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    An Introduction to C & GUI Programming
    An Introduction to C & GUI Programming
    An Introduction to C & GUI Programming
    Ebook225 pages1 hour

    An Introduction to C & GUI Programming

    By Simon Long

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    About this ebook

    The popular C programming language is used for a huge range of applications, from the tiny microcontrollers used in toasters and watches up to complete operating systems. The first half of this book is an introduction to C, and covers the basics of writing simple command-line programs. The second half of the book shows how to use the GTK user interface toolkit with C to create feature-rich GUI applications which can be run on the desktop. No previous experience of C or GTK is required - even if you are an absolute beginner, this book will teach you all you need to know.

    In this book, you'll learn how to:

    • Create simple command-line C programs.
    • Control flow with conditions and loops.
    • Handle variables, strings, and files.
    • Design graphical user interface applications in C.
    • Handle user input with buttons and menus.
    • Use advanced UI features such as data stores and dialogs.

    Updated for GTK3, this book will teach you all you need to know to write simple programs in C and start creating GUIs. Although the examples in this book were developed on a Raspberry Pi running, Raspberry Pi OS, you can follow along on other operating systems, including Linux, macOS, and Windows with the Windows Subsystem for Linux.

    LanguageEnglish
    PublisherRaspberry Pi Press
    Release dateDec 18, 2022
    ISBN9781916868298
    An Introduction to C & GUI Programming
    Author

    Simon Long

    Simon Long is an engineer working for Raspberry Pi. He is responsible for the Raspberry Pi Desktop and its associated applications. Before joining Raspberry Pi, he worked for Broadcom, where he first met Eben Upton, and before that spent ten years working as a software engineer and user interface designer for a major consultancy firm. In his spare time, he enjoys solving those really hard crosswords without any black squares.

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      Book preview

      An Introduction to C & GUI Programming - Simon Long

      Chapter 1

      Getting started

      C is one of the most widely used programming languages – learn how to use it to program the Raspberry Pi!

      What’s so great about C?

      C is a very versatile and widely used programming language. It has been used to write pretty much everything, from low-level routines to control the hardware in embedded microcontrollers to complete operating systems like Linux with graphical user interfaces. In spite of this huge flexibility, it is also relatively simple – the language only has about 20 or so keywords, but there are huge libraries of additional functions that you can call on when you need them. In the first part of this book, we are going to concentrate on learning about the keywords, with a few of the more commonly used library functions; the second part of the book shows how to use the GTK library to make it easy to write graphical interfaces in C.

      Many of the languages that you may have seen, such as Python, are what are called interpreted languages. This means that the code you write is run directly: each line of code is read in and interpreted as you run it. C is different: it’s a compiled language. This means that the code you write, known as the source code, is never run directly. The source code is passed through a program called a compiler, which converts it into a machine-readable version called an executable or a binary; you then run the resulting executable.

      This may seem complex, but it has a few big advantages. First, it means that you don’t need to have a copy of C itself on every computer you want to run your program on; once compiled, the executable is standalone and self-contained. Second, the compilation process will find a lot of errors before you even run the program (but it won’t usually find all of them). Most importantly, the compilation process means that the time-consuming translation of human-readable code into machine-readable instructions has already happened, which means that compiled code generally runs many times faster than interpreted code would.

      CHOOSE YOUR EDITOR

      You can use whatever editor you like to enter code, as long as it saves it as plain text. The Geany editor included in Raspberry Pi OS is a good choice, but you can also use Leafpad, nano, or any others that you prefer.

      WHITESPACE DOESN’T MATTER!

      Unlike Python, whitespace has no significance in C – you can put spaces, tabs, and new lines anywhere you like in a C program to make it readable.

      Hello world – your first C program

      With all that out of the way – which has hopefully made you think that C might be worth learning – let’s have a look at the first program everyone writes in any language, the one that prints ‘Hello World’ on the screen. Incidentally, the tradition of writing a Hello World program was first introduced with the original documentation describing C itself. Just think: no C, no Hello World…

      #include

      void main (void)

      {

      /* A print statement */

      printf (Hello world!\n);

      }

      Hopefully not too frightening! Let’s look at it line by line.

      #include

      This is known as a hash-include. As mentioned above, the C language has a large library of functions that can be included, and we need to use one of them in this program: the formatted print command printf. This is part of the standard input-output library, or stdio for short. So what this line does is to warn the compiler that the program needs the stdio library to be included as part of the compile process.

      void main (void)

      C is a function-based language; every program is made up of a number of functions.

      Each function takes zero or more arguments, and returns a single value. A function definition consists of a specification of what the function returns (in this case, a void), a function name (in this case, main), and a list of arguments enclosed in round brackets (again, a void).

      Every C program has to include a function called main; when you run the compiled program, the main function is the first thing that executes.

      The word void is called a type specifier; a void is a special type which means ‘no value required’. We’ll look more at types in the next chapter.

      So this line defines the main function for this program; it states that the main function takes no arguments, and returns no value.

      The code which makes up the function itself is enclosed between the two curly brackets {} that follow the function definition.

      /* A print statement */

      First, we have a comment telling us what’s going on. Comments in C start with the symbol /*, and end with */ – anything between those two symbols is ignored by the compiler.

      The code itself is just one line:

      printf (Hello world!\n);

      This is a call to the printf (‘print formatted’) function from the stdio library. In this case, it takes a single argument, which is a text string enclosed within double quotes. As mentioned above, function arguments are enclosed in round brackets.

      Note that the line ends with a semicolon. All statements in C must finish with a semicolon; this tells the compiler that this is the end of a statement. One of the most common beginner mistakes in C is to forget a semicolon somewhere!

      What about the string itself? The Hello World! bit is straightforward enough, but what about that \n at the end? Remember this function is called ‘print formatted’? Well, the \n is a bit of formatting; it’s the symbol for a newline character. So this line will print the string ‘Hello World!’, followed by a new line.

      CHECK YOUR BRACKETS

      Unlike whitespace, punctuation is very important in C – make sure you don’t use a curly bracket where a round one is needed, or vice versa.

      Compiling your program

      Let’s compile and run this. Raspberry Pi OS includes a C compiler called gcc, so there’s nothing to install; just start up Raspberry Pi OS on your Raspberry Pi and you’re ready to go. Use your favourite text editor to create a file called hello.c, copy the program above into it, and save it. Then, from a terminal, go into the directory where you saved hello.c and enter:

      gcc –o myprog hello.c

      This calls the gcc C compiler with the option -o myprog, which tells it to create an executable output file called myprog, and to use hello.c as the input source code.

      If you entered your C code correctly (did you make sure the semicolon was there?), this should take a second or so and then return you to the command line. There should now be a file in the current directory called myprog – try running it by typing:

      ./myprog

      Et voilà! You should now have…

      Hello World!

      …written in the terminal.

      A command prompt window titled “pi at raspberry pi”, depicts how a program is compiled. The command g c c hyphen o my prog hello dot c, calls the g c c compiler, and the code dot forward slash my prog, produces the output, Hello World!

      You interact with both the C compiler and your compiled C programs from the command line; you can either do this in a terminal window in the desktop, or by booting your Raspberry Pi straight to the command line

      RUNNING YOUR PROGRAM

      You need to tell Linux that the program you want to run is in the current directory, so don’t forget the ./ before myprog, or it won’t know where to look!

      That’s your first C program written, compiled, and run. In the next chapter, we’ll start using C for something a bit more useful…

      Chapter 2

      Variables and arithmetic

      Doing some real work in C: creating variables and performing mathematical operations on them

      In some languages, you can create variables as you go along and put whatever data you want into them. C isn’t like that: to use a variable in C, you need to have created it first, and at the time you create it, you have to set what type of value it’s going to store. By doing this, a block of memory of the correct size can be allocated by the compiler to hold the variable. This process of creating a variable is known as declaration.

      Integers

      There are several fundamental data types in C, but we’ll start by looking at one of the most commonly used: the int type, used to store an integer value.

      #include

      void main (void)

      {

      int a;

      int b = 3;

      int c;

      a = 2;

      c = a + b;

      printf (The sum of adding %d and %d is %d\n, a, b, c);

      }

      The top three lines inside the main function here are declarations. They tell the compiler that we would like to use variables called a, b, and c respectively, and that each one is of type int, i.e. an integer.

      In the second line, we see an example of an initialisation at the same time as a declaration: this stores an initial value of 3 in the variable b. Note that the values of a and c at this point are undefined; you might assume that a variable which hasn’t had a value stored in it is always 0, but that isn’t the case in C. Before reading the value from a variable or using it in a calculation, you must store a value in it; reading a variable before initialising it is a common error in C.

      The next two lines do some actual work with the variables we have declared.

      a = 2;

      This stores a value of 2 in the variable a, which will now have this value until it’s changed. The reason a is called a variable is that it can vary: you can change its value as often as you like, but only to another integer. The value of a variable can change, but its type is fixed when it is declared.

      c = a + b;

      This line adds a to b, and stores the result in c.

      printf (The sum of adding %d and %d is %d\n, a, b, c);

      This is another use of the formatted print function we saw in the previous chapter. Note the three %d symbols inside the string: these are format specifiers, and they are how you output numbers in C. When the printf function is executed, each %d is replaced

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