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I want to write a library that is dynamically loadable and callable from C code, but I really don't want to write it in C - the code is security critical, so I want a language that makes it easier to have confidence that my code is correct. What are my options?
To be more specific, I want C programmers to be able to #include this, and -l that, and start using my library just as if I had written it in C. I'd like programmers in other languages to be able to use their favourite tools for linking to C libraries to link to it. Ideally I'd like that to be possible on every platform that supports C, but I'll settle for Linux, Windows and MacOS.
Anything that compiles to native code. So you might Google for that - "languages that compile to native code." See, e.g., Programming languages that compile to native code and have the batteries included
C++ is often the choice for this. Compiles to native code and provided you keep your interfaces simple, easy to write an adapter layer.
Objective C and Fortran are also possible.
It sounds like you are looking for a language with ABI compatibility or which can be described as resulting in native code. So long as it can be compiled to a valid object file (typically an .obj or .o file) which is accepted by the linker, that should be the main criteria. You also then want to write a header file as a convenience for any client code which is written in C (or a closely related language/variant thereof).
As mentioned by others, you need a pretty good reason for choosing a language other than C as it is the lingua-franco of low-level/systems software. Assembler is an option, although harder to port between platforms. D is a more portable - but less widespread - alternative which is designed to produce secure, efficient native code with a minimum of fuss. There are many others.
Almost every security critical application I know of is written in C. I don't believe that there are any other language that has higher real status in producing secure applications.
C is being said to be a poor language for security by people who don't understand.
If you want C programmers to use your library, use C. Doing anything else is tying one hand behind your back whilst trying to walk on a balance beam (the gymnastics equipment). Sure, there are dozens of other languages that are CAPABLE of interfacing to C, but it typically involves using a C layer and then stuffing the C data types into a language specific data type (Java Objects, Python Objects, etc, etc), and when the call is finished, you use the same conversion back to a C data type. Just makes it harder to work with, and potentially slower if you don't get all the design decisions right. And people won't understand the source code, so won't like to use it (see more about this below).
If you want security, then write very good code, wearing your "security aspects" hat firmly on at all times, find a security mailing list or website and post it there for review, take the review comments on board, understand the comments, and fix any comments that are meaningful to fix. Distribute the source code to the users, so people can see what your code does. Those that understand security will know what to look for and understand that you have done a good job (or a bad job, whichever is applicable) - and those who don't will hopefully trust the right pople. If it's good, people will use it. If it's "hidden", and not easy to access, you won't get many customers, no matter what language you use.
Don't worry, you won't reveal anything more from releasing source. If there is a flaw in the code, and it is popular (or important) enough, someone will find the flaw, even if you publish only binaries. For those skilled in reverse engineering, not having source code is only a small obstacle.
Security doesn't stem from using a specific language or a specific tool, it stems from good design and good basic understanding of the problems with security.
And remember security by obscurity (whether that means "hidden source code" or "unusual language" or something else obscure) is false security.
You might be interested in ATS, http://ats-lang.sourceforge.net/. ATS compiles via C, can be as efficient as C, and can be used in a way that is ABI-compatible with C. From the project website:
ATS is a statically typed programming language that unifies implementation with formal specification. It is equipped with a highly expressive type system rooted in the framework Applied Type System, which gives the language its name. In particular, both dependent types and linear types are available in ATS. The current implementation of ATS (ATS/Anairiats) is written in ATS itself. It can be as efficient as C/C++ (see The Computer Language Benchmarks Game for concrete evidence) and supports a variety of programming paradigms
ATS's dependent and linear type system helps produce static guarantees about your code, including various aspects of resource management safety.
Chris Double has been writing a series of articles exploring the power of ATS's type system for systems programming here: http://bluishcoder.co.nz/tags/ats/. Of particular note is this article: http://bluishcoder.co.nz/2012/08/30/safer-handling-of-c-memory-in-ats.html
This document covers aspects of calling back and forth between ATS and C code: https://docs.google.com/document/d/1W6DYQApEqKgyBzMbvpCI87DBfLdNAQ3E60u1hUiMoU0
The main downside is that dependently-typed programming is still a daunting prospect, even for non-systems programming. The syntax of the language is also a bit weird: consider lexical quirks such as the use of abst#ype as a keyword. Finally, ATS is to some degree a research project, and I personally don't know whether it would be sensible to adopt for a commercial endeavour.
Theoretically, it's going to be Fortran: less indirection (as in: my array is [here], not just a pointer to here, and this is true of most but not all of your data structures and variables).
However... There are many gotchas and quirks in Fortran: not, perhaps, as many as in C but you probably know your way around C rather better than Fortran. Which is the point behind most of the comments saying 'Know your code' - but do you really know what your compiler is doing?
Knowing you, I'm prepared to take it on trust that you do, for C. Most programers don't. You do not know and cannot know what a local JVM or JIT compiler does, and that's a black hole in your security model if you're using Java or C# r scripting languages.
Ignore anyone who tells you that the hairy-chested he-men of secure computing write their own assembler: they probably don't even know the security errors they're making in any and all nontrivial projects they release. Know your compiler, indeed.
You could write it in lua - providing a C API to a Lua library is relatively straight forward. C++ is also an option, though of course you'd have to write C wrappers and make sure no exceptions can escape your functions. But honestly, if it's security critical the minor inconveniences of the C language shouldn't be that much of a big deal. What you really should be doing is prove the correctness of your program where feasible, and test extensively where it's not.
You can write a library in Java. JNI is normally used to call C from Java, but it can be used the other way around.
There is finally a decent answer to this question: Rust.
I'm looking for a higher-level system language, if possible, suitable for formal verification, that compiles to standard C, so that it can be run cross-platform with (relatively) low overhead.
The two most promising such languages I've stumbled during the past few days are:
BitC - While the design goals of this language match my needs (it even supports the functional paradigm), it is in very unstable state, the documentation is out of date, and, generally, it seems like a very long shot for a real-world project.
Lisaac - It supports Design-by-contract, which is very cool and has a relatively low performance overhead. However the website is dead, there hasn't been a new release since '08 and generally it seems the language is dead.
I'd also like to note that it's not meant for a real-time system, so a GC or, generally, non-determinism (in the real-time sense), is not an issue.
The project involves mainly audio processing, though it has to be cross-platform.
I assume someone would point me to the obvious answer - "plain ol' C". While it is truly cross-platform and very effective, the code quantity would probably be greater.
EDIT: I should clarify that I mean cross-platform AND cross-architecture. That is why I consider only languages, compiled to C in the first place, but if you can point me to another example, I'd be grateful :)
I think you may become interested in ATS. It compiles to C (actually it expresses and explains many C idioms and patterns from a formal type-theoretic perspective, it has even been proposed to prepare a book of sorts to show this -- if only we had more time...).
The project involves mainly audio processing, though it has to be cross-platform.
I don't know much about audio processing, I've been mainly doing some computer graphics stuff (mostly the basic things, just to try it out).
Also, I am not sure if ATS works on Windows (never tried that).
(Disclaimer: I've been working with ATS for some time. It is bulky and large language, and sometimes hard to use, but I very much liked the quality of programs I've been able to produce with it, for instance, see TEST subdirectory in GLES2 bindings for some realistic programs)
The following doesn't strictly adhere to the requirements but I'd like to mention it anyway and it is too long for a comment:
Pypy's RPython can be translated to C. Here's a nice talk about it. It's been used to implement Smalltalk, JavaScript, Io, Scheme, Gameboy (with various degree of completeness), but you can write standalone programs in it. It is known mainly for its implementation of Python language that runs on Intel x86 (IA-32) and x86_64 platforms.
The translation process requires a capable C compiler. The toolchain provides means to infer various things about the code (used by the translation process itself) that you might repurpose for formal verification.
If you know both Python and C you could use cython that translates Python-like syntax to C. It is used to write CPython extensions.
We have a very versatile terminal/sniffer application which can do all sorts of things with TCP, UDP and serial connections.
We are looking to make it extensible -- i.e, allow people to write their own protocol parsers, highlighters, etc.
We created a C-like language for extending the product, and then discovered that for some coders, this presents a steep learning curve.
We are now pondering the question: Should we stick to C or go with something like Ruby or Lua?
C is beautiful for low-level stuff (like parsing binary data), because it supports pointers. But for exactly that reason, it can be tough to learn.
Ruby (etc) are easy to learn, but don't have pointers, so anything that has to do with parsing binary data gets ugly very fast.
What do you think? For extending a product that parses binary data -- Ruby/Lua or C/C++?
Would be great if you could give some background when you respond -- especially if you've done something similar.
Wireshark, the "world's foremost network protocol analyzer", is also a packet sniffer/analyzer, formerly also called Ethereal. It uses Lua to enable writing custom dissectors and taps, see the manual.
However, note that I have not used it, so I cannot tell how nice/effective/easy to learn the API is.
Like TCL, Lua was designed to be tightly integrated with an application. Personally, I find Lua's syntax and idioms to be much easier to deal with than TCL.
Lua is easy to integrate with an existing system, and easy to extend. It is also fairly easy to create safe sandboxes in which user-supplied code can run without full access to the innards of your product.
If you have an API written does it make a difference? The person using the C-like API would only have to understand the difference between passing by value or reference.
Your core does one thing very good, so fine. Let it be that way. I think you should create an API based on std in/out, just like the way of good unix design. Then anyone can extend it in any language of choice.
Tcl was designed with the goal to allow scripting for C programs, so it would be much easier to implement.
http://en.wikipedia.org/wiki/Tcl#Interfacing_with_other_languages
I second Johan's idea. Although in past when I had to do something like this I stuck to
C language APIs and people were restricted to use C language only. But now I look at it,
I realize that it would have been more efficient if we would have done the way Johan describes
PS: And by coincidence it was a protocol testing app using packet sniffer
perl, sed, awk, lex, antler, ... These are languages I'm somewhat familiar with that I'd like to write something like this in. It depends on the data flow, though.
It's really hard to say what the correct thing to use is. Something that I don't think anyone else has mentioned is to keep in mind that the scripts will have bugs. It's very easy to design something like this in such a way that bugs in the scripts (especially run time errors) either just show up a "error in script" or kill the whole system.
You should keep that the scripts should be unit testable and that failures should be reproducible.
I don't think it matters what you do as long as you do one thing, drop the in-house language. It sounds like you choose to make C into a scripting language. One issue I see with this is it will look familiar to C programmers, but not be the same. I can't imagine you have mimicked the semantics of C that would make existing C programmers comfortable. And as you have mentioned, others will find it hard to learn.
The company I am working at have developed their own language. It uses XML for structure so parsing is easy. The language grows "as needed." Meaning if a feature is missing then it will be added. I'm pretty sure it went from an XML database to something that needed control flow. But my point is that if you aren't thinking about building it as a language, then you'll be limiting what users can do with it unintentionally.
Personally I've been looking at how I can get the company to start taking advantage of Lua. And specifically Lua for several reasons. Lua was developed as an extension language that was general purpose. It easily interfaces with the language, including Python and Ruby. It is small and simple for use by non-programmers (not really needed in your case). It is simple enough to replace XML, INI... for configuration settings and powerful enough to replace the need for another programming language.
http://www.lua.org/spe.html
I came across this: Writing a compiler using Turbo Pascal
I am curious if there are any tutorials or references explaining how to go about creating a simple C compiler. I mean, it is enough if it gets me to the level of making it understand arithmetic operations. I became really curious after reading this article by Ken Thompson. The idea of writing something that understands itself seems exciting.
Why did I put up this question instead of asking Google? I tried Google and the Pascal one was the first link. The rest did no seem relevant and added to that... I am not a CS major (so I still need to learn what all those tools like yacc do) and I want to learn this by doing and am hoping people with more experience are always better at these things than Google. I want to read some article written in the same spirit as the one I listed above but that which highlights at least the bootstrapping phases of building a simple C compiler.
Also, I don't know the best way to learn. Do I start off building a C compiler in C or some other language? Do I write a C compiler or some other language? I feel questions like this are better answered once I have some direction to explore. Any suggestions?
Any suggestions?
A compiler consists of three pieces:
A parser
An abstract syntax tree (AST)
An assembly code generator
There are lots of nice parser generators that start with language grammars. Maybe ANTLR would be a good place for you to start. If you want to stick to C roots, try lex/yacc or bison.
There are grammars for C, but I think C in its entirety is complex. You'd do well to start off with a subset of the language and work your way up.
Once you have an AST, you use it to generate the machine code that you'll run.
It's doable, but not trivial.
I'd also check Amazon for books about writing compilers. The Dragon Book is the classic, but there are more modern ones available.
UPDATE: There have been similar questions on Stack overflow, like this one. Check out those resources as well.
I advise you this tutorial:
LLVM tutorial
It is a small example on how to implement a "small language" compiler. The source code is very small and is explained step by step.
There is also the C front end library for the LLVM (Low Level Virtual Machine which represent the internal structure of a program) library:
Clang
For what it's worth, the Tiny C Compiler is a pretty full-featured C compiler in a relatively small source package. You might benefit from studying that source, as it's probably significantly easier to understand than trying to comprehend all of GCC's source base, for instance.
This is my opinion (and conjecture) it will be hard to write a compiler without understanding data structures normally covered in undergraduate (post secondary) Computer Science classes. This doesn't mean you cannot, but you will need to know essential data structures such as linked lists, and trees.
Rather than writing a full or standards compliant C language compiler (at least in the start), I would suggest limiting yourself to a basic subset of the language, such as common operators, integer only support, and basic functions and pointers. One classic example of this was Ron Cain's Small-C, made popular by a series of articles written in Dr. Dobbs Journal in I believe the 1980s. They publish a CD with the James Hendrix's out-of-print book, A Small-C Compiler.
What I would suggest is following Crenshaw's tutorial, but write it for a C-like language compiler, and whatever CPU target (Crenshaw targets the Motorola 68000 CPU) you wish to target. In order to do this, you will need to know basic assembly of which ever target you want to run the compiled programs on. This could include a emulator for a 68000, or MIPS which are arguably nicer assembly instruction sets than the venerable CISC instruction set of the Intel x86 (16/32-bit).
There are many potential books that can be used as starting points for learning compiler / translator theory (and practice). Read the comp.compilers FAQ, and reviews at various online book sellers. Most introductory books are written as textbooks for sophomore to senior level undergraduate Computer Science classes, so they can be slow reading without a CS background. One older book that might be more introductory, but easier to read than "The Dragon Book" is Introduction to Compiler Construction by Thomas Parsons. It is older, so you should be able to find an used copy from your choice of online book sellers at a reasonable price.
So I'd say, try starting with Jack Crenshaw's Let's Build a Compiler tutorial, write your own, following his examples as a guide, and build the basics of a simple compiler. Once you have that working, you can better decide where you wish to take it from that point.
Added:
In regards to the bootstrapping process. Since there are existing C compilers freely available, you do not need to worry about bootstrapping. Write your compiler with separate, existing tools (GCC, Visual C++ Express, Mingw / djgpp, tcc), and you can worry about self-compiling your project at a much later stage. I was surprised by this part of the question until I realized you were brought to the idea of writing your own compiler by reading Ken Thomas' ACM Turing award speech, Reflections on Trusting Trust, which does go into the compiler bootstrapping process. It's a moderated advanced topic, and is also simply a lot of hassle as well. I find even bootstrapping the GCC C compiler under older Unix systems (Digital OSF/1 on the 64-bit Alpha) that included a C compiler a slow and time consuming, error prone process.
The other sort-of question was what a compiler tool like Yacc actually does. Yacc (Yet Another Compiler Compiler or Bison from GNU) is a tool designed to make writing a compiler (or translator) parser easier. Based on the formal grammar for your target language that you input to yacc, it generates a parser, which is one portion of a compiler's overall design. Next is Lex (or flex from GNU) which used to generate a lexical analyzer or scanner, which is often used in combination with the yacc generated parser to form the skeleton of the front-end of a compiler. These tools make writer a front end arguably easier than writing an lexical analyzer and parser yourself. Crenshaw's tutorial does not use these tools, and you don't need to either, many compiler writers don't always use them. Of course Crenshaw admits the tutorial's parser is quite basic.
Crenshaw's tutorial also skips generating an AST (abstract syntax tree), which simplifies but also limits the tutorial compiler. It lacks most if not all optimization, and is very tied to the specific programming language and the particular assembly language emitted by the "back-end" of the compiler. Normally the AST is a middle piece where some optimization can be performed, and serves to de-couple the compiler front-end and back-end in design. For a beginner without a Computer Science background, I'd suggest not worrying about not having an AST for your first compiler (or at least the first version of it). I think keeping it small and simple will help you finish writing a compiler, in its first version, and you can decide from there how you want to proceed then.
You might be interested in the book/course The Elements of Computing Systems:Building a Modern Computer from First Principles.
Note that this isn't about building a "pc" from stuff you bought off newegg. It begins with a description of Boolean logic fundamentals, and builds a virtual computer from the lowest levels of abstraction to progressively higher levels of abstraction. The course materials are all online, and the book itself is fairly inexpensive from Amazon.
In the course, in addition to "building the hardware", you'll also implement an assembler, virtual machine, compiler, and rudimentary OS, in a step-wise fashion. I think this would give you enough of a background to delve deeper into the subject area with some of the more commonly recommended resources listed in the other answers.
In The Unix Programming Environment, Kernighan and Pike walk through 5 iterations of making a calculator working from simple C based lexical analysis and immediate execution to yacc/lex parsing and code generation for an abstract machine. Because they write so wonderfully I can't suggest smoother introduction. It is certainly smaller than C, but that is likely to your advantage.
How do I [start writing] a simple C compiler?
There's nothing simple about compiling C. The best simple C compiler is lcc by Chris Fraser and David Hanson. They spent 10 years working on the design to make it as simple as they possibly could, while still generating reasonably good code. If you have access to a university library, you should be able to get their book.
Do I start off building a C compiler in C or some other language?
Some other language. One time I got to ask Hanson what lessons he and Fraser had learned by spending 10 years on the lcc project. The main thing Hanson said was
C is a lousy language to write a compiler in.
You're better off using Haskell or some dialect of ML. Both languages offer functions over algebraic data types, which is a perfect match to the problems faced by the compiler writer. If you still want to pursue C, you could start with George Necula's CIL, which is a big chunk of a C compiler written in ML.
I want to read some article written in the same spirit as the one I listed above but that which highlights at least the bootstrapping phases...
You won't find another article like Ken's. But Andrew Appel has written a nice article called Axiomatic Bootstrapping: A Guide for Compiler Hackers I couldn't find a free version but many people have access to the ACM Digital Library.
Any suggestions?
If you want to write a compiler,
Use Haskell or ML as your implementation language.
For your first compiler, pick a very simple language like Oberon or like P0 from Niklaus Wirth's book Algorithms + Data Structures = Programs. Wirth is famous for designing languages that are easy to compile.
You can write a C compiler for your second compiler.
A compiler is a complex subject matter that covers aspects of
Input processing involving Lexing, Parsing
Building a symbol store of every variable used such as an Abstract Syntax Tree (AST)
From the AST tree, transpose and build a machine code binary based on the syntax
This is by no means exhaustive as it is an abstract bird's eye view from the top of a mountain, it boils down to getting the syntax notation correct and ensuring that malformed inputs do not throw it off, in fact a good input processing should never fall on its knees no matter how malformed, terrible, abused cases of input that gets thrown at it. And, also in deciding and knowing what output is going to be, is it in machine code, which would imply you may have to get to know the processor instructions intimately...including memory addressing for variables and so on...
Here are some links for you to get started:
There was a Jack Crenshaw's port of his code for C....(I recall downloading it months ago...)
Here's a link to a similar question here on SO.
Also, here's another small compiler tutorial for Basic to x86 assembler compiler.
Tiny C Compiler
Hendrix's Small C Compiler found here.
It might be worthwhile to learn about functional programming, too. Functional languages are well-suited to writing a compiler both in and for. My school's intro compilers class contained an intro to functional languages and the assignments were all in OCaml.
Funny you should ask this today, since just a couple days ago I wrote a lambda calculus interpreter. Lambda calculus is the granddaddy of all functional languages. It's just 200 lines long (in C++, incl. error reporting, some pretty printing, some unicode) and has a two-phase structure, with an intermediate format that could be used to generate code.
Not only is starting small and building up the most practical approach to compilers, it also encourages good, modular, organizational practice.
A compiler is a very large project, although I suppose it wouldn't hurt to try.
I know of at least one C compiler written in Pascal, so it's not the most insane thing you could do. I personally would pick a more modern language in which to implement my C compiler project, both for the simplicity (it's easy to d/l packages for Python, Ruby, C, C++ or Java) and because it will look better on your resume.
In order to do a compiler as a beginner project, though, you will need to drink all of the Agile kool-aid.
Always have something running, even if it doesn't do much of anything. Add things to your compiler only in small steps. ("Frequent releases".) Pick a viciously tiny subset of the language and implement that first. (Support only i = 0; at first and expand things from there.)
If you want a mind-blowing experience that teaches you how to write compilers that compile themselves, you need to read this paper from 1964.
META II a syntax-oriented compiler writing language by Val Schorre.
In 10 pages, it tells you how to write compilers, how to write meta compilers, provides a virtual metacompiler instruction set, and a sample compiler built with the metacompiler.
I learned how to write compilers from this paper back in the late 60s, and used the ideas to construct C-like langauges for several minicomputers and microprocessors.
If the paper is too much by itself (it isn't!) there's an online tutorial which will walk you through the whole thing.
And if getting the paper from the original link is awkward because you are not an ACM member, you'll find that the tutorial contains all the details anyway. (IMHO, for the price, the paper itself is waaaaay worth it).
10 pages!
I would not recommend starting with C as the language to implement, nor with any of the compiler-generator or parser-generator tools. C is a very tricky language, and it's probably a better idea to just make up a language of your own. It can be a little C-like (e.g. use curly backets if you want to indicate the function body, use the same type names, so you don't have to remember what you called everything).
The tools for making compilers and parsers are great, but have the problem of really being a shorthand notation. If you don't know how to create a compiler in longhand, the shorthand will seem cryptic, needlessly restrictive etc. So write your own simple compiler first, then continue on from there. I also recommend you don't start generating actual machine code unless you eat and breathe assembler. Create your own bytecode interpreter with a VM.
As to what language you should use to create your first compiler: It doesn't really matter, as long as the language is fairly complete. You will be reading input text, building data structures from them and writing out binary data. So if a language makes those things easier in any way, that's a point in favor of it. Pick a language you know well, so you can focus on creating the compiler, not learning the language. I usually use an OO language, which makes the syntax tree easier to write, a functional language would probably also work if you are familiar with that.
I've blogged a lot about programming languages, so you might find some useful postings here: http://orangejuiceliberationfront.com/category/language-design/
In particular, http://orangejuiceliberationfront.com/how-to-write-a-compiler/ is a starter on the particulars of parsing common constructs and generating something useful from that, as well as http://orangejuiceliberationfront.com/generating-machine-code-at-runtime/ which talks about actually spitting out Intel instructions that do something.
Oh, regarding bootstrapping of a compiler: You probably won't be able to do that right from the start. There is a fair amount of work involved in creating a compiler. So not only would writing a bootstrapping compiler involve writing the compiler (in some other language), once you have it, you would then have to write a second version of the compiler using itself. That's twice the work, plus the debugging needed in the existing and the bootstrapped new compiler until it all works. That said, once you have a working compiler, it is a good way to test its completeness. OK, maybe not twice the work, but more work. I'd go for the easy successes first, then move on from there.
In any event, have fun!
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Joel and company expound on the virtues of learning C and how the best way to learn a language is to actually write programs using that use it. To that effect, which types of applications are most suitable to the C programming language?
Edit:
I am looking for what C is good at. This likely does not coincide with the best way of learning C.
Code where you need absolute control over memory management. Code where you need to be utterly in control of speed versus memory trade-offs. Very low-level file manipulation (such as access to the raw devices).
Examples include OS kernel, and embedded apps.
In the late 1980s, I was head of the maintenance team on a C system that was more than a million lines of code. It did database access (Oracle), X Windows graphics, interprocess communications, all sorts of good stuff. It ran on VMS and several varieties of Unix. But if I were to recreate that system today, I sure wouldn't use C, I'd use Java. Others would probably use C#.
Low level functions such as OS kernel and drivers. For those, C is unbeatable.
You can use C to write anything. It is a very general purpose language. After doing so for a little while you will understand why there are other "higher level" languages available.
"Learn C", by all means, but don't don't stick with it for too long. Other languages are far more productive.
I think the gist of the people who say you need to learn C is so that you understand the relationship between high level code and the machine internals and what exaclty happens with bits, bytes, program execution, etc.
Learn that, and then move on.
Those 100 lines of python code that were accounting for 80% of your execution time.
Small apps that don't have a UI, especially when you're trying to learn.
Edit: After thinking a little more on this, I'd add the following: if you already know a higher-level language and you're trying to learn more about C, a good route may be to not create a whole new C app, but instead create a C DLL and add functions to it that you can call from the higher language. In this way you can replace simple functions that your high language already has to ensure that you C function does what it should (gives you pre-built testing), lets you code mostly in what you're familiar with, uses the language in a problem you're already familiar with, and teaches you about interop.
Anything where you think of using assembly.
Number crunching (for example, libraries to be used at a higher level from some other language like Python).
Embedded systems programming.
A lot of people are saying OS kernel and device drivers which are, of course, good applications for C. But C is also useful for writing any performance critical applications that need to use every bit of performance the hardware is capable of.
I'm thinking of applications like database management systems (mySQL, Oracle, SQL Server), web servers (apache, IIS), or even we browsers (look at the way chrome was written).
You can do so many optimizations in C that are just impossible in languages that run in virtual machines like Java or .NET. For instance, databases and servers support many simultaneous users and need to scale very well. A database may need to share data structures between multiple users (threads/processes), but do so in a way that efficiently uses CPU caches. In C, you can use an operating system call to determine the size of the cache, and then align a data structure appropriately to the cache line so that the line does not "ping pong" between caches when multiple threads access adjacent, but unrelated data (so called "false sharing). This is one example. There are many others.
A bootloader. Some assembly also required, which is actually very nice..
Where you feel the need for 100% control over your program.
This is often the case in lower layer OS stuff like device drivers,
or real embedded devices based on MCU:s etc etc (all this and other is already mentioned above)
But please note that C is a mature language that has been around for many years
and will be around for many more years,
it has many really good debugging tools and still a huge number off developers that use it.
(It probably has lost a lot to more trendy languages, but it is still huge)
All its strengths and weaknesses are well know, the language will probably not change any more.
So there are not much room for surprises...
This also means that it would probably be a good choice if you have a application with a long expected life cycle.
/Johan
Anything where you need a minimum of "magic" and need the computer to do exactly what you tell it to, no more and no less. Anything where you don't trust the "magic" of garbage collection to handle your memory because it might not be as efficient as what you can hand-code. Anything where you don't trust the "magic" of built-in arrays, strings, etc. not to waste too much space. Anything where you want to be able to reason about exactly what ASM instructions the compiler will emit for a given piece of code.
In other words, not too much in the real world. Most things would benefit more from higher level abstraction than from this kind of control. However, OS code, device drivers, and a few things that have to be near optimal in both space and speed might make sense to write in C. Higher level languages can do pretty well competing with C on speed, but not necessarily on space.
Embedded stuff, where memory-usage and cpu-speed matters.
The interrupt handler part of an OS (and maybe two or three more functions in it).
Even if some of you will now start to bash heavily on me now:
I dont think that any decent app should be written in C - it is way too error prone.
(and yes, I do know what I am talking about, having written an awful lot of code in C myself (OSes, compilers, VMs, network protocols, RT-control stuff etc.).
Using a high level language makes you so much more productive. Speed is typically gained by keeping the 10-90 rule in mind: 90% of CPU time is spent in 10% of your code (which you should optimize first).
Also, using the right algorithm might give more performance than optimizing bits in C. And doing things right in C is so much more trouble.
PS: I do really mean what I wrote in the second sentence above; you can write a complete high performance OS in a high level language like Lisp or Smalltalk, with only a few minor parts in C. Think how the 70's Lisp machines would fly on todays hardware...
Garbage collectors!
Also, simply programs whose primary job is to make operating-system calls. For example, I need to write a short C program called timeout that
Takes a command line as argument, with a number of seconds for that command to run
Forks two child processes, one to run the command and one to sleep for N seconds
When the first of the child processes exits, kills the other, then exits
The effect will be to run a command with a limit on wall-clock time.
I and others on this forum have tried several different solutions using shells and/or perl. All are convoluted and none quite do the right thing. In C the solution will be easy, because all the OS facilities are right where you can get at them.
A few kinds that I can think of:
Systems programming that directly uses Unix/Linux or Win32 system calls
Performance-critical code that doesn't have much string manipulation in it (e.g., number crunching)
Embedded programming or other applications that are severely resource-constrained
Basically, C gives you portable, efficient access to the native capabilities of the machine; everything else is your responsibility. In particular, string manipulation in C is tedious, error-prone, and generally nasty; the most effective way to do extensive string operations with C may be to use it to implement a language that handles strings better...
examples are: embedded apps, kernel code, drivers, raw sockets.
However if you find yourself more productive in C then go ahead and build whatever you wish. Use the language as a tool to get your problem solved.
c compiler
Researches in maths and physics. There are probably two alternatives: C and C++, but such features of the latter as encapsulation and inheritance are not very useful there. One could prefer to use C++ "as a better C" or just stay with C.
Well most people are suggesting system programming related things like OS Kernels , Device Drivers etc. These are difficult and Time consuming. Maybe the most fun thing to with C is console programming. Have you heard of the HAM SDK? It is a complete software development kit for the Nintendo GBA , and making games for it is fun. There is also the CC65 Compiler which supports NES Programming (Althought Not Completely). You can also make good Emulators. Trust Me , C is pretty helpful. I was originally a Python fan, and hated C because it was complex. But after yuoget used to it, you can do anything with C. Now I use CPython to embed Python in my C Programs(if needed) and code mostly in C.
C is also great for portability , There is a C Compiler for like every OS and Almost Every Console And Mobile Device. Ive even seen one that supports some calculators!
Well, if you want to learn C and have some fun at the same time, might I suggest obtaining NXC and a Lego Mindstorms set? NXC is a C compiler for the Lego Mindstorms.
Another advantage of this approach is that you can compare the effort to program the Mindstorms "brick" with C and then try LEJOS and see how it goes with Java.
All great fun.
Implicit in your question is the assumption that a 'high-level' language like Python or Perl (or Java or ...) is fast enough, small enough, ... enough for most applications. This is of course true for most apps and some choice X of language. Given that, your language of choice almost certainly has a foreign function interface (FFI). Since you're looking to learn C, create a module in the FFI built in C.
For example, let's assume that your tool of choice is Python. Reimplement a subset of Numpy in C. Since C is a pretty fast language, and has, in C99, a clear numeric library interface, you'll get the opportunity to experience the power of C in an appropriate setting.
ISAPI filters for Internet Information Server.
Before actually write C code, i would suggest first read good C code.
Choose subject you want to concentrate on, basically any application can be written in C, but i assume GUI application will be not your first choice, and find few open source projects to look into.
Not any open source project is best code to look. I assume that after you will select a subject there is a place for another question, ask for best open source project in the field.
Play with it, understand how it's working modify some functionality...
Best way to learn is learn from somebody good.
Photoshop plugin filters. Lots and lots of interesting graphical manipulation you can do with those and you'll need pure C to do it in.
For instance that's a gateway to fractal generation, fourier transforms, fuzzy algorithms etc etc. Really anything that can manipulate image/color data and give interesting results
Don't treat C as a beefed up assembler. I've done some serious app's in it when it was the natural language (e.g., the target machine was a Solaris box with X11).
Write something with some meat on it. Write a client server chess program, where the AI is on a server and the UI is displaying in X11; once you've done that you will really know C.
I wonder why nobody stated the obvious:
Web applications.
Any place where the underlying libraries are entirely in C is a good candidate for staying in C - openGL, Lua extensions, PHP extensions, old-school windows.h, etc.
I prefer C for anything like parsing, code generation - anything that doesn't need a lot of data structure (read OOP). It's library footprint is very light, because class libraries are non-existent. I realize I'm unusual in this, but just as lots of things are "considered harmful", I try to have/use as little data structure as possible.
Following on from what someone else said. C seems a good language to implement the language in which you write the rest of your software.
And (mutatis mutandis) the virtual machine which runs the rest of your software.
I'd say that with the minuscule runtime environment and it's self-contained nature, you might start by creating some CLI utilities such as grep or tail (or half the commands in Unix). Anything that uses only STDOUT, STDIN and file manipulation is a good candidate.
This isn't exactly answering your question because I wouldn't actually CHOOSE to use C in such an app, but I hope it's answering the question you meant to ask--"what would be a good type of app to use learn C on?"
C isn't actually that bad a language--it's pretty easily to understand your code at an assembly language level which is quite useful, and the language constructs are few, leaving a very sparse language.
To answer your actual question, the very best use of C is the one for which it was created--porting itself (and UNIX) to other CPU architectures. This explains the lack of language features very well; it also explains the existence of Pointers which are really just a construct to make the compiler work less--any code created with pointers could be created without it (just as well optimized), but it becomes much harder to create a compiler to do so.
digital signal processing, all Pure Data extensions are written in C, this can be done in other languages also but has had good success in C