1*09d4459fSDaniel Fojt /* Sizes of structs with flexible array members. 2*09d4459fSDaniel Fojt 3*09d4459fSDaniel Fojt Copyright 2016-2020 Free Software Foundation, Inc. 4*09d4459fSDaniel Fojt 5*09d4459fSDaniel Fojt This file is part of the GNU C Library. 6*09d4459fSDaniel Fojt 7*09d4459fSDaniel Fojt The GNU C Library is free software; you can redistribute it and/or 8*09d4459fSDaniel Fojt modify it under the terms of the GNU General Public 9*09d4459fSDaniel Fojt License as published by the Free Software Foundation; either 10*09d4459fSDaniel Fojt version 3 of the License, or (at your option) any later version. 11*09d4459fSDaniel Fojt 12*09d4459fSDaniel Fojt The GNU C Library is distributed in the hope that it will be useful, 13*09d4459fSDaniel Fojt but WITHOUT ANY WARRANTY; without even the implied warranty of 14*09d4459fSDaniel Fojt MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU 15*09d4459fSDaniel Fojt General Public License for more details. 16*09d4459fSDaniel Fojt 17*09d4459fSDaniel Fojt You should have received a copy of the GNU General Public 18*09d4459fSDaniel Fojt License along with the GNU C Library; if not, see 19*09d4459fSDaniel Fojt <https://www.gnu.org/licenses/>. 20*09d4459fSDaniel Fojt 21*09d4459fSDaniel Fojt Written by Paul Eggert. */ 22*09d4459fSDaniel Fojt 23*09d4459fSDaniel Fojt #include <stddef.h> 24*09d4459fSDaniel Fojt 25*09d4459fSDaniel Fojt /* Nonzero multiple of alignment of TYPE, suitable for FLEXSIZEOF below. 26*09d4459fSDaniel Fojt On older platforms without _Alignof, use a pessimistic bound that is 27*09d4459fSDaniel Fojt safe in practice even if FLEXIBLE_ARRAY_MEMBER is 1. 28*09d4459fSDaniel Fojt On newer platforms, use _Alignof to get a tighter bound. */ 29*09d4459fSDaniel Fojt 30*09d4459fSDaniel Fojt #if !defined __STDC_VERSION__ || __STDC_VERSION__ < 201112 31*09d4459fSDaniel Fojt # define FLEXALIGNOF(type) (sizeof (type) & ~ (sizeof (type) - 1)) 32*09d4459fSDaniel Fojt #else 33*09d4459fSDaniel Fojt # define FLEXALIGNOF(type) _Alignof (type) 34*09d4459fSDaniel Fojt #endif 35*09d4459fSDaniel Fojt 36*09d4459fSDaniel Fojt /* Yield a properly aligned upper bound on the size of a struct of 37*09d4459fSDaniel Fojt type TYPE with a flexible array member named MEMBER that is 38*09d4459fSDaniel Fojt followed by N bytes of other data. The result is suitable as an 39*09d4459fSDaniel Fojt argument to malloc. For example: 40*09d4459fSDaniel Fojt 41*09d4459fSDaniel Fojt struct s { int n; char d[FLEXIBLE_ARRAY_MEMBER]; }; 42*09d4459fSDaniel Fojt struct s *p = malloc (FLEXSIZEOF (struct s, d, n * sizeof (char))); 43*09d4459fSDaniel Fojt 44*09d4459fSDaniel Fojt FLEXSIZEOF (TYPE, MEMBER, N) is not simply (sizeof (TYPE) + N), 45*09d4459fSDaniel Fojt since FLEXIBLE_ARRAY_MEMBER may be 1 on pre-C11 platforms. Nor is 46*09d4459fSDaniel Fojt it simply (offsetof (TYPE, MEMBER) + N), as that might yield a size 47*09d4459fSDaniel Fojt that causes malloc to yield a pointer that is not properly aligned 48*09d4459fSDaniel Fojt for TYPE; for example, if sizeof (int) == alignof (int) == 4, 49*09d4459fSDaniel Fojt malloc (offsetof (struct s, d) + 3 * sizeof (char)) is equivalent 50*09d4459fSDaniel Fojt to malloc (7) and might yield a pointer that is not a multiple of 4 51*09d4459fSDaniel Fojt (which means the pointer is not properly aligned for struct s), 52*09d4459fSDaniel Fojt whereas malloc (FLEXSIZEOF (struct s, d, 3 * sizeof (char))) is 53*09d4459fSDaniel Fojt equivalent to malloc (8) and must yield a pointer that is a 54*09d4459fSDaniel Fojt multiple of 4. 55*09d4459fSDaniel Fojt 56*09d4459fSDaniel Fojt Yield a value less than N if and only if arithmetic overflow occurs. */ 57*09d4459fSDaniel Fojt 58*09d4459fSDaniel Fojt #define FLEXSIZEOF(type, member, n) \ 59*09d4459fSDaniel Fojt ((offsetof (type, member) + FLEXALIGNOF (type) - 1 + (n)) \ 60*09d4459fSDaniel Fojt & ~ (FLEXALIGNOF (type) - 1)) 61