Difference Between Structure and Union (C Programming)

• Memory Allocation: A structure allocates distinct, separate memory space for every single variable inside it.
• Total Size: The total memory size of a structure is the sum of the memory sizes of all its individual members.
• Data Access: Because every variable has its own physical space, you can access, read, and write to all the variables simultaneously without any data getting corrupted.
• Keyword: Defined using the keyword struct.

• Memory Allocation: A union allocates only one shared memory block for all the variables inside it.
• Total Size: The total memory size of a union is equal to the size of its largest member variable. (e.g., If it has an int (4 bytes) and a double (8 bytes), the total size of the union is just 8 bytes).
• Data Access: Because all variables share the exact same physical memory location, you can only safely use one variable at a time. If you store a value in the integer, and then store a value in the double, the integer's data is instantly overwritten and destroyed.
• Keyword: Defined using the keyword union.

You might wonder why anyone would use a Union if it destroys data. Unions are highly specialized. They are used in Embedded Systems programming (like microcontrollers in microwaves or cars) where RAM is extremely scarce (sometimes only 2 Kilobytes), and memory conservation is more important than holding multiple values simultaneously.