/*-
 * Copyright (c) 2019 Taylor R. Campbell
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 *
 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 * ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 * SUCH DAMAGE.
 */

#ifndef	CONTAINER_OF_H
#define	CONTAINER_OF_H

#include <stddef.h>		/* offsetof */

/*
 * container_of(P, T, F)
 * const_container_of(P, T, F)
 *
 *	If P is a pointer to the member F of a structure type T, return
 *	a pointer to the enclosing structure of type T.  For example:
 *
 *		struct foo {
 *			int x;
 *			char y;
 *		};
 *
 *		struct foo f = {...};
 *		char *yp = &f.y;
 *
 *	From yp, we can recover &f by:
 *
 *		struct foo *fp = container_of(yp, struct foo, y);
 *		assert(fp == &f);
 *
 *	const_container_of is similar, but allows P to be a const
 *	pointer and returns a pointer to const T.
 *
 * Technical details of how we detect type mistakes:
 *
 * - The expression 0 ? (PTR) : ... causes the compiler to warn if the
 *   type of *fp does not match the type of struct bar::b_foo.  Used to
 *   use (PTR) - ..., but that doesn't work if the member is an empty
 *   aggregate type.
 *
 * - The further expression container_of_nonconst(0 ? (PTR) : ...)
 *   causes the compiler to warn if PTR is const-qualified, so that
 *   container_of doesn't serve to accidentally upgrade a const pointer
 *   to a non-const pointer.  This works because container_of_nonconst
 *   is declared to be a function that takes a non-const pointer.
 *
 * - Surrounding it all in sizeof(container_of_nonconst(...)) avoids
 *   actually evaluating the subexpression, so it is limited to a
 *   compile-time constant, and we need not actually furnish a
 *   definition of container_of_nonconst.
 *
 * - Multiplying it by zero, 0*sizeof(...), gives us an expression that
 *   has constant value zero, so we can add it to a pointer without
 *   changing the result.
 *
 *   In contrast, for instance, using the comma operator, (sizeof(...),
 *   rest), might lead the compiler to warn about an unused result of
 *   comma operand without side effects.
 */

#define	container_of(PTR, TYPE, FIELD)					\
	((TYPE *)(((char *)(PTR)) - offsetof(TYPE, FIELD))		\
	    + 0*sizeof(container_of_nonconst(0 ? (PTR) :		\
		    &((TYPE *)(((char *)(PTR)) -			\
			    offsetof(TYPE, FIELD)))->FIELD)))

#define	const_container_of(PTR, TYPE, FIELD)				\
	((const TYPE *)(((const char *)(PTR)) - offsetof(TYPE, FIELD))	\
	    + 0*sizeof(0 ? (PTR) :					\
		&((const TYPE *)(((const char *)(PTR)) -		\
			offsetof(TYPE, FIELD)))->FIELD))

/* Useless symbol that serves only to flag const pointers.  */
void	container_of_nonconst(void *);

#endif	/* CONTAINER_OF_H */