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I'm assigned to do a project that consists in changing the quantization in the JPEG source-code, from the quantization tables to Lloyd-Max quantization. The problem is not knowing what to do (I know how to change the quantization), but where to find the code I'm suposed to change.
If someone is familiar with the libjpeg-turbo, could you give me some advice on doing so?
I refrained from responding because it has been a long time since I have prowled around in the LIBJPEG code and I understand that it has been rewritten. The code functions well and is efficient but it is quite torturous to read and understand.
This is a C++ library that apparently was written for instructive purposes. For understandability it is about as good as you are going to get with JPEG:
http://www.colosseumbuilders.com/sourcecode/imagelib403.zip
However, if I remember correctly, this one, like LIbJPEG, combines some steps of the DCT and quantization.
I want to learn hardcore c programming used in Linux kernel but when i read those stuff everything goes on the top of my head. i am not able to understand the code and dat structures they use with pointer to pointer function. its all very confusing with me. I have been trying for long time but i am not able to find a resource where someone has fullyy documented the harware interaction code explaining each and every line of code.
Can anyone point me in right direction how should i go
You've got two problems: understanding some quite sophisticated language constructs: pointers to function pointers I would guess are probably only a small part, I'll bet there are some much gnarlier things going on when you have multiple threads of control. Then there is the problem domain to which these techniques are being applied - really low level stuff interacting with hardware.
You are probably being unrealistic in expecting to learn these two things at the same time. To take an analogy, imagine asking for something to explain Shakespear's plays (say Merchant of Venice) line by line, to someone who neither speaks English, not understands the concepts of lending money or of a legal system.
My recommendation: Study C coding in detail until you understand in general how to use function pointers, and more important why you use them, how to write multi-threaded code, why you need concepts such a mutices and sempahores. Then also read about the general principles of low level programming, for example Deitel & Deitel covers a lot of material about OS development.
I learned a lot about writing modules by reading this book (it's free): http://lwn.net/Kernel/LDD3/
It is a very understandable introduction to kernel development.
You might also want to have a look at this old paper:
http://cm.bell-labs.com/cm/cs/who/dmr/cacm.html
It explains very concisely the concepts in Unix and may help you to keep things simple.
It is summer, and so I have decided to take it upon myself to write a data-compression program, preferably in C code. I have a decent beginners understanding of how compression works. I just have a few questions:
1) Would c be a suitable programming language to accomplish this task?
2) Should I be working in byte's with the input file? Or at a binary level somehow?
If someone could just give me a nudge in the correct direction, I'd really appreciate it. I would like to code this myself however, and not use a pre-existing compression library or anything like that.
You could start by looking at Huffman Encoding. A lot of computer science classes implement that as a project so it should be manageable. C would be appropriate for Huffman encoding, but it might be easier to do it first in a higher-level language so that you understand the concepts.There are slides, hints, and an example project available in Java for a masters-level project at the University of Pennsylvania (search for "huff" on that page).
To answer your questions:
C is suitable.
It depends on the algorithm, or the way you are thinking about `compression'.
My opinion will be, first decide whether you want to do a lossless compression or a lossy compression, then pick an algorithm to implement. Here are a few pointers:
For the lossless one, some are very intuitive, such as the run-length encoding,
e.g., if there is 11 as and 5 bs, you just encode them as 11a5b.
Some algorithms use a dictionary, please refer to LZW encoding.
Finally, I do recommend Huffman encoding since it is very straight-forward, simple and helpful to gain experience in learning algorithm (for your educational purpose).
For lossy ones, Discrete Fourier Transform (DFT), or wavelet, is used in JPEG compression. This is useful to understand multimedia compression.
Wikipedia page is a good starting point.
Yes, C is well suited for this kind of work.
Whether you work with bytes or bits will depend on the algorithm that you decide to implement. For example, Huffman coding is inherently bit-oriented whereas many other compression algorithms are not.
C is a great choice for writing a compression program. You can use plenty of other languages too, though.
Your computer probably can't directly address units of memory smaller than a byte (pretty much by definition), so working with bytes is probably a good choice. Some of how you work with the data will be affected by the compression algorithm you choose.
Good luck!
1) Would c be a suitable programming language to accomplish this task?
Yes.
2) Should I be working in byte's with the input file? Or at a binary level somehow?
They're the same, so the question makes no sense.
not use a pre-existing compression library
Can you use a pre-existing compression algorithm? There are dozens and "compression algorithm" -- when used with Google -- will reveal a great deal of helpful information.
I know about the existance of question such as this one and this one. Let me explain.
Afet reading Joel's article Back to Basics and seeing many similar questions on SO, I've begun to wonder what are specific examples of situations where knowing stuff like C can make you a better high level programmer.
What I want to know is if there are many examples of this. Many times, the answer to this question is something like "Knowing C gives you a better feel of what's happening under the covers" or "You need a solid foundation for your program", and these answers don't have much meaning. I want to understand the different specific ways in which you will benefit from knowing low level concepts,
Joel gave a couple of examples: Binary databases vs XML, and strings. But two examples don't really justify learning C and/or Assembly. So my question is this: What specific examples are there of knowing C making you a better high level programmer?
My experience with teaching students and working with people who only studied high-level languages is that they tend to think at a certain high level of abstraction, and they assume that "everything comes for free". They can become very competent programmers, but eventually they have to deal with some code that has performance issues and then it comes to bite them.
When you work a lot with C, you do think about memory allocation. You often think about memory layout (and cache locality if that's an issue). You understand how and why certain graphics operations just cost a lot. How efficient or inefficient certain socket behaviors are. How buffers work, etc. I feel that using the abstractions in a higher level language when you do know how it is implemented below the covers sometimes gives you "that extra secret sauce" when thinking about performance.
For example, Java has a garbage collector and you can't directly assign things to memory directly. And yet, you can make certain design choices (e.g., with custom data structures) that affect performance because of the same reasons this would be an issue in C.
Also, and more generally, I feel that it is important for a power programmer to not only know big-O notation (which most schools teach), but that in real-life applications the constant is also important (which schools try to ignore). My anecdotal experience is that people with skills in both language levels tend to have a better understanding of the constant, perhaps because of what I described above.
In addition, many higher level systems that I have seen interface with lower level libraries and infrastructures. For instance, some communications, databases or graphics libraries. Some drivers for certain devices, etc. If you are a power programmer, you may eventially have to venture out there and it helps to at least have an idea of what is going on.
Knowing low level stuff can help a lot.
To become a racing driver, you have to learn and understand the basic physics of how tyres grip the road. Anyone can learn to drive pretty fast, but you need a good understanding of the "low level" stuff (forces and friction, racing lines, fine throttle and brake control, etc) to get those last few percent of performance that will allow you to win the race.
For example, if you understand how the CPU architecture works in your computer, you can write code that works better with it (e.g. if you know you have a certain CPU cache size or a certain number of bytes in each CPU cache line, you can arrange your data structures and the way that you access them to make the best use of the cache - for example, processing many elements of an array in order is often faster than processing random elements, due to the CPU cache). If you have a multi-core computer, then understanding how low level techniques like threading work can gave huge benefits (just as not understanding the low level can lead to disaster in threading).
If you understand how Disk I/O and caching works, you can modify file operations to work well with it (e.g. if you read from one file and write to another, working on large batches of data in RAM can help reduce I/O contention between the reading and writing phases of your code, and vastly improve throughput)
If you understand how virtual functions work, you can design high-level code that uses virtual functions well. If used incorrectly they can severely hamper performance.
If you understand how drawing is handled, you can use clever tricks to improve drawing speed. e.g. You can draw a chessboard by alternately drawing 64 white and black squares. But it is often faster to draw 32 white sqares and then 32 black ones (because you only have to change the drawing colour twice instead of 64 times). But you can actually draw the whole board black, then XOR 4 stripes across the board and 4 stripes down the board in white, and this can be much faster still (2 colour changes, and only 9 rectangles to draw instead of 64). This chessboard trick teaches you a very important programming skill: Lateral thinking. By designing your algorithm well, you can often make a big difference to how well your program operates.
Understanding C, or for that matter, any low level programming language, gives you an opportunity to understand things like memory usage (i.e. why is it a bad thing to create several million heavy objects), how pointers/object references work, etc.
The problem is that as we've created ever increasing levels of abstraction, we find ourselves doing a lot of 'lego block' programming, without understanding how the legos actually function. And by having almost infinite resources, we start treating memory and resources like water, and tend to solve problems by throwing more iron at the situation.
While not limited to C, there's a tremendous benefit to working at a low level with much smaller, memory constrained systems like the Arduino or old-school 8-bit processors. It lets you experience close to the metal coding in a much more approachable package, and after spending time squeezing apps into 512K, you will find yourself applying these skills at a larger level within your day to day programming.
So the language itself is not important, but having a deeper appreciation for how all of the bits come together, and how to work effectively at a level closer to the hardware is a set of skills beneficial to any software developer.
For one, knowing C helps you understand how memory works in the OS and in other high level languages. When your C# or Java program balloons on memory usage, understanding that references (which are basically just pointers) take memory too, and understand how many of the data structures are implemented (which you get from making your own in C) helps you understand that your dictionary is reserving huge amounts of memory that aren't actually used.
For another, knowing C can help you understand how to make use of lower level operating system features. You don't need this often, but sometimes you may need memory mapped files, or to use marshalling in C#, and C will greatly help understand what you're doing when that happens.
I think C has also helped my understanding of network protocols, but I can't put my finger on specific examples. I was reading another SO question the other day where someone was complaining about how C's bit-fields are 'basically useless' and I was thinking how elegantly C bit fields represent low-level network protocols. High level languages dealing with structures of bits always end up a mess!
In general, the more you know, the better programmer you will be.
However, sometimes knowing another language, such as C, can make you do the wrong thing, because there might be an assumption that is not true in a higher-level language (such as Python, or PHP). For example, one might assume that finding the length of a list might be O(N) where N is the length of the list. However, this is probably not the case in many high-level language instances. In Python, for most list-like things the cost is O(1).
Knowing more about the specifics of a language will help, but knowing more in general might lead one to make incorrect assumptions.
Just "knowing" C would not make you better.
But, if you understand the whole thing, how native binaries work, how does CPU work with it, what are architecture limitations, you may write a code which is easier for CPU.
For example, how L1/L2 caches affect your work, and how should you write your code to have more hits in L1/L2 caches. When working with C/C++ and doing heavy optimizations, you will have to go down to that kind of things.
It isn't so much knowing C as it is that C is closer to the bare metal than many other languages. You need to be more aware of how to allocate/deallocate memory because you have to do it yourself. Doing it yourself helps you understand the implications of many decisions that you make.
To me any language is acceptable as long as you understand how the compiler/interpreter (basically) maps your code onto the machine. It's a bit easier to do in a language that exposes this directly, but you should be able to, with a bit of reading, figure out how memory is allocated and organized, what sort of indexing patterns are more optimal than others, what constructs are more efficient for particular applications, etc.
More important, I think, is a good understanding of operating systems, memory architectures, and algorithms. If you understand how your algorithm works, why it would be better to choose one algorithm or data structure over another (e.g., HashSet vs. List), and how your code maps onto the machine, it shouldn't matter what language you are using.
This is my experience of how I learnt and taught myself programming, specifically, understanding C, this is going back to early 1990's so may be a bit antique, but the passion and the drive is important:
Learn to understand the low level principles of the computer, such as EGA/VGA programming, here's a link to the Simtel archive on the C programmer's guide to the PC.
Understanding how TSR's work
Download the whole archive of Bob Stout's snippets which is a big collection of C code that does one thing only - study them and understand it, not alone that, the collection of snippets strives to be portable.
Browse at the International Obfuscated C Code Contest (IOCCC) online, and see how the C code can be abused and understand the intracies of the language. The worst code abuse is the winner! Download the archives and study them.
Like myself, I loved the infamous Ponzo's C Tutorial which helped me immensely, unfortunately, the archive is very hard to find. If anyone knows of where to obtain them, please leave a comment and I will amend this answer to include the link. There is another one that I can remember - Coronado's [Generic?] C Tutorial, again, my memory on this one is hazy...
Look at Dr. Dobb's journal and C User Journal here - I do not know if you can still get them in print but they were a classic, can remember the feeling of holding a printed copy in my hand and tearing off home to type in the code to see what happens!
Grab an ancient copy of Turbo C v2 which I believe you can get from borland.com and just play with 16bit C programming to get a feel and mess with the pointers...sure it is ancient and old but playing with pointers on it is fine.
Understand and learn Pointers, link here to the legacy Simtel.net - a crucial link to achieving C Guru'ship for want of a better word, also you will find a host of downloads pertaining to the C programming language - I remember actually ordering the Simtel CD Archive and looking for the C stuff...
A couple of things that you have to deal directly with in C that other languages abstract away from you include explicit memory management (malloc) and dealing directly with pointers.
My girlfriend is one semester from graduating MIT (where they mainly use Java, Scheme, and Python) with a Computer Science degree, and she is currently working at a company whose codebase is in C++. For the first few days she had a difficult time understanding all the pointers/references/etc.
On the other hand, I found moving from C++ to Java very easy, because I was never confused about pass-references-by-value vs pass-by-reference.
Similarly, in C/C++ it is much more apparent that primitives are just the compiler treating the same sets of bits in different ways, as opposed to a language like Python or Ruby where everything is an object with its own distinct properties.
A simple (not entirely realistic) example to illustrate some of the advice above. Consider the seemingly harmless
while(true)
for(Iterator iter = foo.iterator(); iter.hasNext();)
bar.doSomething( iter.next() )
or the even higher level
while(true)
for(Baz b: foo)
bar.doSomething(b)
A possible problem here is that each time round the while loop a new object (the iterator) is created. If all you care about is programmer convenience, then the latter is definitely better. But if the loop has to be efficient or the machine is resource constrained then you are pretty much at the mercy of the designers of your high level language.
For example, a typical complaint for doing high-performance Java is having execution stop while garbage (such as all those allocated Iterator objects) is reclaimed. Not very good if your software is charged with tracking incoming missiles, auto-piloting a passenger jet, or just not leaving the user wondering why the GUI has stopped responding.
One possible solution (still in the higher-level language) would be to weaken the convenience of the iterator to something like
Iterator iter = new Iterator();
while(true)
for(foo.initAlreadyAllocatedIterator(iter); iter.hasNext();)
bar.doSomething(iter.next())
But this would only make sense if you had some idea about memory allocation...otherwise it just looks like a nasty API. Convenience always costs somewhere, and knowing lower-level stuff can help you identify and mitigate those costs.
I'm a web coder: I currently enjoy AS3 and deal with PHP. I own a Nintendo DS and want to give C a go.
From a higher level, what basic things/creature comforts are going to go missing?
I can't find [for... in] loops, so I assume they aren't there. It looks like I'm going to have to declare things religiously, and I assume I have no objects (which I dealt with in PHP a while ago).
Hash tables? Funny data types?
To sum it up, you'll basically get:
Typed variables
Functions
Pointers
Standard libraries
Then, you make the rest -- that may be a little too simplified, but that's a rough idea of what to face.
It can be daunting to begin with and there may be a learning curve to overcome. Here's a few speed bumps you may encounter:
String? What string?
One big thing to get used to would be strings. There is no such thing as a string in C. A string is a "null-terminated character array" (sometimes called C strings), which basically means an array of type char with the final element being a \0 (char value 0).
In memory, a char array of length 4 containing Hi! would appear as:
char[0] == 'H'
char[1] == 'i'
char[2] == '!'
char[3] == '\0'
Also, strings don't know their own length (no such things as "objects" that come for free in C), so the use of standard library call strlen would be required, which more or less is a for loop that goes through the string until it hits a \0 character. (This means it's an O(N) operation -- longer the string, longer it takes to find the length, unlike O(1) operation of most string implementation in modern languages.)
Garbage collection?
No such thing is as a garbage collector in C. In fact, you need to allocate and deallocate memory yourself:
/* Allocate enough memory for array of 10 int values. */
int* array_of_ints = malloc(sizeof(int) * 10);
/* Done with the array? Don't forget to free the memory! */
free(array_of_ints);
Failing to clean up after allocation of memory can lead to things called memory leaks which I'm sure you've heard of before.
Pointers!
And as always, when we talk about C, we can't forget about pointers. The whole concept of references to variables and dereferencing pointers can be a serious headache-inducing concept, but once you get a hang of it, it's actually not too bad.
Except for the times when you expect it to work one way, but you find out that you didn't quite understand pointers well enough and it actually does something else -- as they say, been there, done that.
Oh, and pointers are probably going to be one of the first times you'll actually see a program crash bad enough that the operating system will yell at you. A segmentation fault is not something the computer likes a lot.
Types
All variables in C will have types. C is a statically-typed language, meaning that variable types will be checked at compile time. This might take some getting used to at the beginning, but can also be seen as a good thing, as it can reduce runtime errors such as type errors where you try to assign a number to a string.
However, it is possible to perform typecasts, so it is possible to cast a int type (which are integer values) to a double type (a floating type value). However, it is not possible to try to cast an int directly to a string like char*.
So, for example, in some languages the following is allowed:
// Example of a very weakly-typed pseudolanguage with implicit typecasts:
number n = 42
string s = "answer: "
string result = s + n // Result: "answer: 42"
In C, one would have to call an itoa function to get a char* representation of an int, then use strcat to concatenate two strings.
Conclusion
Those things said, learning C coming from a higher language can be very eye-opening and probably challenging to begin with, but once you get a hang of it, it can be pretty fun to work with.
I'd recommend starting to experiment with a C compiler, and have a good book or reference.
I think many people will recommend the K&R book, which is indeed an excellent book.
At first, I didn't think recommending K&R as the first C book would be a good idea because it may be a little bit on the difficult side, but on second thought, I think it is a very comprehensive and well-written book that can be good for getting into C if you already have some programming experience.
Good luck!
Well ... You might be in for something of a culture shock. These are the 32 standard keywords in C, and that includes the basic types.
C's standard library is pretty functional (more so than people perhaps expect), but very very thin when compared to what higher-level languages give you. There is no hash table in sight, and you are correct to assume that C does not have syntactic or semantic support for objects.
It is possible to write pretty object-oriented code anyway, but you will have to jump through a few hoops, and do much more manually since the language won't help you. See for instance the GTK+ UI toolkit for an example of a well-designed object-oriented C library/API.
I'm a web coder: I currently enjoy AS3 and deal with PHP. I own a Nintendo DS and want to give C a go.
Why do you want to do C programming?
What are your reasons, what do you hope to achieve?
Is it in order to write software for the Nintendo DS?
From a higher level, what basic things/creature comforts are going to go missing?
Given your background, I think you'll personally miss the lack of dynamic typing support, in other words you will have to be very explicit in your C programs, your data must be specified with proper types, so that the compiler knows what type of data you are working with. This also applies to any sort of memory management, i.e. basically anything once you start working with data structures that are non PODs.
For example, where you would do something like this in php:
function multiply(x) {
return (x*x);
}
You would have to do something like this in C:
int multiply(int x) {
return (x*x);
}
While these may seem fairly similar, there are big differences, namely typing restrictions: the php version will also work with floating point values, while in C you would have to explicitly provide versions for different types and ranges of values (C types are constrained to certain ranges).
I can't find [for... in] loops, so assume they aren't there
in C, it looks more like the following:
int c;
for (c=0;c<=10;c++) {
// loop body
}
it looks like I'm going to have to declare things religiously
Yes, very much so - much more so, than you'll appreciate
and I assume I have no objects (which I dealt with in PHP a while ago).
correct, no objects - but OOP can still be emulated using other ways, such as function(struct obj)
Depending on your goals and motivation, I think you may find C a pretty frustrating language to start serious programming with, you may want to look into some of the related alternatives like for example Java instead.
Dynamic arrays and garbage collection. It's not built in to C so you'll need to roll your own or use a pre-existing solution.
The standard procedure is that you manage the memory yourself which might sound like something horrible but it really isn't. For example in AS3 and PHP you can create an array and forget it when you're done with it. In C you'll have to make sure to deallocate it yourself or memory will leak and bad stuff can/will happen.
You'll particularly miss automatic memory management, and semantically meaningful datatypes such as strings, tables &c. However, learning C well is quite instructive, even though you probably don't want to use it for application-level programming, so I suggest you grab a "K&R" (Kernighan and Ritchie's seminal book) and give it a go -- you'll find plenty of free libraries on the web to use and study as you proceed beyond that, though you'll have to discipline yourself to use proper memory management heuristics... happy learning!
I was just doing some research online, and it seems there's a viable possibility to use lua for developing on the "nintendo DS", this may in fact be the easiest way for someone familiar with high level languages to get started doing embedded development, without sacrificing too much HLL power and without experiencing the inevitable culture shock when migrating from a HLL to C: microlua, here are the API docs.
So you might want to give it a go, possibly using an emulator for starters.
Keep us posted!
I'm pretty sure you want to be looking at C++, not C. C++ is basically object oriented C.
What you'll REALLY miss is the ability to rapidly prototype and test changes. You can't just change a line of code and run. Even using build tools like "make" a recompile can often take several minutes. This is even worse when you consider that it's really easy to make mistakes in C/C++. On large projects I reckon I spend more time compiling than actually coding. As a long-term user of script languages this is my biggest issue with using C.
Moving directly from a higher-level language running on a machine with effectively infinite resources to a DS is going to be a challenge, and not just because of the language.
The Nintendo DS has only 4MB of RAM, a 66MHz ARM-7, no operating system, and the development libraries available (such as libnds) provide only a thin abstraction over the hardware itself.
So, in addition to having to deal with manual memory management, a simpler language with fewer creature comforts, static typing, lack of objects, and the need to run a compile step before you can see any changes, you also have to deal with memory fragmentation, a very slow CPU by modern standards, and needing to interact with the hardware directly in order to do anything useful.
Writing code for the DS, the only other option is C++. You can't use a lot of the advanced features that make C++ worthwhile on such a limited system. You'd be writing C code using a C++ compiler.
That said, it's a lot of fun. You can screw around with the hardware all you like, and there's no need to interface with the operating system, because there isn't one.
C is the next level above straight assembler and allows you to operate close to the metal. This gives power to do amazing stuff but also to easily shoot yourself in the foot!
One such example is direct memory access and the perils and wonder of pointer arithmetic. Pointers are very powerful, fast, and handy however require careful management. See this SO question for an example.
Also as mentioned by the other answerers you will have to do your own memory management. Again powerful and painful.
I would recommend studying up a good textbook and find some quality example code. The key thing is to learn the patterns that make all this stuff hang together correctly and elegantly (well, as much as possible). A good debugger will also really help and get familiar with the standard C libraries too.
You may notice your applications crashing at the drop of a hat initially but perservere as C is definitely worth at least dabbling in. You will understand some of the amazing abstractions higher level languages provide and what is really going on under the hood.
We need more homebrew developers. I am a GBA/NDS and many other embedded platform developer and hope to see that you continue with this. I would say skip to arm assembler and then back up to C or any other language you like, once you know how the processor works, languages are just syntax.
I assume your prior experience covers the programming mindset, breaking things down into bite sized chunks and then writing code to perform those chunks. Then another module that links those together and so on. Then C is just another language, a very very simple language, no need to dive into the corners of it, drive down the middle. It is a good habit to declare variables, etc, and here you will have to. The compilers will tell you when you have forgotten something. You are not going to need big concepts, big structures, language magic, this is embedded, you are resource limited, write some bytes here, read a register there, extract a bit from the data to see if a button has been pressed, write a register in response to move a sprite, etc.
I think you will find the NDS much harder than C at first, there are two processors and some infrastructure to get the simplest of working binaries. Granted there are many many examples out there as well. I generally (and still do) recommend starting with the GBA then graduate to the NDS. bite size chunks.
A lot of things from OOP is the same or almost the same in PHP and C#.
You don't play with pointers in C# (compared to C++) so I would definitely recommend going with C# if you want to play with C.
What C are you talking about?
C#
foreach(string item in itemsCollection)
{
...
}
PHP
foreach($itemsCollection as $key=>$value)
{
...
}
etc.
I like C# because it is strongly typed and your types are automatically checked while you write a code... The possibility of trying to save integer into string or vice versa is zero compared to PHP where you can save anything into anything...