Question about the C API

[stuaxo]

I had an issue with a while back CairoCFFI, which seemed to come back to how pycairo exports things from it's .so.

PycairoContext_FromContext seems to be exported on some (but not all) distros and windows.

I noticed that PyCairo changed to the capsule API from something else previously, could this have changed how the C API is exported in it's .so ?

(And so the distros where I could access PycairoContext_FromContext be using the other mechanism) ?

[lazka]

No idea, maybe it depends on the distro specific compiler flags? They are not needed really, so it shouldn't matter.

If you want to access them through Python it should be possible to get all the function pointers through cairo.CAPI and PyCObject_AsVoidPtr/PyCapsule_GetPointer with ctypes, but I haven't tried that.

What are you trying to do?

[lazka]

Something like this:

import sys
import ctypes
from ctypes import c_void_p, py_object, c_char_p

# see pycairo.h
class CAPI(ctypes.Structure):
    _fields_ = [
        ("Context_Type", py_object),
        ("Context_FromContext", ctypes.PYFUNCTYPE(py_object, c_void_p, py_object, py_object)),
    ]

def get_capi():
    import cairo

    if sys.version_info[0] == 2:
        PyCObject_AsVoidPtr = ctypes.PYFUNCTYPE(c_void_p, py_object)(
            ('PyCObject_AsVoidPtr', ctypes.pythonapi))
        ptr = PyCObject_AsVoidPtr(cairo.CAPI)
    else:
        PyCapsule_GetPointer = ctypes.PYFUNCTYPE(c_void_p, py_object, c_char_p)(
            ('PyCapsule_GetPointer', ctypes.pythonapi))
        ptr = PyCapsule_GetPointer(cairo.CAPI, b"cairo.CAPI")

    ptr = ctypes.cast(ptr, ctypes.POINTER(CAPI))
    return ptr.contents


def main():
    capi = get_capi()

    import cairocffi
    import cairo

    surface = cairocffi.ImageSurface(cairocffi.FORMAT_ARGB32, 10, 10)
    ctx = cairocffi.Context(surface)
    ptr = ctx._pointer
    cairocffi.cairo.cairo_reference(ptr)
    ptr = int(cairocffi.ffi.cast('uintptr_t', ptr))
    print(capi.Context_FromContext(ptr, capi.Context_Type, None))


main()

[lazka]

I've extended the example above to transfer an object from cairocffi to pycairo.

[stuaxo]

Do you know where the distro specific flags can be found ?

Then I can try and dig out my old tickets on this and remember what was happening on which distros :)

[lazka]

No idea. The symbols aren't public API, so you shouldn't use them.

[stuaxo]

Indeed it is the converting to and from raw Cairo contexts for interoperability with cairocffi.

Cairocffi has a util.py with methods for this, but they use ctypes to get fields, it seems like a issue enough operation to support from our end.

In shoebot we ship an older version of this:
https://github.com/shoebot/shoebot/blob/master/shoebot/cairocffi_util.py

It is fine in Debian and Ubuntu, but not available on Fedora and Windows for a while now.

[lazka]

Indeed it is the converting to and from raw Cairo contexts for interoperability with cairocffi.

Cairocffi has a util.py with methods for this, but they use ctypes to get fields, it seems like a issue enough operation to support from our end.

By "our end" you mean pycairo? If yes, what's the use case?

Anyway, this should work for you hopefully: https://bpaste.net/show/fba9c849a004

[lazka]

The reason I'm asking is because I see the gain if you have an older pycairo and want to use some newer functions with cairocffi. But adding something like this to pycairo wont help those being stuck on older versions.

If there is anything unclear/missing etc. feel free to post here or open a new issue.