Why does struct.unpack need 2 extra bytes?

October 9, 2023

I have the following struct.unpack call that fails:

struct.unpack('14x4x2i2xh8x2i', b'\0'*46)

Traceback (most recent call last):
  File "<pyshell#88>", line 1, in <module>
    struct.unpack('14x4x2i2xh8x2i', b'\0'*46)
struct.error: unpack requires a buffer of 48 bytes

I’ve added up the number of bytes required by the format multiple times, and I always come up with a total of 46 bytes. But obviously the error says it’s not enough. Why am I coming up short?

The following does produce the expected output:

struct.unpack('14x4x2i2xh8x2i', b'\0'*48)

(0, 0, 0, 0, 0)

>Solution :

By default struct expects types to be aligned. Since i is 4 bytes, it’s 2 bytes of padding are assumed to get the offset to a multiple of 4.

You can use the = flag at the beginning of the format string to indicate that values are packed rather than aligned.

struct.unpack('=14x4x2i2xh8x2i', b'\0'*46)

See the documentation of Byte Order, Size, and Alignment. You should probably actually use < rather than =, since BMP integers are stored in little-endian format.