Struct(s) and Union(s)

systems memory Structs (Structures) and Unions are mechanisms for creating custom data types. In Systems Programming, understanding their exact memory layout is critical for interacting with hardware protocols and binary file formats.

Structs (struct)

A way to group related variables of different types under one name.

struct Point {
    int x;
    int y;
};

• Memory: Members are stored sequentially in memory (with potential gaps, see Padding).

Unions (union)

A data type where all members share the same memory location. The size of the union is the size of its largest member.

union Data {
    int i;
    float f;
    char str[4];
};

• Use Case: low-level type punning (interpreting bits of a float as an int) or saving memory when variables are mutually exclusive.

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Memory Alignment & Padding

The Central Processing Unit(s) (CPU) prefers to access memory at aligned addresses (e.g., 4-byte integers at addresses divisible by 4). To enforce this, compilers insert Padding Bytes.

Example:

struct Bad {
    char c;   // 1 byte
    // 3 bytes PADDING inserted here
    int i;    // 4 bytes
};
// sizeof(struct Bad) = 8 bytes (not 5!)

Packing: To force exact layout (e.g., for network headers), you must tell the compiler to stop padding.

struct __attribute__((packed)) NetworkHeader {
    char id;
    int size;
};
// sizeof = 5 bytes (Performance penalty on some CPUs)

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Bit Fields

C allows you to specify the exact number of bits a member uses. Crucial for hardware registers.

struct Flags {
    unsigned int is_visible : 1; // Uses 1 bit
    unsigned int is_admin   : 1; // Uses 1 bit
    unsigned int reserved   : 30;
};

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