C #2 | The First Program

For the complete list of the tutorial on this blog, you can check these following links:

1. Introduction
2. First C-Program
3. Types and Variables
4. Conditional Statements
5. Function and Math Operations
6. Arrays
7. Pointers
8. I/O in C

Wise and experienced programmers will always tell you this, the only way to learn a programming language is to write programs. It is true and I will just jumpstart by creating the very first simple C program. This program will simply prints some words, and play with how to bring the texts appear on the screen with printf function.

First thing to know is that, C-language are written in .c and .h file extensions. .c is the compile-able file that consist every functions and routines while .h is where you stores function names, variables, headers etc. You can live with .c alone but sometimes in order for programmer to be able to hide the intellectual property of, e.g the functions inside .c, programmers merely write the function name inside .h and let the compiler transform .c with the definition of the function into either static or dynamic library. Thus, everyone who wants to use corresponding function, they will be provided by just the .h and the library, without requiring them to have the original .c code.

line:01 demonstrates the use of a # preprocessor. A preprocessor provides the ability for the inclusion of header files, macro expansions, conditional compilation, and line control. In many C implementations, it is a separate program invoked by the compiler as the first part of translation. Check…

Preprocessor is a separate step in the compilation process, thus is not part of the compiler. If you invoke a preprocessor in C, it will substitute the text with the one that is referred by the preprocessor, whether it is a macro function or a file header. Here a list of preprocessor that will be used frequently on this tutorial.

line:03 is where you can find the first function called main. A bracket () after the function name collects the argument. Empty bracket means no argument passed onto the function.

line:04 and line:08 contain the function braces {}. Statements of a function are enclosed in braces. { begins the statements and } ends the statements.

line:05 shows the invocation of function printf and an argument "Hi, world\n". This function is called from the library stdio.h that is described from line:01. printf prints output to the screen, in this case, strings from our argument between quotes " ". \n tells printf to give a newline after printing the argument.

line:07 with return statement terminates the execution. However, as the name implies, return gives certain value back to the calling function. If you call a function A within function main, and you invoke return value from within A, then main will get the value that computed inside A. More detailed about return statement can be found on Wiki page.

How to Comments

When you put some information inside /* and */, you just made a comment. This is what we called with multi-line comment, or sometimes also called a block comment. Comments are used by the programmers to explain functionality of certain parts of codes, or to keep notes for themselves.

On line:01, it shows how block comment works. Any characters between /* and */ are ignored by the compiler. On line:9 and 10, a single line comments are provided. A single line comment starts with // and everything after it is ignored by the compiler until end of the line. This is originated from C++ style comment, but nowadays most of C compilers support this comment.

Quick Explanation on printf()

Originall, printf() is declared with,

int printf(const char *format, ...);

format in here is the string that contains the text to be displayed. In sort, this is the text you wanted to see. It can be accompanied by the variables. An example of printf("Hi World") will bring texts “Hi World” to be displayed on terminal. You can add format tags to determine data types, and point it out to the corresponding variables. If you have int a and want to show it after “Hi World”, then the command will be printf("Hi World %d", a). The tag is %, and d represents signed decimal integer. %f is format tag for floating point data type, %s for characters, and %x for hexadecimal integer. To check all of available format tags for C, you can see it here. For the tutorial about variables, please refer to the end of this tutorial.

Spaces on C

Also, one thing to notice about C-syntax is, it does not care about spaces.

Those lines will resulted on same result with previous codes. But, remember humans will read your codes as well. So, keep it mind to make readable codes for your colleagues, friend or bosses.

How to Run C-program

In order to execute a C program, we have to make sure main() function does exist in our code. For now, it is located in main.c C file. Then, there are two important steps to make it executable, by compiling and linking.

Nowadays, there are lot of GUI (Graphical User Interface) development tools available for you to use. If you are full time Windows user, then probably most of the time you will use Visual Studio. For Mac, Xcode probably is the best IDE (Integrated Development Environment) to use. UNIX based users probably know how good Eclipse CDT is, probably also considered as IDE with the most complete features yet. Still, there are many 3rd party IDEs out there for everyone to check. But, one thing to notice here is, every operating systems and IDE has different C standard and thus, sometimes you have to design cross-platform friendly codes if you want your code to be executable (compile-able) in different platforms. However, you’ll gonna deal with specific platform dependancies that probably will hinder you from working efficiently. Myself, I’d rather stick with UNIX C standard (C99) as I’m using Mac OSX. I’m using GCC as my compiler and it comes as native compiler in most of UNIX based operating systems like Linux. GCC is also available in Windows with help of MinGW.

By using IDEs, programmers do not need to bring down their feet into command line and deal with manual compilation. But, ones should understand that GUI based IDEs are basically using same commands with automation. Also, on learning phase, compiling using command line compiler will let you learn how units of code are compiled, linked and mapped together. You have to tell explicitely compiler which files to compile and library to link. You will gain better understanding on how your whole program works. An IDE will save you lot of time, but probably if you’re still learning, then it will keep you ignorant and that’s something that we don’t want you to become.

You then take it a step further and learn to create your own make-file. This is a valuable skill. Your make-file makes the build cycle approach the convenience of “IDE magic” while still retaining full understanding and learning. There will be a specific tutorial for makefile, however if you want to jump as soon as possible, please check Makefile guide.

How to Compile

We’re gonna use gcc for all of the compilation. gcc is the C and C++ compiler developed by GNU project. It is widely used as the default compiler of UNIX-like systems. If you are using Mac, you may get it automatically installed by installing Xcode Command Line Tools.

Open command line or terminal if you’re using Mac, and navigate to your source codes directory. First, check whether you have gcc installed or not.

$ gcc --version

If you’re using Mac OSX (Yosemite or later), most of the chance you will have this result instead,

Configured with: --prefix=/Applications/Xcode.app/Contents/Developer/usr --with-gxx-include-dir=/usr/include/c++/4.2.1
Apple LLVM version 7.0.0 (clang-700.0.72)
Target: x86_64-apple-darwin15.0.0
Thread model: posix

This is because from Mac OSX 10.9 onwards, the gcc that came has LLVM clang as back-end, still we can use gcc command for general compilation.

To compile main.c to get executable program myapp, then the gcc or clang command will be,

$ gcc main.c -o myapp

To execute the program myapp, you can type

$ ./myapp

And this result will appear,

Hi World

If you’re dealing with compiling multiple file sources, then refer to Tutorial 5 which talks about functions in C.

Next, we will go through the basic but one of the core in C, variables and data types.