C Union

A union is like a structure, but all its members share the same memory. So only one member can hold a valid value at a time. The size of a union is the size of its biggest member.

Example

#include <stdio.h>

union Data {
    int i;
    float f;
    char text[8];
};

int main(void) {
    union Data d;

    d.i = 25;
    printf("i = %d\n", d.i);

    d.f = 3.5f;                 // overwrites the same memory
    printf("f = %.1f\n", d.f);
    return 0;
}

Output:

After d.f is stored, the value of d.i is not valid any more.

Union vs structure size

#include <stdio.h>

struct S { int i; float f; char text[8]; };
union  U { int i; float f; char text[8]; };

int main(void) {
    printf("struct: %zu bytes\n", sizeof(struct S));
    printf("union : %zu bytes\n", sizeof(union U));
    return 0;
}

Output:

Difference

Point	Structure	Union
Memory	Each member has its own memory	All members share one memory
Size	Sum of all members (plus padding)	Size of the biggest member
Values	All members can hold values together	Only one member at a time
Keyword	struct	union

Where is a union useful?

• When memory is very small (embedded systems).
• To read the same data in different ways, for example, as an int or as 4 bytes.
• To store a value that can be of different types, together with a variable which tells the current type.

Next: C typedef.

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