mirrors/git
1
0
Fork 0
Code Issues Pull Requests Packages Projects Releases Wiki Activity

OFFSETOF_VAR macro to simplify hashmap iterators

Browse Source 
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>
This commit is contained in:
Eric Wong
2019-10-06 23:30:41 +00:00
committed by Junio C Hamano
parent c8e424c9c9
commit 23dee69f53
15 changed files with 56 additions and 45 deletions
Download Patch File Download Diff File Expand all files Collapse all files

13
git-compat-util.h
View File

@ -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