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I've always wanted to learn how to manipulate .DLLs. I looking for tutorials, but I couldn't find a single one that can explain everything to me line-by-line. If there is a chance that I would find something, it wouldn't be in C (I'm looking for C tuts. Not C++ ones.)
I will explain in a general way how they work here. For the whole story, start at the MSDN documentation of DLLs. Also read documentation for your C compiler for the specific details about how to create and use them. And when you have trouble, ask specific questions at SO with supporting code so we can be more helpful.
At the simplest level (and deliberately omitting details) a DLL is a collection of functions that can be loaded into a process's memory space at run time with LoadLibrary() and subsequently called by the process.
The DLL file itself is formatted like a normal Windows executable file, but it does not have WinMain or main and as a result cannot be invoked without a container process. So it is created with the linker, just like any other executable. (This is a small lie. There actually is an entry point called DllMain(), but it is used for advanced features, is called when the DLL is loaded and unloaded, and still doesn't allow a DLL to be invoked directly from the command prompt or "executed" by CreateProcess() or similar API calls.)
DLLs can be loaded automatically at run time. This is the most common form, and allows the C runtime library to be loaded by every program from a single common copy. To do this, you simply link the application with the right libraries and options, and the system takes care of loading the DLL and fixing up all symbols that point into it.
DLLs can also be loaded at run time. This is commonly done to allow extensions to an existing application. One classic example of this is a Windows screen saver. A .SCR file is really just a renamed DLL that exports certain functions that provide the features of the screen saver. The window manager looks up which screen saver is configured at run time, load its .SCR file as a DLL, and calls the functions it provides.
You load a DLL by calling LoadLibrary(), and you find out the addresses of functions it contains with with GetProcAddress(). Note that it is entirely the responsibility of the calling application to know how to call the function, and there are lots of interesting ways that can go wrong. The most obvious case is that you must provide the expected number of parameters of the right types expected by the function in the DLL.
One of the reasons that COM was invented was to provide a framework that would allow the DLL to describe what functions it offers and the application to ask what parameters are expected by any particular function. The .NET framework provides the same sorts of introspective features but in many ways much more successfully. None of that sort of protection is available (another lie, it is, but it is very tricky to implement COM in pure C) to a plain C program and a DLL written in plain C.
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I would like to customize the login screen in a Linux OS. Not just the bacground image, but everything from buttons to textboxes, even the login method (so not a username + password, but something of my own).
The flavor of Linux doesn't really matter, it only needs to have a graphical interface. I have some experience with Ubuntu though.
However, this would be my first time developing something for Linux -- I have developed applications for Windows before, mostly in C# -- but I am up to the challange to learn something new. So please try to be a bit easier on me, me being something of a noob at the moment.
So, how can I do this?
The "login screen" shown on GNU/Linux systems to login to a graphical session is part of the desktop environment you are using. Various alternatives exist for the zoo of alternative environments out there. Examples are kdm as offered by the KDE desktop environment, gdm as offered by Gnome, xdm, wdm, many others exist. Start by reading the wikipedia article about kdm to get an impression.
You'd have to implement a replacement for that part which serves the same purpose, if the existing options to configure it are not enough. So best probably is to start reading about those. That should give you an impression about the requirements.
Roughly explained you have to create an executable that can be executed by the system and configure that as "desktop manager" in the systems configuration. The exact location of that configuration again depends on the distribution, but the general approach always is the same.
The logic of such a program has to somehow answer the question of whether a user is allowed to login to the system by whatever means chosen. Typically this is done by using PAM in background ("Pluggable authentication modules"). Maybe start by reading an introduction to PAM to gain insight into how things work, which may be a bit confusing at first glance due to its endless flexibility. That framework is used on modern unixoid systems to separate the requirements for that logic from the actual implementation. So you really want to learn about how PAM works too. It is an unbelievable flexible framework where already endless strategies exist, so countless adapters for all sorts of existing authorities like databases, LDAP servers, simple file based authorities, IMAP servers, you name it: everything you can imagine. So chances are that you do not have to implement anything, but just can pick something implemented by someone else and configure / tailor it to your needs.
The logic itself does not have to be complex, it just has to answer that simple question in an clear way: is the user who requests access granted or not. For that it will (or will not) have to consult some existing authority. How exactly that works obviously is completely up to you. The real difficult thing probably is to access and control the graphics hardware without having a running graphical desktop environment to build on (since that is only started afterwards). So you probably have to deal with some hardware abstraction layer. Which again means that you probably want to be using some language like C or C++ for that job, scripting languages are less well suited for such job, also because they introduce all sorts of additional requirements of what has to be installed on a system which becomes unusable if some element inside that chain of utilities break due to the lack of a login screen :-)
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After some time coding in c in a simple console, I decided I wanted to try and code an actual Win32 application. However, upon selecting the option, the sheer amount of unknown code that surfaced on my IDE (Visual Studio 2013) just to open a blank window was overwhelming, as I don't understand half of it's meaning or even what to do, since, even simple printf commands yield no result... Can someone point me to a way to understand the differences between console and application? Or at least someway I can insert my current coding knowledge in an application environment?
Working with the Windows API can be quite the experience for a newcomer. The act of opening a window does, certainly, involve a lot of boilerplate code. The good news is a lot of that boilerplate rarely changes, and when it does it'll be for very special circumstances that you can go research. That's why Visual Studio spits the whole mess out for you by default.
There are plenty of resources to learn with, including Microsoft itself: https://msdn.microsoft.com/en-us/library/bb384843.aspx
To help situate you, understand that a lot of what you're looking at that's probably confusing are typedefs used on primitive types. This helps maintain backwards compatibility with programs created for older versions of Windows, and adds semantic meanings to types. A lot of the preprocessor stuff you see in header files is actually conditional compilation - it's checking what environment you're compiling in, what version of Windows, etc. and then providing the correct typedefs so that what actually compiles works on the desired Windows platform. Stuff that's greyed out in VS2013 is stuff that Intellisense sees isn't going to be compiled based on the current #defines and a lot of it may be relatively ancient. A big part of the Windows API for a long time was a strong desire to not break backwards compatibility. This was a huge advantage for Windows, because it means my program you wrote for Windows 3.1 isn't going to be hosed by Windows 95 rolling out. Or XP, 2000, Vista, 7, 8, 10, etc. It does mean a lot of stuff makes it into the API and stays there.
They try to hide a lot of that in the headers, but you'll also see deprecated input variables to functions and the like (this parameter should always be NULL...etc), and you just have to read the documentation. Thank the internet.
For example, an LPCSTR is just a const char*, but the LPCSTR signifies that it's a null-terminated constant string. In fact, you may see that's it's not a const char*, but a CONST CHAR* where CONST and CHAR are #defines themselves to make sure the correct keywords and types are used. In your case it'll probably end up being just normal const char*.
My suggestion, rather than diving into about as complicated a C-API as possible, is to look at something like SDL. SDL is a much simpler C API designed to provide an interface to the operating system's windowing, graphics, and input, while hiding the dirty details of the API, be it Windows, Mac, or Linux. https://www.libsdl.org/
It uses openGL for its graphics. If you're making any kind of game you'll be using some kind of graphics API to talk to the video card. The native Windows graphics API is DirectX, but openGL is the very commonly used cross-platform API. Both APIs allow you to make use of a computer's video hardware to render graphics.
Edit: I'll add, since I went off and voiced an opinion on libraries, which is why these types of questions are probably frowned upon, that I think it's fair to say that SDL is the de facto standard for C third-party multimedia libraries. Also commonly mentioned is SFML, which provides much the same functionality but is more object-oriented and written in C++.
In Visual Studio, when you create a win32 app project, click on next, and click on empty project. This will eliminate all the stuff that Visual Studio creates for a win32 app, but then you start with no files. You'll probably want to start with some simple example C program for windows from a book or web site.
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There are websites as collections of python/perl/R libraries. Why there is not an equivalent one for c?
I searched the internet and only found a small website calling itself CCAN. There are only a few libraries in that website.
If I need extra libraries for c programming, where can I find them? Is there an well organized website of the c libraries?
Thanks.
If I need extra libraries for c programming, where can I find them? Is there an well organized website of the c libraries?
No known to me outside of CCAN.
The problem here is that C doesn't have any even loose specification for libraries. Compare that to e.g. packages in Java or Python or Perl.
And even then, C is quite bare bone itself leaving many things for libraries to implement themselves. I/O abstraction, memory management, multi-threading, OS integration - minor differences in how libraries work with any of the resources might make them incompatible, preventing them being used in the same project.
I have seen in past some 3rd party commercial libraries for C, covering quite a lot of functionality, but frankly I can't recommend them and honestly do not even remember their names - for they often were causing more problems than really helping. (OK, I'm lying: they were rarely causing unsolvable problems: it's the numerous workarounds which were causing often the problems later.)
Otherwise, for C you might want to check the Glib and (do not get me wrong) to also check the C standard as in my experience few actually know many of the utilities already in the standard library itself. And well, Google is your friend: lots of public domain code is there for you to simply throw as-is into your project.
I don't know of anybody who's studied this in detail, though I would be curious to see the studies. I'm sure it has to do with the nature of the C programming community itself.
I think a large (maybe the primary?) part of the answer is: before the WWW, there was no such thing as a single resource for obtaining libraries for a particular language. People obtained their libraries, and knowledge of libraries, via many different means: through BBSes, mailing lists, newsgroups, and periodicals. The C community dates from this time, of course, and I've noticed a similar difference in culture regarding other languages from this period and before.
I think another part of the answer has to do with the general decentralization of C culture itself. There's no one C compiler, no one C development community, that serves as a hub and a potential point for projects to attach themselves to. And the C development community is huge, which further drives this decentralization and splintering.
In the case of C libraries, OS distributions actually do a pretty good job of collecting useful C/C++ libraries out there. (With the unfortunate exception of Windows, I believe.) They do a better job in these languages than most others, probably since C and C++ are such important systems languages on these platforms.
As far as CCAN goes, I think what would make a more worthwhile project, given the number of different distributors of C code out there, is to have a single site that links to the various libraries on their own native sites, rather than trying to get them to upload straight to CCAN. I think there's a use for this in and apart from Google, which will give you a lot of noise if you try just browsing for libraries. The question is, would you and the bulk of the C communities out there embrace such a site if it existed?
You might be amused to see how CPAN got its start: http://www.brainbell.com/tutors/Perl/CPAN_History.htm
CPAN evolved just as its community did. So the same thing could happen in the C/C++ world if the leadership and interest is there. But it hasn't happened yet.
use http://www.google.com/codesearch?q=lang:%22C%22 variant of http://www.google.com/codesearch
=> i.e. add lang:"C" in the search query
Use these web-sites:
Debian "Testing": Subsection libs
The Free Country: Free C/C++ Libraries
Free Software Directory: Category/Library
SourceForge Software Map: Software Development/Libraries/C
Upstream Tracker: List of C/C++ Shared Libraries
There is a Maven-like repository and dependency management system called Biicode.
There isn't a huge collection of libraries on there yet, but you can add forks of open source projects yourself or inform original authors about it.
EDIT: the company behind biicode is dead
EDIT2: the spiritual successor seems to be conan.io
There is a C package manager which looks promising called CLib:
github:
https://github.com/clibs/clib
tutorial:
https://dev.to/noah11012/clibs-a-package-manager-for-c-4jmi
Why do you need a website for a collection of C libraries? Just use Google.
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I'm not going to go into detail on the "media player" part except the fact that it will obviously use plug-ins, which will be a simple dynamic library that is loaded at runtime. Now, I could link those plug-ins dynamically to their dependencies, or I could link them statically. Both have their advantages and disadvantage - I'm not counting Linux here, as that will use shared libraries.
The single advantage I see with using shared libraries is that the library can be updated independently of the program. On Windows, this is rarely an advantage, as the library will be next to the application using it (thanks to no official C++ ABI). On Windows, to help with reducing DLL hell and share the C libaries, I would have to use SxS, which isn't a really nice citizen.
As for static libraries, I see one big advantage: link-time optimizations. Those ave been supported by ICC and VC++ for quite a while now and GCC has a branch for them. Since I will probably be using VC++ on Windows there would be a noticeable performance improvement as the compiler (well, the actual "compiler" just converts C++ to an intermediate language, so the compiler here is the "linker") has perfect knowledge of the code and can optimize lots of stuff this way. This is the option I'm leaning towards.
My question is, which one would be the best in my specific case?
There is no concern of other applications using them as I don't count Linux in this issue (though I have no knowledge of OS X) or multiple instances (who runs the same media player twice anyway?), binary compatibility (as I'll distribute everything with the application) or easy of updating (on Windows I'll use a very efficient binary diff patcher for distributing updates).
I'm not completely sure of what your "specific" case is.
If your "media player" is intended for a well know unique client (or small set of clients) who will update all the media player at once or any given plugin at once under your complete supervision then I'll go for static libraries.
If that's not the case I'll go for dynamic libraries. Optimizations are good but are not as good as customer/user satisfaction. There's nothing worse than updating your xxx library to the latest version and that all the sudden things just stop to work. If you haven't control over when and how updates are made be as flexible as you can.
Response to the comment:
Usually dynamic libraries are backwards compatible for minor release versions while static linking might be dependent on a concrete version and break if you try to link it to another release. With Dynamic Linking your program might even work as long as the calls you use doesn't change while static linking might depend on a change of the function offset in the library.
For example, static libraries might be loaded in runtime into the address space of the process at an static offset. Of course, knowing this offset allows for certain optimizations but if you update the library then either you update ALL the plugins using that library or probably the unupdated plugins won't work (as function offset might very well have changed).
I assume you load them in runtime, although if that can be named static linking is in the air, in any other case you will just have the library on each plugin and there will be no "sharing".
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Any online C compiler that you know which can do the following:
Compile and execute the C programs online
File handling
System functions like exec(), system(), fork()
Or any compiler which does not need installation procedure (I mean that you can just copy paste a folder to run the compiler easily)
Note:Please do not propose Turbo C.I know some online compilers at codepad.org (gcc).
I was using codeide.com but its out of service now I think.
And as asked above online compiler will be a best advice that you can give for me.
My environment is Windows ... Thanks in advance.
Any advices on other Language compilers are also invited
Just install something like tcc. It's seriously not worth messing around with an online compiling site if you're going to be compiling files on any sort of regular basis.
Comeau has an online C/C++ compiler, but it's mainly to evaluate their compiler.
or cygwin
I'd rather recommend to install cygwin, you'll get an unix-like environment, with gcc. Then setting up a Makefile - or even just a shell script - to be able to compile is not a big deal.
You asked for other languages: Here's one for Lisp (not a compiler, but an interpreter)
Online Lisp interpreter in Flash
rextester
Maybe my answer is a bit late, but we have created an online compiler and IDE where you can run and create your files using just a browser.
At the moment we support a few languages (C, C++, ObjC, Java, Pascal, Fortran) and a simple file system, but we will enable many more features for better coding and debugging during the near future.
All the features offered at the moment are completely free and there is no registration needed (you can register though in order to keep your files online).
You can try our service here: www.sourcelair.com
DJGPP Public Access Cross-Compiler (C/C++, DOS32, based on GCC)
yet another online compiler:
http://cmpe150-1.cmpe.boun.edu.tr
It supports sytax highlight, indentation etc. I wrote it as a part of my MS thesis
PS: did not test fork command
We have another compiler for C and C++ here: http://www.codepad.org/
I have nothing for C. For other languages, this is a ruby interpreter. But honestly the best online development environment is the browser itself. Javascript is an advanced language. Combined with technologies like CSS and DHTML and frameworks like jQuery or Prototype you can build graphics applications. It is not hard to find debuggers (like Firebug) also.
Of course you can't interact with the file system. To overcome this you could write a plug-in for a browser (notably Firefox). There are many resources available for this and although it is not as straight forward as pure javascript, it is easier than most people believe.
For Python and Sage, try out http://live.codenode.org. It is also open source under the BSD license, so you can be download and run it from your own computer, more info is here: http://codenode.org
just a sudgestion.
I do not know why you do not want simply to install a compiler. However did you considered the possibility of using a portable one?
http://en.wikipedia.org/wiki/Portable_C_Compiler
best,
Ste
One of the better lists for C++ is isocpp Getting Started page. Unfortunately Cameau's seems to be disabled for now. LiveWorkspace has been in maintenance mode for a while and it is not clear when it is coming back, which is unfortunate since it has a simple interface and when it was working allowed you to switch between gcc, clang and intel very easily.
Of the ones that are left Coliru is the most powerful, you have a full command line available and you can save files and therefore uses multiple files in your project.
The isocpp list somehow is missing codepad which although rather primitive along with Coliru allows you to use boost.
Coliru, ideone and codepad all support many other languages as well. The list of languages supported by ideone and codepad is pretty large and is obvious on the main pages with Coliru you don't have a list but besides C and C++ it also supports python, perl and ruby.