'll make it work :-)
I have written a small custom web server application in C running on Linux. When the application receives a request it calls fork() and handles the request in a separate process, which is chrooted into a specific directory containing the files I want to make available.
I want to port the application to Windows, but neither fork() nor chroot() are available on this platform, and there don't seem to be any direct equivalents. Can you point me to a simple (and preferably well written) example of code that will provide this functionality in Windows? My C isn't all that good, so the simpler the better.
First of all, the Windows equivalent of chroot is RUNAS which is documented here. If you need to do this from a program, then studying this C++ source code should help you understand how to use the Windows API. It is not precisely the same as chroot() but Windows folk use it to create something like a chroot jail by creating a user with extremely limited permissions and only giving that user read permission on the application folder, and write permission on one folder for data.
You probably don't want to exactly emulate fork() on Windows because it doesn't sound like you need to go that far. To understand the Windows API for creating processes and how it differs from fork(), check Mr. Peabody Explains fork(). The actual current source code for Cygwin's fork implementation shows you the current state of the art.
The Microsoft documentation for CreateProcess() and CreateThread() are the place to look for more info on the differences between them.
And finally, if you don't want to learn all the nitty-gritty platform details, just write portable programs that work on Windows and Unix, why not just use the Apache Portable Runtime library itself. Here are some docs on process creation with some sample code, in C, to create a new process.
There's no such thing as fork() on Windows. You need to call CreateProcess() - this will start a separate process (mostly equivalent to calling fork() and then immediately exec() for the spawned process) and pass the parameters to it somehow. Since you seem to have all the data to process in a dedicated directory you can make use of lpCurrentDirectory parameter of CreateProcess() - just pass the directory path you previously used with chroot() there.
The absolutely simplest way of doing it is using Cygwin, the free Unix emulation layer for Windows. Download it and install a complete development environment. (Choose in the installer.) If you are lucky, you will be able to compile your program as is, no changes at all.
Of course there are downsides and some might consider this "cheating" but you asked for the simplest solution.
Without using a compatibility framework (Interix, Cygwin, ...) you're looking at using the Windows paradigm for this sort of thing.
fork/vfork is a cheap operation on UNIXes, which is why it's used often compared to multi-threading. the Windows equivalent - CreateProcess() - is by comparison an expensive operation, and for this reason you should look at using threads instead, creating them with CreateThread(). There's a lot of example code out there for CreateThread().
In terms of chroot(), Windows doesn't have this concept. There's libraries out there that claim to emulate what you need. However it depends why you want to chroot in the first place.
Reading comments, if it's simply to stop people going up the tree with ../../../../(etc), chroot would do the job, but it's no substitue for parsing input in the first place and making sure it's sane: i.e., if too many parents are specified, lock the user into a known root directory. Apache almost certainly does this as I've never had to create a chroot() environment for Apache to work...
Using fork/chroot is simply not how things are done on Windows. If you are concerned about security in subprocesses, maybe some form of virtualization or sandboxing is what you want to use. Passing complex information to the subprocess can be done by some form of RPC-solution.
It sounds to me as if you have designed your application in the Unix way, and now you want to run in on Windows without having to change anything. In that case, you may want to consider using Cygwin, but I'm not sure if/how Cygwin emulates chroot.
Consider SUA ( aka Windows Services for Unix ). It has nearly everything you need to port applications.
man chroot(interix)