python - 属性错误: free when taking ownership or freeing cdata

Question

如果我想获得在 C 中进行 malloc 的指针的所有权，则 docs for the Python cffi package和 this answer假设使用 ffi.gc 和 lib.free 作为析构函数。但是，当我这样做时，我在调用 lib.free 时收到 AttributeError: free 。 lib.free 在哪里定义的？

from tempfile import TemporaryDirectory
from weakref import WeakKeyDictionary

from cffi import FFI

common_header = """
typedef struct {
  int32_t length;
  double* values;
} my_struct;
"""

# FFI
ffi = FFI()

ffi.cdef(common_header + """
int func(my_struct*);
""")
ffi.set_source('_temp', common_header + """
int func(my_struct *input) {
  double* values = malloc(sizeof(double) * 3);
  input->length = 3;
  input->values = values;
  return 0;
}
""")

with TemporaryDirectory() as temp_dir:
    lib_path = ffi.compile(tmpdir=temp_dir)

    lib = ffi.dlopen(lib_path)

    func = lib.func

# Using the library
my_struct = ffi.new('my_struct*')
func(my_struct)

# Taking ownership of the malloced member
global_weakkey = WeakKeyDictionary()
global_weakkey[my_struct] = ffi.gc(my_struct.values, lib.free)
# AttributeError: free

Answer 1

文档并没有强调这一点，但您需要将 free 作为库的 cdef 的一部分公开，就像任何其他函数一样，以便在Python 端。将 void free(void *ptr); 放入对 ffi.cdef 的调用中，并且 free 函数将通过 lib 可用。编译后免费:

from tempfile import TemporaryDirectory
from weakref import WeakKeyDictionary

from cffi import FFI

common_header = """
typedef struct {
  int32_t length;
  double* values;
} my_struct;
"""

# FFI
ffi = FFI()

ffi.cdef(common_header + """
int func(my_struct*);
void free(void *ptr); // <-- Key to lib.free working later
""")
ffi.set_source('_temp', common_header + """
int func(my_struct *input) {
  double* values = malloc(sizeof(double) * 3);
  input->length = 3;
  input->values = values;
  return 0;
}
""")

with TemporaryDirectory() as temp_dir:
    lib_path = ffi.compile(tmpdir=temp_dir)

    lib = ffi.dlopen(lib_path)

    func = lib.func

# Using the library
my_struct = ffi.new('my_struct*')
func(my_struct)

# Taking ownership of the malloced member
global_weakkey = WeakKeyDictionary()
global_weakkey[my_struct] = ffi.gc(my_struct.values, lib.free)