[PATCH v2 10/10] hashmap_for_each_entry(): workaround MSVC's runtime check failure #3

From: Junio C Hamano <gitster@pobox.com>

The OFFSETOF_VAR(var, member) macro is implemented in terms of offsetof(typeof(*var), member) with compilers that know typeof(), but its fallback implemenation compares &(var->member) and (var) and count the distance in bytes, i.e.

    ((uintptr_t)&(var)->member - (uintptr_t)(var))

MSVC's runtime check, when fed an uninitialized 'var', flags this as a use of an uninitialized variable (and that is legit---uninitialized contents of 'var' is subtracted) in a debug build.

After auditing all 6 uses of OFFSETOF_VAR(), 1 of them does feed a potentially uninitialized 'var' to the macro in the beginning of the for() loop:

    #define hashmap_for_each_entry(map, iter, var, member) \
            for (var = hashmap_iter_first_entry_offset(map, iter, \
                                                    OFFSETOF_VAR(var, member)); \
                    var; \
                    var = hashmap_iter_next_entry_offset(iter, \
                                                    OFFSETOF_VAR(var, member)))

We can work around this by making sure that var has _some_ value when OFFSETOF_VAR() is called. Strictly speaking, it invites undefined behaviour to use NULL here if we end up with pointer comparison, but MSVC runtime seems to be happy with it, and most other systems have typeof() and don't even need pointer comparison fallback code.

Signed-off-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
---
 hashmap.h | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)