python ctypes structure wrong byte size

So I’m trying to figure out why the size of my ctypes.Structure is not what it should be. My code is the following, theres also the calculation of what the size should be afaik.

class FILE_HEAD(ctypes.Structure):
    _fields_ = [
        ("name", ctypes.c_char * 4),                    # 4 bytes
        ("size", ctypes.c_int),                         # 4 bytes
        ("Cal_l", ctypes.c_double),                     # 8 bytes
        ("Cal_r", ctypes.c_double),                     # 8 bytes
        ("Speed_ChL", ctypes.c_byte),                   # 1 byte
        ("Speed_Pulses_ChL", ctypes.c_int),             # 4 bytes
        ("Speed_factor_ChL", ctypes.c_double),          # 8 bytes
        ("Quantity_ChL", ctypes.c_char * 3),            # 3 bytes
        ("Description_ChL", ctypes.c_char * 32),        # 32 bytes
        ("Unit_ChL", ctypes.c_char * 8),                # 8 bytes
        ("Speed_ChR", ctypes.c_byte),                   # 1 byte
        ("Speed_Pulses_ChR", ctypes.c_int),             # 4 bytes
        ("Speed_factor_ChR", ctypes.c_double),          # 8 bytes
        ("Quantity_ChR", ctypes.c_char * 3),            # 3 bytes
        ("Description_ChR", ctypes.c_char * 32),        # 32 bytes
        ("Unit_ChR", ctypes.c_char * 8)                 # 8 bytes
    ]                                                   # = 136 bytes

So I think that Structure should have a size of 136 bytes. But when I let python print the size of an instance of the structure print ctypes.sizeof(COMI_HEAD) I get 144. I do not know where those 8 bytes come from.

I’ve filled that with the following data and wrote that to a file to see all the bytes and analyze where the bytes are.

comi = FILE_HEAD()
comi.name = "COMI"
comi.size = ctypes.sizeof(comi) - 8
comi.Cal_l = 342.324
comi.Cal_r = 342.324
comi.Speed_ChL = ctypes.c_byte(1)
comi.Speed_Pulses_ChL = 123
comi.Speed_factor_ChL = 123.456
comi.Quantity_ChL = "Tes"
comi.Description_ChL = "Test Desc"
comi.Unit_ChL = "t/t"
comi.Speed_ChR = ctypes.c_byte(1)
comi.Speed_Pulses_ChR = 123
comi.Speed_factor_ChR = 123.456
comi.Quantity_ChR = "Tes"
comi.Description_ChR = "Test Desc"
comi.Unit_ChR = "t/t"

Here is what my HEX-Viewer shows me. I’ve marked red, which bytes I think are the 8 bytes that are too much, but I do not know where these 8 bytes come from. The first 3 bytes I marked come directly after the Speed_ChL which should be 1 byte but looks like its 4 bytes. The next 5 bytes I’ve marked are at the end of the file. Theres this string Unit_ChR which should be 8 bytes but looks like it is 13 bytes.

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Can someone tell me where my mistake is? What am I doing wrong here? Any help is appreciated!

Best answer

Ok so I’ve found the solution. Thanks to Andreas who told me that my problem has to do with the offset. So the solution is to add a _pack_ = 1 to the structure like that.

class FILE_HEAD(ctypes.Structure):
    _pack_ = 1
    _fields_ = [
        ("name", ctypes.c_char * 4),                    # 4 bytes
        ("size", ctypes.c_int),                         # 4 bytes
        ("Cal_l", ctypes.c_double),                     # 8 bytes
        ("Cal_r", ctypes.c_double),                     # 8 bytes
        ("Speed_ChL", ctypes.c_byte),                   # 1 byte
        ("Speed_Pulses_ChL", ctypes.c_int),             # 4 bytes
        ("Speed_factor_ChL", ctypes.c_double),          # 8 bytes
        ("Quantity_ChL", ctypes.c_char * 3),            # 3 bytes
        ("Description_ChL", ctypes.c_char * 32),        # 32 bytes
        ("Unit_ChL", ctypes.c_char * 8),                # 8 bytes
        ("Speed_ChR", ctypes.c_byte),                   # 1 byte
        ("Speed_Pulses_ChR", ctypes.c_int),             # 4 bytes
        ("Speed_factor_ChR", ctypes.c_double),          # 8 bytes
        ("Quantity_ChR", ctypes.c_char * 3),            # 3 bytes
        ("Description_ChR", ctypes.c_char * 32),        # 32 bytes
        ("Unit_ChR", ctypes.c_char * 8)                 # 8 bytes
    ]                                                   # = 136 bytes