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| author | Eric Wong <e@80x24.org> | 2019-10-06 23:30:41 +0000 |
|---|---|---|
| committer | Junio C Hamano <gitster@pobox.com> | 2019-10-07 10:20:11 +0900 |
| commit | 23dee69f53cf5024ca79e0b707dcb03c63f33bef (patch) | |
| tree | e99f2fd2a4e1fe622451eeaafa8c37b40413ec10 /git-compat-util.h | |
| parent | c8e424c9c94d97b18cd335be17f32a8ce94a5b7f (diff) | |
| download | git-23dee69f53cf5024ca79e0b707dcb03c63f33bef.tar.gz | |
OFFSETOF_VAR macro to simplify hashmap iterators
While we cannot rely on a `__typeof__' operator being portable
to use with `offsetof'; we can calculate the pointer offset
using an existing pointer and the address of a member using
pointer arithmetic for compilers without `__typeof__'.
This allows us to simplify usage of hashmap iterator macros
by not having to specify a type when a pointer of that type
is already given.
In the future, list iterator macros (e.g. list_for_each_entry)
may also be implemented using OFFSETOF_VAR to save hackers the
trouble of using container_of/list_entry macros and without
relying on non-portable `__typeof__'.
v3: use `__typeof__' to avoid clang warnings
Signed-off-by: Eric Wong <e@80x24.org>
Reviewed-by: Derrick Stolee <stolee@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Diffstat (limited to 'git-compat-util.h')
| -rw-r--r-- | git-compat-util.h | 13 |
1 files changed, 13 insertions, 0 deletions
diff --git a/git-compat-util.h b/git-compat-util.h index 4a23b9090b..8605cb4202 100644 --- a/git-compat-util.h +++ b/git-compat-util.h @@ -1337,4 +1337,17 @@ static inline void *container_of_or_null_offset(void *ptr, size_t offset) #define container_of_or_null(ptr, type, member) \ (type *)container_of_or_null_offset(ptr, offsetof(type, member)) +/* + * like offsetof(), but takes a pointer to a a variable of type which + * contains @member, instead of a specified type. + * @ptr is subject to multiple evaluation since we can't rely on __typeof__ + * everywhere. + */ +#if defined(__GNUC__) /* clang sets this, too */ +#define OFFSETOF_VAR(ptr, member) offsetof(__typeof__(*ptr), member) +#else /* !__GNUC__ */ +#define OFFSETOF_VAR(ptr, member) \ + ((uintptr_t)&(ptr)->member - (uintptr_t)(ptr)) +#endif /* !__GNUC__ */ + #endif |