Learning from Java to C... ...What is the biggest challenge?_问答_开发者

I had experience on Java, because of some results, I need to code in C, is it dif开发者_运维知识库ficult to switch from Java to C? And what is the biggest different between these two languages?

Without question, first and foremost, it's the manual memory management.

Second is that C has no objects so C code will tend to be structured very differently to Java code.

Edit: a little anecdote: back 15 or so years ago when it was common to log on to your local ISP on a UNIX command prompt when PPP was still pretty new and when university campuses still had an abundance of dumb terminals to UNIX servers, many had a program called fortune that would run when we logged on and output a random geeky platitude. I literally laughed out loud one day when I logged in an read:

C -- a programming language that combines the power of assembly language with the flexibility of assembly language.

It's funny because it's true: C is the assembly language of modern computing. That's not a criticism, merely an observation.

Perhaps the most difficult concept is learning how to handle pointers and memory management yourself. Java substantially abstracts many concepts related to pointers, but in C, you'll have to understand how pointers are related to one another, and to the other concepts in the language.

Besides pointers and memory management, C is not an object-oriented language. You can organize your code to meet some object based concepts, but you will miss some features like inheritance, interfaces and polimorphism.

Java has a massive standard library, while C's is tiny. You'll find yourself re-inventing every kind of wheel over and over again. Even if you're writing the fifteenth linked list library of your career, you might very well make the same mistakes you've made before.

C has no standard containers besides arrays, few algorithms, no standard access to networking, graphics, web anything, xml anything, and so on. You have to really know what you're doing not to accidentally invoke undefined behaviour, cause memory corruption, resource leaks, crashes, etc. It's not for tourists.

Good luck.

Apart from Standard C being a non-OO language, the standard library being very small (and in some places, just plain bad), the goriness of manual memory handling, the complete lack of threading utilities (or even awareness of multithreading), the lax "type system" and being built for single-byte character sets, I think the greatest conceptual difference is that you have to have a clear notion of ownership of objects (or memory chunks, as it becomes in C).

It's always good practice to specify ownership of objects, but for non-GCed languages it is paramount. When you pass a pointer to another function, will that function assume ownership of the pointer or will it just "borrow" it from you for the duration of the call? When you write a function taking a pointer argument, does it make sense for you to assume ownership of the pointer, or does the pointee continue living after the function terminates?

People have covered the big differences: memory management, pointers, no fancy objects (just plain structs). So I will list a couple of minor things:

You have to declare things at the beginning of a block of code, not just when you want to use them for the first time
Automatic type conversion between pointers, booleans, ints and just about anything. This is typical C code: if (!ptr) { /* null pointer detected */ }
No bounds-checking in any sense, and fancy pointer arithmetic allowed. It is explicitly legal to reference things outside their bounds: ptr2 = ptr + 10; ptr2[-10] ++; is equivalent to ptr[0] ++;.
Strings are zero-terminated char arrays. Forgetting this and the previous point causes all sorts of bugs and security holes.
Headers should be separated from implementation code (.h and .c), and must explicitly reference each other via #include statements, avoiding any circular dependencies. There is a preprocessor, and compile-time macros (such as #includes) are a vital part of the language.
And finally -- C has a goto statement. But, if you ever use it, Dijkstra will rise from the grave and haunt you.

Java is a garbage collected environment, C is not. C has pointers, Java does not. Java methods use pass by reference more often, implicitly, whereas C you must explicitly indicate when you are passing by reference. Lot more considerations to be sure.