Understand the key differences between unions and structures, particularly in how they store data in memory.
Unions and structures are both user-defined data types in C that can hold a collection of members of different types. However, they have a fundamental difference in how they manage memory, which leads to very different use cases. A **structure** allocates enough memory to store all of its members simultaneously. Each member has its own unique memory location within the structure's memory block. The total size of a structure is at least the sum of the sizes of all its members (it might be slightly larger due to memory alignment). This means you can store values in all members of a structure at the same time. A **union**, on the other hand, allocates only enough memory to store its largest member. All members of a union share the same memory space. This means a union can only hold a value for one of its members at any given time. When you assign a value to one member of a union, you overwrite any data that was previously stored in any other member. The primary use case for a union is memory saving. If you have a data structure where you know you will only need to use one of several possible members at a time, a union is more memory-efficient than a structure. Another common use is to interpret the same block of raw memory in different ways, for example, accessing the individual bytes of an integer. In summary: use a structure when you need to store multiple related values at once; use a union when you need to store one value from a set of possible types in the same memory location.