The branch main has been updated by imp: URL: https://cgit.FreeBSD.org/src/commit/?id=640ddf8571291ab2965de85f67e6065a4d7efbf8
commit 640ddf8571291ab2965de85f67e6065a4d7efbf8 Author: Faraz Vahedi <[email protected]> AuthorDate: 2026-05-07 10:25:56 +0000 Commit: Warner Losh <[email protected]> CommitDate: 2026-06-20 00:23:29 +0000 libc: Enforce lock-free atomic_flag and C23-safe initialisation Select the `atomic_flag` backing type according to the C standard requirements that `atomic_flag` operations be lock-free. C11 §7.17.1.5 defines `atomic_flag` as: > a structure type representing a lock-free, primitive atomic flag and §7.17.8.2 further requires: > Operations on an object of type atomic_flag shall be lock free Therefore: - Prefer `atomic_bool` when `ATOMIC_BOOL_LOCK_FREE == 2` - Fall back to `atomic_uchar` when `ATOMIC_CHAR_LOCK_FREE == 2` - Trigger a translation failure if neither type is lock-free Adjust `ATOMIC_FLAG_INIT` for C23 initialisation rules: - Use `{ ATOMIC_VAR_INIT(0) }` in pre-C23 modes - Use `{ 0 }` in C23 and later Preserve `atomic_flag_test_and_set_explicit()` semantics by normalising the exchanged value with `!= 0`, ensuring consistent boolean results regardless of whether the underlying representation is `atomic_bool` or `atomic_uchar`. Signed-off-by: Faraz Vahedi <[email protected]> Reviewed by: imp Pull Request: https://github.com/freebsd/freebsd-src/pull/2185 --- sys/sys/stdatomic.h | 16 ++++++++++++---- 1 file changed, 12 insertions(+), 4 deletions(-) diff --git a/sys/sys/stdatomic.h b/sys/sys/stdatomic.h index bbdd196515e0..3e4bdd505418 100644 --- a/sys/sys/stdatomic.h +++ b/sys/sys/stdatomic.h @@ -377,21 +377,29 @@ __extension__ ({ \ /* * 7.17.8 Atomic flag type and operations. - * - * XXX: Assume atomic_bool can be used as an atomic_flag. Is there some - * kind of compiler built-in type we could use? */ typedef struct { +#if ATOMIC_BOOL_LOCK_FREE == 2 atomic_bool __flag; +#elif ATOMIC_CHAR_LOCK_FREE == 2 + atomic_uchar __flag; +#else +#error "atomic_flag is required to be lock-free" +#endif } atomic_flag; +#if __ISO_C_VISIBLE < 2023 #define ATOMIC_FLAG_INIT { ATOMIC_VAR_INIT(0) } +#else +#define ATOMIC_FLAG_INIT { 0 } +#endif static __inline _Bool atomic_flag_test_and_set_explicit(volatile atomic_flag *__object, memory_order __order) { - return (atomic_exchange_explicit(&__object->__flag, 1, __order)); + + return (atomic_exchange_explicit(&__object->__flag, 1, __order) != 0); } static __inline void
