struct error unpack requires a string argument of length 4

Question

Python says I need 4 bytes for a format code of "BH":

struct.error: unpack requires a string argument of length 4

Here is the code, I am putting in 3 bytes as I think is needed:

major, minor = struct.unpack("BH", self.fp.read(3))

"B" Unsigned char (1 byte) + "H" Unsigned short (2 bytes) = 3 bytes (!?)

struct.calcsize("BH") says 4 bytes.

EDIT: The file is ~800 MB and this is in the first few bytes of the file so I'm fairly certain there's data left to be read.

User · Answer

By default  on many platforms the short will be aligned to an offset at a multiple of 2  so there will be a padding byte added after the char   To disable this  use  struct unpack   BH   data   This will use standard alignment  which doesn t add padding    gt  gt  gt  struct calcsize   BH   3   The   character will use native byte ordering   You can also use  lt  or  gt  instead of   to force little-endian or big-endian byte ordering  respectively

User · Answer

The struct module mimics C structures   It takes more CPU cycles for a processor to read a 16-bit word on an odd address or a 32-bit dword on an address not divisible by 4  so structures add  pad bytes  to make structure members fall on natural boundaries   Consider   struct                     11     char a       012345678901     short b      ------------     char c       axbbcxxxdddd     int d       This structure will occupy 12 bytes of memory  x being pad bytes    Python works similarly  see the struct documentation     gt  gt  gt  import struct  gt  gt  gt  struct pack  BHBL  1 2 3 4    x01 x00 x02 x00 x03 x00 x00 x00 x04 x00 x00 x00   gt  gt  gt  struct calcsize  BHBL   12   Compilers usually have a way of eliminating padding   In Python  any of   lt    will eliminate padding    gt  gt  gt  struct calcsize   BHBL   8  gt  gt  gt  struct pack   BHBL  1 2 3 4    x01 x02 x00 x03 x04 x00 x00 x00    Beware of letting struct handle padding   In C  these structures   struct A         struct B       short a          int a      char b           char b                        are typically 4 and 8 bytes  respectively   The padding occurs at the end of the structure in case the structures are used in an array   This keeps the  a  members aligned on correct boundaries for structures later in the array   Python s struct module does not pad at the end    gt  gt  gt  struct pack  LB  1 2    x01 x00 x00 x00 x02   gt  gt  gt  struct pack  LBLB  1 2 3 4    x01 x00 x00 x00 x02 x00 x00 x00 x03 x00 x00 x00 x04

[python] struct.error: unpack requires a string argument of length 4

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