C Struct

3 min read

Compositive data type that has only public member variables. In C, (unlike C++) this is its only functionality.

Syntax

struct <optional name> {
	<type declaration>;
	<type declaration>;
	...
} <optional variable list>

The struct tag names the type, e.g.

struct car {
	char model[100];
	int year;
}

Names following the struct tag define instances of the struct, i.e. allocate memory for the instances:

struct car car1, car2;

or all in one block

struct car {
	char model[100];
	int year;
} car1, car2;

or like the following where the struct car and instance car are in difference namespaces

struct car {
	char model[100];
	int year;
} car;

Note that this is invalid; we are assigning an array to another array, which will cause a compiler error because the character array is a const character pointer, which is thus immutable.

car.model = "Ford"; // problem line
car.year = 1972;

But this is valid, because the compiler recognizes what is happening.

car = {"Ford", 1972};

Storage

  • Rule 1: Members are stored in the same order as they are initialized in source code
    • Even if it is not optimal, like in this example
  • Rule 2: Datatypes must be aligned on a multiple of their size; Natural Alignment*
    • Think about how pointer arithmetic works. You want data to be aligned on natural boundaries. If you don’t do this you need hardware to handle it.
    • If the character array came first, we would not waste bytes on filler for maintaining natural alignment
    • We align it in this way, sacrificing space for time
      • Otherwise you would have to use multiple loads to get one var, if it was not naturally aligned
  • Rule 3: The struct is filled out to the end to meet the most strict alignment of its members.
    • In this case of int which is 4 bytes. This is because: imagine what happens if we need to make an array of this struct.
    • The size of this struct is 4 x 7 Note: Compiler maintains a list of the members, their types, and an offset from the beginning of the struct

Another Example

What is the size of struct a?

struct a {
	char x, y;// sizeof char = 1 byte
	int a, b; // sizeof int = 4 bytes
	double d; // sizeof double = 8 bytes
	char z;   // sizeof char = 1 byte
}

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