Re: Calling a function with a custom datatype argument

On 15/01/2010, Rosalyn Hatcher <[email protected]> wrote:
>
> How do I create the ut_error_message_handler datatype in python and thus
> pass a correctly typed argument to the function?

Short answer: you can't, ctypes doesn't provide a way of dealing with
varargs functions.

There are a couple of ways to cheat though. Firstly, by just using the
function you can pass in any kind of pointer, so you can for instance
use printf from your c library as it has the same signature. Secondly,
you can misdeclare ut_error_message_handler omitting the va_list part
and use that as the signature for any functions you'll want to pass.

    printf_like = ctypes.CFUNCTYPE(ctypes.c_int, ctypes.c_char_p)
    ut_set_error_message_handler = ctypes.CFUNCTYPE(printf_like, printf_like)(
        ("ut_set_error_message_handler", udunits2))
    printf = printf_like(("printf", libc))
    ut_write_to_stderr = printf_like(("ut_write_to_stderr", udunits2))
    ut_ignore = printf_like(("ut_ignore", udunits2))

    old_handler = ut_set_error_message_handler(ut_write_to_stderr)

Note, this still means you can only use callbacks implemented in C,
but assuming I've understood correctly and ut_write_to_stderr is an
actual function in the library, that might be all you need.

Martin

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