LANGUAGE » C

Union

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Basic usage ​

Members of unions share the same memory. Used if only one member at time is needed, or for low level memory optimizations.

c
union Person {
    char name[8];
    long age;
};

Examples ​

Union Person that shares the same memory for name and age. Size is 8 bytes as union, but would be 16 if it were struct.

c
#include <stdio.h>
#include <string.h>

union Person {
    char name[8];
    long age;
};

int main() {
    union Person user;
    printf("Size of user: %lu. Size of user.name: %lu. Size of user.age: %lu.\n", sizeof(user), sizeof(user.name), sizeof(user.age));
    strcpy(user.name, "Yuki");
    printf("Value of user.name: %s. Value of user.age: %ld.\n", user.name, user.age);
    user.age = 18;
    printf("Value of user.name: %s. Value of user.age: %ld.\n", user.name, user.age);
    return 0;
}

Changing the bytes individually of a bigger variable. See more examples here.

c
#include <stdio.h>

union HW_Register {
    struct {
        unsigned char byte4;
        unsigned char byte3;
        unsigned char byte2;
        unsigned char byte1;
    } bytes;
    unsigned int dword;
};

int main() {
    union HW_Register reg;
    reg.dword = 0x12345678;
    printf("Dword: %08x\nByte1: %d, Byte2: %d, Byte3: %d, Byte4: %d\n", reg.dword, reg.bytes.byte1, reg.bytes.byte2, reg.bytes.byte3, reg.bytes.byte4);
    reg.bytes.byte1 = 0;
    reg.bytes.byte3 = 0;
    printf("Dword: %08x\nByte1: %d, Byte2: %d, Byte3: %d, Byte4: %d\n", reg.dword, reg.bytes.byte1, reg.bytes.byte2, reg.bytes.byte3, reg.bytes.byte4);
    return 0;
}