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Introduction To C Programming

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Published in: C / C++
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Introduction to C, Structure of C programming, Compilation and Linking process in C are explained.

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Qualification: B.Tech/B.E. ( Saveetha Engineering College, Chennai - 2017)

Teaches: Basic Computer, MS Office, School Level Computer, Biology, Physics, English, EVS, Mathematics, Computer, Instrumentation, C / C++

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  1. Introduction to C Every full C program begins inside a function called "main". A function is simply a collection of commands that do "something". The main function is always called when the program first executes. From main, we can call other functions, whether they be written by us or by others or use built-in language features. To access the standard functions that comes with your compiler, you need to include a header with the #include directive. What this does is effectively take everything in the header and paste it into your program. Let's look at a working program: #include int main() printf( "I am alive! Beware.\n" ); getchar(); return 0; Let's look at the elements of the program. The #include is a "preprocessor" directive that tells the compiler to put code from the header called stdio.h into our program before actually creating the executable. By including header files, you can gain access to many different functions--both the printf and getchar functions are included in stdio.h. The next important line is int main(). This line tells the compiler that there is a function named main, and that the function returns an integer, hence int. The "curly braces" ( { and }) signal the beginning and end of functions and other code blocks. If you have programmed in Pascal, you will know them as BEGIN and END. Even if you haven't programmed in Pascal, this is a good way to think about their meaning. The printf function is the standard C way of displaying output on the screen. The quotes tell the compiler that you want to output the literal string as-is (almost). The '\n' sequence is actually treated as a single character that stands for a newline (we'll talk about this later in more detail); for the time being, just remember that there are a few sequences that, when they appear in a string literal, are actually not displayed literally by printf and that '\n' is one of them. The actual
  2. effect of '\n' is to move the cursor on your screen to the next line. Notice the semicolon: it tells the compiler that you're at the end of a command, such as a function call. You will see that the semicolon is used to end many lines in C. The next command is getchar(). This is another function call: it reads in a single character and waits for the user to hit enter before reading the character. This line is included because many compiler environments will open a new console window, run the program, and then close the window before you can see the output. This command keeps that window from closing because the program is not done yet because it waits for you to hit enter. Including that line gives you time to see the program run. Finally, at the end of the program, we return a value from main to the operating system by using the return statement. This return value is important as it can be used to tell the operating system whether our program succeeded or not. A return value of 0 means success. The final brace closes off the function. You should try compiling this program and running it. You can cut and paste the code into a file, save it as a .c file, and then compile it. If you are using a command-line compiler, such as Borland C++ 5.5, you should read the compiler instructions for information on how to compile. Otherwise compiling and running should be as simple as clicking a button with your mouse (perhaps the "build" or "run" button). The Structure of a C programs is given below: Include Header file Section Global Declaration Section / * Comments * / main() / * Comments * /
  3. Include Header file section: C program depends upon some header files for function definition that are used in program. Eg. #inlcude Global Declaration section: This section declares some variables that are used in more than one function. These variables are known as global variables. Function main: Every program written in C language must contain main function. It is the starting point of every C program. Declaration part: It contains the entire variables that are used in the executable part. The initialization of variables are also done here. Executable part: This part contains the statements following the declaration of the variables. User defined functions: The functions defined by the users are called user defined functions. They are generally defined after the main function. Comments: To understand the flow of the program, the programmer can include comments in the program. Programming Rules: 1. 2. 3. 4. 5. All statements must be in lower case letters. Blank spaces may be inserted between the words. The opening and closing braces must be balanced. The statements can be written anywhere between the opening and closing braces. Every statement must be terminated by a semicolon (;). Compilation Compilation refers to the processing of source code files (.c, .cc, or .cpp) and the creation of an 'object' file. This step doesn't create anything the user can actually run. Instead, the compiler merely produces the machine language instructions that correspond to the source code file that was compiled. For instance, if you compile (but don't link) three separate files, you will have
  4. three object files created as output, each with the name .o or .obj (the extension will depend on your compiler). Each of these files contains a translation of your source code file into a machine language file -- but you can't run them yet! You need to turn them into executables your operating system can use. Linking Linking refers to the creation of a single executable file from multiple object files. In this step, it is common that the linker will complain about undefined functions (commonly, main itself). During compilation, if the compiler could not find the definition for a particular function, it would just assume that the function was defined in another file. If this isn't the case, there's no way the compiler would know -- it doesn't look at the contents of more than one file at a time. The linker, on the other hand, may look at multiple files and try to find references for the functions that weren't mentioned. You might ask why there are separate compilation and linking steps. First, it's probably easier to implement things that way. The compiler does its thing, and the linker does its thing -- by keeping the functions separate, the complexity of the program is reduced. Another (more obvious) advantage is that this allows the creation of large programs without having to redo the compilation step every time a file is changed. Instead, using so called "conditional compilation" it is necessary to compile only those source files that have changed; for the rest, the object files are sufficient input for the linker. Finally, this makes it simple to implement libraries of pre- compiled code: just create object files and link them just like any other object file. (The fact that each file is compiled separately from information contained in other files, incidentally, is called the "separate compilation model".) To get the full benefits of condition compilation, it's probably easier to get a program to help you than to try and remember which files you've changed since you last compiled. (You could, of course, just recompile every file that has a timestamp greater than the timestamp of the corresponding object file.) If you're working with an integrated development environment (IDE) it may already take care of this for you. If you're using command line tools, there's a nifty utility called make that comes with most *nix distributions. Along with conditional compilation, it has
  5. several other nice features for programming, such as allowing different compilations of your program -- for instance, if you have a version producing verbose output for debugging. Knowing the difference between the compilation phase and the link phase can make it easier to hunt for bugs. Compiler errors are usually syntactic in nature -- a missing semicolon, an extra parenthesis. Linking errors usually have to do with missing or multiple definitions. If you get an error that a function or variable is defined multiple times from the linker, that's a good indication that the error is that two of your source code files have the same function or variable.

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