On 03.04.2013 12:53, Nikita Nemkin wrote:
offsetof() is not supported by current Cython, and I have not found any
workaround (except hardcoding offsets for a specific architecture and
compiler, but this is obviously wrong).
offsetof() would certainly be very useful, but in the meantime
offsetof(Struct, field) can be replaced with:
<Py_ssize_t>&(<Struct*>NULL).field
It's not ANSI C, but is portable enough.
This will dereference a NULL pointer.
Also, Py_ssize_t is not guaranteed to be long enough to store a pointer
(but Py_intptr_t is). Use Py_intptr_t when you cast pointers to integers.
Another option (for extern or public structs only) is to abuse
renaming:
enum:
Struct_offsetof_field1 "offsetof(Struct, field1)"
This will fail if "Struct" is name mangled by Cython. Basically it
requires that it is defined outside of the Cython code, e.g. in a header
file.
To be valid C, offsets must be computed:
cdef Struct tmp
cdef Py_ssize_t offset
offset = <Py_ssize_t> (<Py_intptr_t>&(tmp.field) - <Py_intptr_t>&(tmp))
For that reason, most C compilers defines offsetof as a builtin function.
Sturla
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