Programming in C

DATA TYPES IN C

The type or data type of a variable determines the set of values that the variable can take and the set of operations that can be applied to those values. Every variable in C must be associated with a data type, which informs the compiler about the memory requirement, range of values, and valid operations.

Data types in C can be broadly classified as shown in Fig. 8.7.

Classification of Data Types in C

C data types are classified into the following categories:

1. Primitive / Basic Data Types

char
int
float
double
void

2. Derived Data Types

Array
Function
Pointer

3. User-Defined Data Types

Structure
Union
Enumeration

4. Valueless Data Type

void

C provides a standard minimal set of basic (primitive) data types, from which more complex data types can be constructed.

Basic Data Types in C

C has five basic data types, as shown in Fig. 8.8:

Data Type	Keyword
Character	char
Integer	int
Floating-point	float
Double precision floating-point	double
Valueless	void

Sizes and Ranges of Basic Data Types

The size and range of data types depend on the machine architecture.

For a 16-bit Computer

char → 8 bits (–128 to 127)
int → 16 bits (–32768 to 32767)
float → 32 bits (1.17549 × 10⁻³⁸ to 3.40282 × 10³⁸)
double → 64 bits (2.22507 × 10⁻³⁰⁸ to 1.79769 × 10³⁰⁸)
void → valueless

For a 32-bit Computer

char → 8 bits (–128 to 127)
int → 32 bits (–2147483648 to 2147483647)
float → 32 bits
double → 64 bits
void → valueless

According to IEEE standards, the precision of:

float ≈ 6 digits
double ≈ 15 digits

Void Data Type

The void data type specifies an empty set of values.
It has no values and only one operation, i.e., assignment.

Used as the return type of functions that do not return any value
Does not refer to any object
Is an incomplete type

According to ISO/IEC:

“The void type comprises an empty set of values; it is an incomplete type that cannot be completed.”

Type Modifiers and Qualifiers

C provides:

4 type specifiers (modifiers)
3 type qualifiers

Important Rules

Modifiers apply mainly to int
signed and unsigned can be applied to char
long can also be applied to double
If the base type is omitted, int is assumed
void has no modifiers

Type Specifiers and Qualifiers Classification

1. Size Specifiers

short
long

2. Sign Specifiers

signed
unsigned

3. Type Qualifiers

const
volatile
restrict

Size Specifiers (short and long)

The size specifiers alter the size of the base data type.

Rules for Integer Sizes

Minimum size of short int → 2 bytes
int ≥ short int
long int ≥ int
Minimum size of long int → 4 bytes

Typical Sizes

16-bit systems:
short int = int = 2 bytes, long int = 4 bytes
32-bit systems (gcc):
short int = 2 bytes, int = long int = 4 bytes

The long qualifier is also applicable to double.

C99 Extended Integer Types

C99 introduced:

long long int
unsigned long long int

These occupy at least 64 bits.

Type Qualifiers

const

Makes a variable read-only
Value must be assigned at declaration
Enables compiler optimization

Example:


const float pi = 3.14156;

Note:
const does not make a variable a constant; it only prevents modification through that variable.

volatile

Indicates a variable that can change unexpectedly
Commonly used for hardware-linked variables
Prevents compiler optimization that may remove required memory access

restrict

Helps compilers generate faster executables
Mainly useful in performance-critical programs

Character Data Type (char)

A char variable occupies one byte and stores the ASCII code of characters.

'A' → ASCII 65
'a' → ASCII 97

C treats char as a sub-range of int.
Thus, characters can be used in arithmetic expressions.

Signedness of char

Default signed char range: –128 to 127
unsigned char range: 0 to 255

Unsigned char is useful for portable programs and non-character data storage.

Integer Data Types

Signed Integer Types

short
int
long
long long

Most systems use 2’s complement representation.

2’s Complement Rules

Leftmost bit is the sign bit
Negative numbers are obtained by:
- Complementing bits
- Adding 1

Other encodings (1’s complement, sign-magnitude) are also permitted.

Unsigned Integer Types

Use straight binary representation
Range: 0 to 2ⁿ – 1
Frees the sign bit for magnitude

Example use cases:

Memory addresses
Population counts
Stadium seating capacity

Floating-Point Data Types

float

4 bytes
Approx. 7 significant digits
Used for fractional numbers

⚠ Floating-point values are approximations, not exact values.

double

8 bytes
15 digits precision
Used in scientific and engineering computations
Provides larger range and higher accuracy

Boolean Data Type (_Bool)

Introduced in C99.

Stores only 0 (false) or 1 (true)
Any non-zero value becomes 1
Header file <stdbool.h> defines:
- bool
- true
- false

Conclusion

Thus, data types in C define how data is stored, interpreted, and manipulated. Proper understanding of basic data types, modifiers, and qualifiers is essential for writing efficient, portable, and error-free C programs, especially for large-scale and system-level applications.

📖 Reference

The content for this subject is prepared by referring to the standard textbook
“Computer Fundamentals and Programming in C”
by Pradip Dey and Manas Ghosh,
Second Edition, Oxford University Press (2018).

The explanation is exam-oriented and strictly aligned with the textbook.

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2.5 DATA TYPES