David Dong

Java/C/C#/Python

POST

Python struct.pack()

May 3, 2019

Python uses struct.pack() to pack python data type into binary data (Byte Stream), which means convert a value to a string based on the format.

Because there is no byte type in Python, where the string is equivalent to a byte stream, or byte array.

struct.pack()

The function prototype:

struck.pack(format, data1, data2,...)

format: define the converted format. data1,data2,…: the data needs to be packed.

The format includes

Format	C Type	Python Type	Standard Size
x	pad byte
c	char	string of length 1	1
b	signed char	integer	1
B	unsigned char	integer	1
?	_bool	bool	1
h	short	integer	2
H	unsigned short	integer	2
i	int	integer	4
I	unsigned int	integer	4
l	long	integer	4
L	unsigned long	integer	4
q	long long	integer	8
Q	unsigned long long	integer	8
f	float	float	4
d	double	float	8
s	char[]	string
p	char[]string
P	void *	integer

To exchange data with the structure in C, it is also necessary to consider that some C or C + + compilers use byte alignment, usually a 32-bit system with 4 bytes as the unit. Therefore, a struct can be converted according to the byte order of the local machine. The alignment can be changed by the first character in the format. The definition is as follows:

Format	Byte Order	Size	alignment
@	native byte	native	native
=	native	standard	none
<	little-endian char	standard	none
>	big-endian char	standard	none
!	network (= big-endian)	standard	none

Use it in the first place of format.

for example.

import struct

a = 11
b = 12

print(len(struct.pack("ii",a,b)))
8
print(struct.pack("ii",a,b))
b'\x0b\x00\x00\x00\x0c\x00\x00\x00'

struct.unpack()

struct.unpack() unpacks the byte stream into Python data type.
The function prototype:

struct.unpack(fmt, string)

This function return a tuple.

For example.

a = 11
b = 12
packdata = struct.pack("ii",a,b)
c,d = struct.unpack("1i1i",packdata)
print((c,d))
(11,12)

struct.calcsize()

Struct.calcsize() is used to calculate the length of the result corresponding to the format string.