Data Types

This study guide condenses Chapter 4 for developers coming from high-level, garbage-collected languages like JavaScript, Python or Go. In C, data types directly define memory layout, how bits are interpreted, and how fast the hardware can work with them.

1. Key concepts

Data types

A data type defines what kind of value a variable holds, how its bit pattern is read, and exactly how many bytes it occupies. Unlike JavaScript or Python, C is statically typed: every variable's type is declared before use. C groups its types into:

• Basic types: arithmetic types, meaning integers and floating-point numbers.
• Enumerated types: enum, integer values with symbolic names.
• The void type: no value. Used for return types, empty parameter lists and generic pointers.
• Derived types: pointers, arrays, structures, unions and functions.

The sizeof operator gives the storage size of a type or variable in bytes, as a value of type size_t, printed with %zu.

#include <stdio.h>

int main(void) {
    int num = 10;
    printf("int:    %zu bytes\n", sizeof(int));
    printf("num:    %zu bytes\n", sizeof num);
    printf("double: %zu bytes\n", sizeof(double));
    return 0;
}

Integers

Integer types store whole numbers. Their sizes depend on the platform; the standard only guarantees minimum ranges. On most 64-bit systems short is 2 bytes, int is 4 and long long is 8, while long is 8 bytes on Linux and macOS but 4 on Windows.

Float and double

float is a 4-byte single-precision IEEE 754 number with about 6 to 7 significant decimal digits. double uses 8 bytes and gives about 15 to 16. Floating-point literals such as 3.14 are double unless you add the f suffix.

Fixed-width integers

C99 added <stdint.h>, with exact-width types such as int32_t and uint8_t. Use them when the exact size matters: file formats, network protocols and hardware registers.

Glossary