The C and C++ Include Header Files

/usr/include/linux/stddef.h


$ cat -n /usr/include/linux/stddef.h

     1	/* SPDX-License-Identifier: GPL-2.0 WITH Linux-syscall-note */
     2	#ifndef _LINUX_STDDEF_H
     3	#define _LINUX_STDDEF_H
     4	
     5	
     6	
     7	#ifndef __always_inline
     8	#define __always_inline __inline__
     9	#endif
    10	
    11	/**
    12	 * __struct_group() - Create a mirrored named and anonyomous struct
    13	 *
    14	 * @TAG: The tag name for the named sub-struct (usually empty)
    15	 * @NAME: The identifier name of the mirrored sub-struct
    16	 * @ATTRS: Any struct attributes (usually empty)
    17	 * @MEMBERS: The member declarations for the mirrored structs
    18	 *
    19	 * Used to create an anonymous union of two structs with identical layout
    20	 * and size: one anonymous and one named. The former's members can be used
    21	 * normally without sub-struct naming, and the latter can be used to
    22	 * reason about the start, end, and size of the group of struct members.
    23	 * The named struct can also be explicitly tagged for layer reuse, as well
    24	 * as both having struct attributes appended.
    25	 */
    26	#define __struct_group(TAG, NAME, ATTRS, MEMBERS...) \
    27		union { \
    28			struct { MEMBERS } ATTRS; \
    29			struct TAG { MEMBERS } ATTRS NAME; \
    30		} ATTRS
    31	
    32	#ifdef __cplusplus
    33	/* sizeof(struct{}) is 1 in C++, not 0, can't use C version of the macro. */
    34	#define __DECLARE_FLEX_ARRAY(T, member)	\
    35		T member[0]
    36	#else
    37	/**
    38	 * __DECLARE_FLEX_ARRAY() - Declare a flexible array usable in a union
    39	 *
    40	 * @TYPE: The type of each flexible array element
    41	 * @NAME: The name of the flexible array member
    42	 *
    43	 * In order to have a flexible array member in a union or alone in a
    44	 * struct, it needs to be wrapped in an anonymous struct with at least 1
    45	 * named member, but that member can be empty.
    46	 */
    47	#define __DECLARE_FLEX_ARRAY(TYPE, NAME)	\
    48		struct { \
    49			struct { } __empty_ ## NAME; \
    50			TYPE NAME[]; \
    51		}
    52	#endif
    53	
    54	#ifndef __counted_by
    55	#define __counted_by(m)
    56	#endif
    57	
    58	#endif /* _LINUX_STDDEF_H */