3.2. Data Types

All the data types defined by C are made up of units of memory called bytes. On most computer architectures a byte is made up of eight bits, each bit stores a one or a zero. These eight bits with two states give 256 combinations (28). So an integer which takes up two bytes can store a number between 0 and 65535 (0 and 216. Usually however, integer variables use the first bit to store whether the number is positive or negative so their value will be between -32768 and +32767.

As we mentioned, there are eight basic data types defined in the C language. Five types for storing integers of varying sizes and three types for storing floating point values (values with a decimal point). C doesn't provide a basic data type for text. Text is made up of individual characters and characters are represented by numbers. In the last example we used one of the integer types: int. This is the most commonly used type in the C language.

The majority of data used in computer programs is made up of the integer types, we'll discuss the floating point types a little later. In order of size, starting with the smallest, the integer types are char, short, int, long and long long. The smaller types have the advantage of taking up less memory, the larger types incur a performance penalty. Variables of type int store the largest possible integer which does not incur this performance penalty. For this reason, int variables can be different depending what type of computer you are using.

The char data type is usually one byte, it is so called because they are commonly used to store single characters. The size of the other types is dependent on the hardware of your computer. Most desktop machines are "32-bit", this refers to the size of data that they are designed for processing. On "32-bit" machines the int data type takes up 4 bytes (232). The short is usually smaller, the long can be larger or the same size as an int and finally the long long is for handling very large numbers.

The type of variable you use generally doesn't have a big impact on the speed or memory usage of your application. Unless you have a special need you can just use int variables. We will try to point out the few cases where it can be important in this book. A decade ago, most machines had 16-bit processors, this limited the size of int variables to 2 bytes. At the time, short variables were usually also 2 bytes and long would be 4 bytes. Nowadays, with 32-bit machines, the default type (int) is usually large enough to satisfy what used to require a variable of type long. The long long type was introduced more recently to handle very large numeric values.

Some computers are better at handling really big numbers so the size of the data types will be bigger on these machines. To find out the size of each data type on your machine compile and run this piece of code. It uses one new language construct sizeof(). This tells you how many bytes a data type takes up.

Example 3-2. sizeof_types.c

  printf("sizeof(char) == %d\n", sizeof(char));
  printf("sizeof(short) == %d\n", sizeof(short));
  printf("sizeof(int) == %d\n", sizeof(int));
  printf("sizeof(long) == %d\n", sizeof(long));
  printf("sizeof(long long) == %d\n", sizeof(long long));

  return 0;