Python struct.error：结构格式中的错误字符

首先我想知道这个python语句的作用：

struct.unpack("!%sH" % (len(data) / 2), data))

其次，我必须使用 python 发出 ICMP 请求消息，问题是我已经得到了老师给我的一些帮助代码：

def step4(code, server, port):
    s = socket.socket(socket.AF_INET, socket.SOCK_RAW, socket.getprotobyname('icmp'))

    CHARMAP = nonprintable_to_dots()

    packet = create_packet(int(code))

    s.sendto(packet, (server, port))

    while True:
        msg = s.recv(1600)

        if not msg:
            break

        ihl = (ord(msg[0]) & 0x0F) * 4
        icmp = msg[ihl:]

        hexdump(icmp, True)

        if cksum(icmp[:]) != 0:
            print("wrong ckecksum!!")

def nonprintable_to_dots():
    return str.join('', [c if c in printable[:-5] else '.' for c in map(chr, range(256))])

def hexdump(frame, with_time=False):
    def to_chr(bytes):
        retval = bytes.translate(CHARMAP)
        return retval[:8] + ' ' + retval[8:]

    def to_hex(bytes):
        retval = str.join(' ', ["%02X" % ord(x) for x in bytes])
        return retval[:23] + ' ' + retval[23:]

    if with_time:
        print('--' + time.strftime("%H:%M:%s"))

    for i in range(0, len(frame), 16):
        line = frame[i:i + 16]
        print('%04X  %-49s |%-17s|' % (i, to_hex(line), to_chr(line)))

    print

def cksum(data):

    def sum16(data):
        "sum all the the 16-bit words in data"
        if len(data) % 2:
            data += '\0'

        return sum(struct.unpack("!%sH" % (len(data) / 2), data))

    retval = sum16(data)                       # sum
    retval = sum16(struct.pack('!L', retval))  # one's complement sum
    retval = (retval & 0xFFFF) ^ 0xFFFF        # one's complement

    return retval

问题是，理论上所有这些代码都是正确的，所以我只应该执行“create_packet”函数，我将其包含在此处：

def create_packet(code):
    ICMP_REQUEST = 8
    checksum = 0
    identifier = 7
    sequence_number = 7

    message_header = struct.pack("!BBHHH", ICMP_REQUEST, 0, checksum, identifier, sequence_number)

    message_payload = struct.pack("!I", code)

    checksum = cksum(message_header + message_payload)

    message_header = struct.pack("!BBHHH", ICMP_REQUEST, 0, checksum, identifier, sequence_number)

    return message_header + message_payload

每当我执行脚本时，我总是会收到此错误：

Traceback (most recent call last):
    File "gymkhana.py", line 256, in <module>
        main()                                                                                                                
    File "gymkhana.py", line 19, in main
        step4(identifier, server, ginkana_port)
    File "gymkhana.py", line 181, in step4
        packet = create_packet(int(code))
    File "gymkhana.py", line 211, in create_packet
        checksum = cksum(message_header + message_payload)
    File "gymkhana.py", line 248, in cksum
        retval = sum16(data)                       # sum
    File "gymkhana.py", line 246, in sum16
        return sum(struct.unpack("!%sH" % (len(data) / 2), data))                                                           
struct.error: bad char in struct format

作为第一个问题的答案，这一行

struct.unpack("!%sH" % (len(data) / 2), data)    # in Python 2
struct.unpack("!%sH" % (len(data) // 2), data)   # in Python 3

说，采取n字节数data (example:4字节）并将它们解释为n/2 个无符号短整数 (example:2 个整数），每个 2 个字节。这!开头是关于字节顺序的，表示大端字节序。%s根据值转换为一位或多位数字len(data) / 2，所以在示例中，它与执行相同

struct.unpack("!2H", data)

Your bad char in struct format例外是因为您提供的代码使用/除法运算符。这在 Python 2 中是有效的。但在 Python 3 中它需要是//。那是因为/在Python 2中表示整数除法，但在Python 3中表示浮点除法。所以在Python 3中，

"!%sH" % (len(data) / 2)

出来为

struct.unpack("!2.0H", data)

这解释了你的bad char error.