Unions in C

A union is a user-defined data type similar to a structure, but all members share the same memory location. The size of a union is equal to the size of its largest member.

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Defining and Using a Union

union Data {
    int i;
    float f;
    char c;
};

union Data d;
d.i = 65;
printf("int: %d\n", d.i);   /* 65 */

d.f = 3.14;
printf("float: %f\n", d.f); /* 3.14 */
/* d.i is now corrupted — only one member valid at a time */

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Structure vs Union

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Size Example

struct S {
    int i;     /* 4 bytes */
    float f;   /* 4 bytes */
    char c;    /* 1 byte  */
};             /* sizeof = 12 (with padding) */

union U {
    int i;     /* 4 bytes */
    float f;   /* 4 bytes */
    char c;    /* 1 byte  */
};             /* sizeof = 4 (largest member) */

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Practical Use: Variant Records

Unions are useful when a variable can hold different types at different times:

struct Shape {
    int type;   /* 0 = circle, 1 = rectangle */
    union {
        struct { float radius; } circle;
        struct { float width, height; } rect;
    } data;
};

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Initialising a Union

Only the first member can be initialised in the declaration:

union Data d = {42};   /* initialises d.i = 42 */