Python ctypes 和 librsvg 错误

Question

我正在尝试使用 Python 的 ctypes 包装 librsvg 的基本功能，但出现段错误。

C：

// pycairo excerpt
typedef struct {
  PyObject_HEAD
  cairo_t *ctx;
  PyObject *base; /* base object used to create context, or NULL */
} PycairoContext;

// librsvg excerpt
RsvgHandle * rsvg_handle_new_from_file (const gchar * file_name, GError ** error);
// ...
gboolean rsvg_handle_render_cairo (RsvgHandle * handle, cairo_t * cr);

Python ctypes：

from ctypes import *
from ctypes import util


librsvg = cdll.LoadLibrary('/brew/lib/librsvg-2.2.dylib')
libgobject = cdll.LoadLibrary('/brew/lib/libgobject-2.0.dylib')

libgobject.g_type_init()


class RSVGDimensionData(Structure):

    _fields_ = (
        ('width', c_int),
        ('height', c_int),
        ('em', c_double),
        ('ex', c_double)
    )

class PycairoContext(Structure):

    _fields_ = (
        ('PyObject_HEAD', c_byte * object.__basicsize__),
        ('ctx', c_void_p),
        ('base', c_void_p)
    )


class RSVGHandle(object):

    def __init__(self, path):
        self.path = path
        self.error = ''
        self.handle = librsvg.rsvg_handle_new_from_file(self.path, self.error)

    def render_cairo(self, context):
        context.save()
        z = PycairoContext.from_address(id(context))
        librsvg.rsvg_handle_render_cairo(self.handle, z.ctx)
        context.restore()


import cairo

h = RSVGHandle('bank.svg')
s = cairo.ImageSurface(cairo.FORMAT_ARGB32, 100, 100)
ctx = cairo.Context(s)


# segmentation fault....
h.render_cairo(ctx)

错误发生在这一行：librsvg.rsvg_handle_render_cairo(self.handle, z.ctx)

知道这有什么问题吗？

Answer 1

问题是没有定义返回类型的规范；仅对结果使用 c_void_p 不足以解决这种情况下的问题。你需要放在

librsvg.rsvg_handle_new_from_file.restype = c_void_p

合适的地方。然后它（也）可以在 OSX 中以 32 位或 64 位运行。

但我发现增强基本包装更有帮助，以便处理从文件创建句柄时可能出现的错误。以下是执行此操作的基本包装器。它还以几乎相同的方式复制了标准 rsvg 绑定的基本用法。

from ctypes import CDLL, POINTER, Structure, byref, util
from ctypes import c_bool, c_byte, c_void_p, c_int, c_double, c_uint32, c_char_p

class _PycairoContext(Structure):
    _fields_ = [("PyObject_HEAD", c_byte * object.__basicsize__),
                ("ctx", c_void_p),
                ("base", c_void_p)]

class _RsvgProps(Structure):
    _fields_ = [("width", c_int), ("height", c_int),
                ("em", c_double), ("ex", c_double)]

class _GError(Structure):
    _fields_ = [("domain", c_uint32), ("code", c_int), ("message", c_char_p)]


def _load_rsvg(rsvg_lib_path=None, gobject_lib_path=None):
    if rsvg_lib_path is None:
        rsvg_lib_path = util.find_library('rsvg-2')
    if gobject_lib_path is None:
        gobject_lib_path = util.find_library('gobject-2.0')
    l = CDLL(rsvg_lib_path)
    g = CDLL(gobject_lib_path)
    g.g_type_init()

    l.rsvg_handle_new_from_file.argtypes = [c_char_p, POINTER(POINTER(_GError))]
    l.rsvg_handle_new_from_file.restype = c_void_p
    l.rsvg_handle_render_cairo.argtypes = [c_void_p, c_void_p]
    l.rsvg_handle_render_cairo.restype = c_bool
    l.rsvg_handle_get_dimensions.argtypes = [c_void_p, POINTER(_RsvgProps)]

    return l    

_librsvg = _load_rsvg()


class Handle(object):
    def __init__(self, path):
        lib = _librsvg
        err = POINTER(_GError)()
        self.handle = lib.rsvg_handle_new_from_file(path.encode(), byref(err))
        if self.handle is None:
            gerr = err.contents
            raise Exception(gerr.message)
        self.props = _RsvgProps()
        lib.rsvg_handle_get_dimensions(self.handle, byref(self.props))

    def render_cairo(self, ctx):
        """Returns True is drawing succeeded."""
        z = _PycairoContext.from_address(id(ctx))
        return _librsvg.rsvg_handle_render_cairo(self.handle, z.ctx)

示例用法请参见 https://stackoverflow.com/a/14928770/1832154。

Answer 2

librsvg.rsvg_handle_render_cairo 需要指针，并且正在获取整数。不确定这里的整个故事，但这种修改至少不会出现段错误。

试试这个

 librsvg.rsvg_handle_render_cairo(c_void_p(self.handle), c_void_p(z.ctx))

注意，我将两个参数包装在 c_void_p 中，使它们成为 void * 指针。不理想，但似乎有效。