Using enums in ctypes.Structure

Question

I have a struct I'm accessing via ctypes:

struct attrl {
   char   *name;
   char   *resource;
   char   *value;
   struct attrl *next;
   enum batch_op op;
};

So far I have Python code like:

# struct attropl
class attropl(Structure):
    pass
attrl._fields_ = [
        ("next", POINTER(attropl)),
        ("name", c_char_p),
        ("resource", c_char_p),
        ("value", c_char_p),

But I'm not sure what to use for the batch_op enum. Should I just map it to a c_int or ?

Answer 1

Using c_int or c_uint would be fine. Alternatively, there is a recipe in the cookbook for an Enumeration class.

Answer 2

At least for GCC enum is just a simple numeric type. It can be 8-, 16-, 32-, 64-bit or whatever (I have tested it with 64-bit values) as well as signed or unsigned. I guess it cannot exceed long long int, but practically you should check the range of your enums and choose something like c_uint.

Here is an example. The C program:

enum batch_op {
    OP1 = 2,
    OP2 = 3,
    OP3 = -1,
};

struct attrl {
    char *name;
    struct attrl *next;
    enum batch_op op;
};

void f(struct attrl *x) {
    x->op = OP3;
}

and the Python one:

from ctypes import (Structure, c_char_p, c_uint, c_int,
    POINTER, CDLL)

class AttrList(Structure): pass
AttrList._fields_ = [
    ('name', c_char_p),
    ('next', POINTER(AttrList)),
    ('op', c_int),
]

(OP1, OP2, OP3) = (2, 3, -1)

enum = CDLL('./libenum.so')
enum.f.argtypes = [POINTER(AttrList)]
enum.f.restype = None

a = AttrList(name=None, next=None, op=OP2)
assert a.op == OP2
enum.f(a)
assert a.op == OP3