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py/misc: Change sizeof to offsetof for variable-length alloc.

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This fixes the case where e.g.

    struct foo_t {
      mp_obj_t x;
      uint16_t y;
      char buf[];
    };

will have `sizeof(struct foo_t)==8`, but `offsetof(struct foo_t, buf)==6`.

When computing the size to allocate for `m_new_obj_var` we need to use
offsetof to avoid over-allocating. This is important especially when it
might cause it to spill over into another GC block.

This work was funded through GitHub Sponsors.

Signed-off-by: Jim Mussared <jim.mussared@gmail.com>
This commit is contained in:
Jim Mussared
2023-11-03 14:19:55 +11:00
committed by Damien George
parent c85db05244
commit b6a9778484
11 changed files with 24 additions and 24 deletions
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py/objtype.c
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@@ -1155,7 +1155,7 @@ mp_obj_t mp_obj_new_type(qstr name, mp_obj_t bases_tuple, mp_obj_t locals_dict)
// (currently 10, plus 1 for base, plus 1 for base-protocol).
// Note: mp_obj_type_t is (2 + 3 + #slots) words, so going from 11 to 12 slots
// moves from 4 to 5 gc blocks.
mp_obj_type_t *o = m_new_obj_var0(mp_obj_type_t, void *, 10 + (bases_len ? 1 : 0) + (base_protocol ? 1 : 0));
mp_obj_type_t *o = m_new_obj_var0(mp_obj_type_t, slots, void *, 10 + (bases_len ? 1 : 0) + (base_protocol ? 1 : 0));
o->base.type = &mp_type_type;
o->flags = base_flags;
o->name = name;