C will not coddle you okay, raise your hand if you think Matlab was coddling C will require your syntax to be even more perfect than Matlab. C will make you define every variable with a Type, and not let you ever change these in a given program. C will assume you are a master of everything you do. Further, C is a very basic language. Note: to be honest, all of these things have been written in C and are available as libraries, but the core C language is in some sense, bare boned.
It was and still is in some circumstances the language of choice in Operating System Development including all of Unix. Inside a function, you can initialize a static variable using the static keyword. I said "inside a function" because global variables are static by default, so there's no need to add the keyword.
What's a static variable? A static variable is initialized to 0 if no initial value is specified, and it retains the value across function calls. If we call incrementAge once, we'll get 1 as the return value. If we call it more than once, we'll always get 1 back, because age is a local variable and it's re-initialized to 0 on every single function call.
In this section I want to talk more about the difference between global and local variables. A local variable is defined inside a function, and it's only available inside that function. A global variable can be accessed by any function in the program. Access is not limited to reading the value: the variable can be updated by any function. The main difference with local variables is that the memory allocated for variables is freed once the function ends.
Well, typedef gets really useful when paired with two things: enumerated types and structures. Using the typedef and enum keywords we can define a type that can have either one value or another. C comes with a bool type, so this example is not really practical, but you get the idea. Every item in the enum definition is paired to an integer, internally.
So in this example monday is 0, tuesday is 1 and so on. A structure is a collection of values of different types. Arrays in C are limited to a type, so structures can prove to be very interesting in a lot of use cases. You can declare variables that have as type that structure by adding them after the closing curly bracket, before the semicolon, like this:. In this case I declare a single person variable named flavio , and an array of 20 person named people. Structures are very useful because we can pass them around as function parameters, or return values, embedding various variables within them.
Each variable has a label. It's important to note that structures are passed by copy , unless of course you pass a pointer to a struct, in which case it's passed by reference. In your C programs, you might need to accept parameters from the command line when the command launches. For simple needs, all you need to do to do so is change the main function signature from.
Note that there's always at least one item in the argv array: the name of the program. If this was our program, we'd have argc being 4 and argv being an array containing. If the name of our program is hello and we run it like this:. If we pass some random parameters, like this:. This system works great for simple needs. For more complex needs, there are commonly used packages like getopt. Simple programs can be put in a single file. But when your program grows larger it's impossible to keep it all in just one file.
You can move parts of a program to a separate file. Then you create a header file. A header file looks like a normal C file, except it ends with. Instead of the implementations of your functions and the other parts of a program, it holds the declarations.
The preprocessor goes and looks up the stdio. To include your own header files, you'll use quotes, like this:. Now in the main. Don't forget that to compile a program composed by multiple files, you need to list them all in the command line, like this:. And with more complex setups, a Makefile is necessary to tell the compiler how to compile the program. The preprocessor is a tool that helps us a lot when programming with C.
It is part of the C Standard, just like the language, the compiler, and the standard library. It parses our program and makes sure that the compiler gets all the things it needs before going on with the process. For example, it looks up all the header files you include with the include directive. It also looks at every constant you defined using define and substitutes it with its actual value.
That's just the start. I mentioned those 2 operations because they are the most common ones. The preprocessor can do a lot more. That's common to all the preprocessor directives. If a line starts with , that's taken care of by the preprocessor. One of the things we can do is to use conditionals to change how our program will be compiled, depending on the value of an expression.
Symbolic constants are very useful because we can give names to values without creating variables at compilation time. With define we can also define a macro.
The difference between a macro and a symbolic constant is that a macro can accept an argument and typically contains code, while a symbolic constant is a value:. Notice the parentheses around the arguments: this is a good practice to avoid issues when the macro is replaced in the precompilation process. The big difference with functions is that macros do not specify the type of their arguments or return values, which might be handy in some cases.
The preprocessor also defines a number of symbolic constants you can use, identified by the 2 underscores before and after the name, including:. Send any feedback, errata, or opinions at hey flaviocopes. If this article was helpful, tweet it. Learn to code for free. Get started. Forum Donate. Flavio Copes. This approach will give you a well-rounded overview of the language. I remember it being my second programming language ever, after Pascal. I want to introduce the first C program now, which we'll call "Hello, World!
But what is a function, anyway? A function is a routine that takes one or more arguments, and returns a single value. How do we execute a C program? In any case, when you open the terminal window you can type gcc , and this command should return an error saying that you didn't specify any file: That's good.
It means the C compiler is there, and we can start using it. You can use any editor, but for the sake of simplicity I'm going to use the nano editor in the command line: Type the program: Now press ctrl-X to exit: Confirm by pressing the y key, then press enter to confirm the file name: That's it, we should be back to the terminal now: Now type gcc hello.
Now type. Now if you call ls -al hello , you can see that the program is only 12KB in size: This is one of the pros of C: it's highly optimized, and this is also one of the reasons it's this good for embedded devices that have a very limited amount of resources.
Variables and types C is a statically typed language. This means that any variable has an associated type, and this type is known at compilation time. When you create a variable in C, you have to specify the type of a variable at the declaration.
In this example we initialize a variable age with type int : int age; A variable name can contain any uppercase or lowercase letter, can contain digits and the underscore character, but it can't start with a digit. Integer numbers C provides us the following types to define integer values: char int short long Most of the time, you'll likely use an int to store an integer.
If you are programming C on an Arduino, different board will have different limits. Unsigned integers For all the above data types, we can prepend unsigned to start the range at 0, instead of a negative number.
Using floating point numbers, we represent numbers as decimal numbers times powers of You might see floating point numbers written as 1. The following types: float double long double are used to represent numbers with decimal points floating point types. We have a compiler that converts the source code into bytecode. This can be simulated by clicking on the Compile button. Once the bytecode has been created, that same bytecode can be used without any changes on any operating system that has a virtual machine interpreter for the programming language.
Note that each of the virtual machine interpreters have different runtime library code, because each operating system has different runtime libraries. This is how the virtual machine language gets around platform dependency problems.
Once again, note that the bytecode does not need to be recompiled to run on any of the different operating systems. The only reason to recompile a program is if you changed the source code. Hopefully, you can see how virtual machine language programs will have better performance than interpreted language programs. The virtual machine languages convert the source code to an average machine code before the program is ever run.
Virtual machine languages don't quite match the performance of compiled languages because the bytecode still has to be loaded by the virtual machine before running. The source code for a Java program is a text file that ends in ". Suppose you typed out the following file, " Hello. The Java compiler is named javac.
The javac program is unique in that it does not produce actual machine code. Instead it produces something called bytecode. Unlike machine code, bytecode is not platform specific. The bytecode produced on a Windows machine is the same bytecode that is produced on a Linux machine. This means that the bytecode can be run without recompiling on any platform that has a Java interpreter.
If the compilation into bytecode is successful, the bytecode will be contained in a file called " Hello. To run this bytecode, the Java interpreter is invoked in the following way.
You've read these rules many times before and resolved to do them the next time you started code development. This time, do them—they will help you to avoid many long hours of debugging:. Accessing objects before they have a defined state can lead to strange effects. Not only does avoiding these effects require that you make sure you've set all your ints and floats to a defined state, you also have to make sure that your complex type functions, such as typedefed structs , are initialized first.
For instance, declare an object like the one in Listing 1 in the header. In Listing 1 , the object has the typical init flag and two function pointers for reading and updating data. Click on image to enlarge. Then, in the C module, initialize the object to a level for first usage.
In this case, the init flag was set to false. The variable is static here, since I have only one instance of it:. Later on during construction of the object, you can check if things were not initialized, such as shown in Listing 2. Even though they finally compile the code, modern compilers are always complaining about strange constructs.
Do not ignore these complaints. More often than not, they are right. Also do not ignore return values since they indicate the first time something has gone wrong. If you ignore such warnings, you'll have a ticking time bomb in your system that will explode at a later point. If you follow these procedures, you'll have more time left at the end of the project. After all, the end of the project is the point at which money and time are running out and people are overstressed, especially if the product isn't working and is shipping late.
Now you'll have time to help them set things right. Tip 2—Use enums as error types Every module should have a specific error return type that explains what the problem is in detail, at the time it occurs.
Such an error type already has been decoded and the cause determined. That means you only have to know what the first element in the chain is. No matter how many more errors you add between the first and the last, all you have to do is check the range or iterate. Also, this last enum value gives you the total number of entries. More on this later. Tip 3—Expect to fail Failures happen. So plan for it and use it to your advantage. In programming, it's safer to assume that the failure is not the exception in a string of successes but exactly the opposite: The good case is the only exception in a string of more commonly occurring error cases.
Tip 4—Check input values: never trust a stranger If your modules expect input data from other modules, you should never trust a stranger. That is, at the outmost layer of your software architecture, check all input values for consistency. The check has to be at the outmost layer since it must be detected as soon as possible.
Otherwise you could, for instance, dereference an invalid pointer given to you at one of your lower layers. The result: The crash dump reports that it was your software's problem, but later, after many hours of debugging, you find out that someone has given you invalid input. Here is an example using an enum error type mapped to a string representation for trace output to a console window, shown in Listing 5. The lookup table representing the strings is defined as shown in Listing 6.
Safe access to the string map that does not allow any out of bounds access is shown in Listing 7. Unfortunately enums in C are integers.
That means you could hand over any value of integer to the interface accessing the array, an error that can be avoided. Also getting the string out of the string array is a very simple offset addressing operation, which is really fast in C. So, that was range checking. You should also check for NULL pointers if someone gives you an address value.
You cannot check pointers for anything other than the NULL value, but this is better than nothing. Tip 5—Write once, read many times When we read other people's code, we're thankful for any good line of comment or more readable code; most of the time, however, the original coder hasn't been so kind to us. If the variables are called i, j, and k, you'll soon have a mental break down. Often the longest variable name is pbuf. It will be faster to rewrite it. So what can you do? First of all, if you write code, write it to be as readable as a newspaper.
Well-written code requires only a few lines of comments. Also consider that although code is nothing for compilers, it needs to be readable by human beings. Don't be lazy at typing new variable names and, if required, add the unit to the name. For example, do not call parameters Size , Length , Temperature , or Angle.
Instead, since all those parameters have a unit, call them:. Not using units in your application programming interface's definitions can also cause major design failures. If you've written code that requires some renaming, I recommend the use of the open-source Eclipse Development Environment www.
It has a great feature called Refactoring that renames any kind of object everywhere in the code.
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