Correct, portable way to interpret buffer as a struct

I guess this is what I’ve been trying to avoid, but I finally went and took a look at the C99 standard myself. Here’s what I’ve found (emphasis added):
§6.3.2.2 void

1 The (nonexistent) value of a void expression (an expression that has type void) shall not
be used in any way, and implicit or explicit conversions (except to void) shall not be
applied to such an expression. If an expression of any other type is evaluated as a void
expression, its value or designator is discarded. (A void expression is evaluated for its
side effects.)

§6.3.2.3 Pointers

1 A pointer to void may be converted to or from a pointer to any incomplete or object
type. A pointer to any incomplete or object type may be converted to a pointer to void
and back again; the result shall compare equal to the original pointer.

And §3.14

1 object
region of data storage in the execution environment, the contents of which can represent
values

§6.5

An object shall have its stored value accessed only by an lvalue expression that has one of
the following types:
— a type compatible with the effective type of the object,
— a qualified version of a type compatible with the effective type of the object,
— a type that is the signed or unsigned type corresponding to the effective type of the object,
— a type that is the signed or unsigned type corresponding to a qualified version of the effective type of the object,
— an aggregate or union type that includes one of the aforementioned types among its
members (including, recursively, a member of a subaggregate or contained union), or
— a character type.

§6.5

The effective type of an object for an access to its stored value is the declared type of the
object, if any. If a value is stored into an object having no declared type through an
lvalue having a type that is not a character type, then the type of the lvalue becomes the
effective type of the object for that access and for subsequent accesses that do not modify the stored value. If a value is copied into an object having no declared type using
memcpy or memmove, or is copied as an array of character type, then the effective type
of the modified object for that access and for subsequent accesses that do not modify the
value is the effective type of the object from which the value is copied, if it has one. For
all other accesses to an object having no declared type, the effective type of the object is
simply the type of the lvalue used for the access.

§J.2 Undefined Behavior

— An attempt is made to use the value of a void expression, or an implicit or explicit
conversion (except to void) is applied to a void expression (6.3.2.2).

Conclusion

It is ok (well-defined) to cast to-and-from a void*, but not ok to use a value of type void in C99. Therefore the “real world example” is not undefined behavior. Therefore, the explicit casting method can be used with the following modification, as long as alignment, padding, and byte-order is taken care of:

void receive_message(void *bytes, size_t len) {
    assert(len >= sizeof(struct message);
    struct message *msg = (struct message*) bytes;
    /* And now use the message */
    if (msg->command == SELF_DESTRUCT)
        /* ... */
}