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Struct initialization in C is the process of binding initial values to the contiguous memory locations representing the members of a user-defined struct type at the point of variable declaration.

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Ordered Initialization (C89)

The traditional method assigns values to struct members strictly in the order they are defined. The initializer list is enclosed in braces {} and separated by commas. 
struct Point {
    int x;
    int y;
    float z;
};

/* Values map to x, y, and z sequentially */
struct Point p1 = {10, 20, 5.5f};

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Designated Initialization (C99)

Designated initializers use the dot operator (.) to explicitly name the members being initialized. This approach is order-independent, allowing you to initialize members in any sequence. 
struct Configuration {
    int baud_rate;
    int parity;
    int stop_bits;
};

/* Order is independent of struct definition */
struct Configuration cfg = {
    .stop_bits = 1,
    .baud_rate = 9600
};

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Partial Initialization and Zero-Padding

If an initializer list provides fewer values than the struct has members, the compiler implicitly initializes all remaining unlisted members to zero (integer 0, floating-point 0.0, or NULL for pointers). 
struct Buffer {
    int id;
    char data[256];
    int *ptr;
};

/* id = 1, data array is all '\0', ptr = NULL */
struct Buffer buf1 = {1};

/* Idiomatic zero-initialization of the entire struct */
struct Buffer buf2 = {0};

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Nested Struct Initialization

When a struct contains another struct or an array, the initializer lists can be nested using additional braces. In C99, designated initializers can be chained to reach deep members directly. 
struct Transform {
    struct Point position;
    struct Point rotation;
};

/* Ordered nested initialization */
struct Transform t1 = {{0, 0, 0.0f}, {90, 0, 0.0f}};

/* Designated nested initialization */
struct Transform t2 = {
    .position = {.x = 0, .y = 0, .z = 0.0f},
    .rotation.x = 90 /* Chained designator */
};

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Compound Literals (C99)

Compound literals create an unnamed struct object in memory. While technically an expression rather than a declaration initializer, they are used to initialize struct pointers or assign complete sets of values to existing struct variables post-declaration. 
struct Point p2;

/* Assignment using a compound literal */
p2 = (struct Point){.x = 5, .y = 10, .z = 1.0f};

/* Initializing a pointer to an anonymous struct */
struct Point *p_ptr = &(struct Point){1, 2, 3.0f};

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Storage Duration Rules

The initial state of a struct depends on its storage duration if no explicit initializer is provided:
  • Automatic Storage Duration: Local structs declared without an initializer contain indeterminate (garbage) values. Reading them before assignment invokes undefined behavior.
  • Static/Thread Storage Duration: Global or static structs declared without an initializer are guaranteed by the C standard to be implicitly zero-initialized.
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