如何构造unpack c null终止字符串？

Question

I used tcp to send a data to python server. The data is like:

struct protocol
{
    unsigned char prot;
    int id;
    char name[32];
}

Look at the name field, it is a null terminated string max size is 32. Now I use strcpy .

protocol p;
memset(&p, 0, sizeof(p));
strcpy(name, "abc");

Now I unpack it using python.

prot,id,name = struct.unpack("@Bi32s")

Now the len(name) is 32. But I need get the string of "abc" when the length is 3.

How can I do that?

Answer 1

After the unpacking you can just do a:

name = name.split('\0', 1)[0]

Alternatively you could use the ctypes module:

name = ctypes.create_string_buffer(name).value

Answer 2

Partition it with null character ( '\\0' ) after the unpack:

>>> prot, id, name = struct.unpack('@Bi32s', b'\0\0\0\0\0\0\0\0abc' + b'\0' * 29)
>>> name, _, _ = name.partition('\0')
>>> name
'abc'

Alternative using ctypes :

>>> from ctypes import *
>>>
>>> class Protocol(Structure):
...     _fields_ = [("prot", c_char),
...                 ("id", c_int),
...                 ('name', c_char * 32)]
...
>>> # sock.recv_into(buf) in real program
... buf = create_string_buffer(b'\0\0\0\0\0\0\0\0abc' + b'\0' * 29)
>>> p = cast(buf, POINTER(Protocol))
>>> p[0].name
'abc'

Answer 3

Simply get the substring up to the first \\0 :

prot,id,name = struct.unpack("@Bi32s")
name= name[:name.index("\0")]

This has the particularity that it will check and fail (throw ValueError ) if no \\0 appears inside the string.

Answer 4

How about using rstrip to remove the padding?

prot,id,name = struct.unpack("@Bi32s")
name = name.rstrip(b'\0')

This will also let you use all 32 bytes to store the name without a zero terminator. However, this does rely on ALL padding bytes being set to zero.