anonymous structures/unions in C with example

What are anonymous structures/unions?

Structures/unions with no names are called anonymous structures/unions. They are also called unnamed unions/structures.

As there are no names, we cannot create variables of anonymous structures/unions, but we can use them in nested structures or unions.

Real World Example of anonymous structures/unions:

A status register of a microcontroller can have multiple bits. Sometimes we read the value of each bit, and sometimes the whole register. We can use anonymous structure/unions in this to easily refer to both.

Code:

	#include <stdio.h>
	typedef union {
	struct {
	unsigned char c:1; //carry flag
	unsigned char r:1; //reset flag
	unsigned char o:1; //overflow flag
	unsigned char s:1; //sign flag
	unsigned char z:1; //zero flag
	unsigned char i:1; //interrupt flag
	unsigned char t:1; //timer flag
	unsigned char n:1; //negative flag
	};
	unsigned char value;
	}status_register;
	int main()
	{
	status_register reg;
	reg.c = 1;
	reg.r = 1;
	reg.n = 1;
	printf("Status register value:0x%02x\n", reg.value);
	}

view raw anonymous.c hosted with ❤ by GitHub

Output:

$ ./anonymous

Status register value:0x83