If you want to develop applications with GTK+, a graphical toolkit used by the GNOME desktop environment, it is essential that you are comfortable with the C programming language. This article is meant to give you a short refresher on the basics of C that you will need to know when developing GTK+ applications.

Basic C program structure

Every C program is composed of one or more functions in the following format. The function receives a number of variable parameters, runs the commands in the function, and then returns a variable of the given type. Note that you can omit the return value by using void as your function type.

type function (parameters)
{
  local variables
  commands
}

Here is a practical example (don’t worry if you don’t understand it yet):

int my_function(int a)
{
  char b;

  b='r';
  printf("%d %c",a,c);
}

Another example function is shown below. The main() function is the only one that is required by every C program. It receives two parameters (argc and argv), which contain the command line parameters entered when the program begins and the number of parameters. The function should return an integer to exit. The following code will print out “x = 0 and y = 1”. (The printf() function will be covered later in this section!)

#include <stdio.h>

int main (int argc, char *argv[])
{
  int x, y;
  
  x = 0;
  printf ("x = %d ", x);
  
  y = 1;
  printf ("and y = %d", y);
  
  return 0;
}

gcc -o result result.c

Variable types

C provides a number of variable types. You may notice that these are very basic types, all holding numbers, but they can be used to represent any piece of data. The following table gives an overview of the types that are available to you.

C Data Types

It’s important to think about where you place the declaration when creating variables. The scope of a variable refers to which parts of the application can access the variable. In general, variables use the following scope rules in C:

1. You can only access a variable that has been declared before referencing it. In other words, the declaration of a variable must appear above its use.
2. A variable cannot be accessed outside the set of brackets where it was declared. For example, if a variable was declared inside of a for loop, it cannot be accessed outside of that loop.

You can declare variables outside any function, such as above the main() function. This is called a global variable. Also, a variable declared within a function is local to that function.

Arrays

The variable types previously mentioned are useful, but what if you want a thousand integers? It would take quite some time to create a variable for each of these integers. To solve this problem, you could create an array of integers. The following program creates an array, and then fills it up with the numbers 1 to 1000. Notice that arrays are indexed beginning from zero.

int arrayOfInt[1000], i;

for (i = 0; i < 1000; i++)
  arrayOfInt[i] = i + 1;

It is also possible to create arrays with multiple dimensions. The following applications creates an array that has 1000 columns and 1000 rows, and fills it up with the sum of the indexes.

int multiArray[1000][1000], i, j;

for (i = 0; i < 1000; i++)
  for (j = 0; j < 1000; j++)
    arrayOfInt[i][j] = i + j;

The maximum number of dimensions is defined by the compiler, but most applications do not use more than three at the most. If you reach the maximum (GCC’s maximum is 29), you should consider rethinking your approach to the problem at hand!