When are anonymous structs and unions useful in C11?

C11 adds, among other things, 'Anonymous Structs and Unions'.

I poked around but could not find a clear explanation of when anonymous structs and unions would be useful. I ask because I don't completely understand what they are. I get that they are structs or unions without the name afterwards, but I have always (had to?) treat that as an error so I can only conceive a use for named structs.

Anonymous union inside structures are very useful in practice. Consider that you want to implement a discriminated sum type (or tagged union), an aggregate with a boolean and either a float or a char* (i.e. a string), depending upon the boolean flag. With C11 you should be able to code

typedef struct {
    bool is_float;
    union {
       float f;
       char* s;
    };
} mychoice_t;

double as_float(mychoice_t* ch) 
{ 
   if (ch->is_float) return ch->f;
   else return atof(ch->s);
}

With C99, you'll have to name the union, and code ch->u.f and ch->u.s which is less readable and more verbose.

Another way to implement some tagged union type is to use casts. The Ocaml runtime gives a lot of examples.

The SBCL implementation of Common Lisp does use some union to implement tagged union types. And GNU make also uses them.

A typical and real world use of anonymous structs and unions are to provide an alternative view to data. For example when implementing a 3D point type:

typedef struct {
    union{
        struct{
            double x; 
            double y;
            double z;
        };
        double raw[3];
    };
}vec3d_t;

vec3d_t v;
v.x = 4.0;
v.raw[1] = 3.0; // Equivalent to v.y = 3.0
v.z = 2.0;

This is useful if you interface to code that expects a 3D vector as a pointer to three doubles. Instead of doing f(&v.x) which is ugly, you can do f(v.raw) which makes your intent clear.

struct bla {
    struct { int a; int b; };
    int c;
};

the type struct bla has a member of a C11 anonymous structure type.

struct { int a; int b; } has no tag and the object has no name: it is an anonymous structure type.

You can access the members of the anonymous structure this way:

struct bla myobject;
myobject.a = 1;  // a is a member of the anonymous structure inside struct bla   
myobject.b = 2;  // same for b
myobject.c = 3;  // c is a member of the structure struct bla

Another useful implementation is when you are dealing with rgba colors, since you might want access each color on its own or as a single int.

typedef struct {
	union{
		struct {uint8_t a, b, g, r;};
		uint32_t val;
	};
}Color;

Now you can access the individual rgba values or the entire value, with its highest byte being r. i.e:

int main(void)
{
	Color x;
	x.r = 0x11;
	x.g = 0xAA;
	x.b = 0xCC;
	x.a = 0xFF;

	printf("%X\n", x.val);

	return 0;
}

Prints 11AACCFF

I'm not sure why C11 allows anonymous structures inside structures. But Linux uses it with a certain language extension:

/**
 * struct blk_mq_ctx - State for a software queue facing the submitting CPUs
 */
struct blk_mq_ctx {
	struct {
		spinlock_t		lock;
		struct list_head	rq_lists[HCTX_MAX_TYPES];
	} ____cacheline_aligned_in_smp;
    
    /* ... other fields without explicit alignment annotations ... */

} ____cacheline_aligned_in_smp;

I'm not sure if that example strictly necessary, except to make the intent clear.

EDIT: I found another similar pattern which is more clear-cut. The anonymous struct feature is used with this attribute:

#if defined(RANDSTRUCT_PLUGIN) && !defined(__CHECKER__)
#define __randomize_layout __attribute__((randomize_layout))
#define __no_randomize_layout __attribute__((no_randomize_layout))
/* This anon struct can add padding, so only enable it under randstruct. */
#define randomized_struct_fields_start	struct {
#define randomized_struct_fields_end	} __randomize_layout;
#endif

I.e. a language extension / compiler plugin to randomize field order (ASLR-style exploit "hardening"):

struct kiocb {
	struct file		*ki_filp;

	/* The 'ki_filp' pointer is shared in a union for aio */
	randomized_struct_fields_start

	loff_t			ki_pos;
	void (*ki_complete)(struct kiocb *iocb, long ret, long ret2);
	void			*private;
	int			ki_flags;
	u16			ki_hint;
	u16			ki_ioprio; /* See linux/ioprio.h */
	unsigned int		ki_cookie; /* for ->iopoll */

	randomized_struct_fields_end
};

Well, if you declare variables from that struct only once in your code, why does it need a name?

struct {
 int a;
 struct {
  int b;
  int c;
 } d;
} e,f;

And you can now write things like e.a,f.d.b,etc.

(I added the inner struct, because I think that this is one of the most usages of anonymous structs)