Python crashes on field access ctypes - winapi

Question

I'm using the ctypes module to call GetTcpTable2.
I've been slowly converting the example here in C++ to Python; but am getting crash during a field access.
if __name__ == "__main__":
ptcp_table = POINTER(MIB_TCPTABLE2)()
ptcp_table = cast(create_string_buffer(sizeof(MIB_TCPTABLE2)),
POINTER(MIB_TCPTABLE2))
ip_addr = in_addr()
size = c_ulong(sizeof(MIB_TCPTABLE2))
retval = GetTcpTable2(ptcp_table, byref(size), TRUE)
if retval == ERROR_INSUFFICIENT_BUFFER:
ptcp_table = cast(create_string_buffer(size.value),
POINTER(MIB_TCPTABLE2))
if not ptcp_table:
#throw error
pass
retval = GetTcpTable2(ptcp_table, byref(size), TRUE)
if retval == NO_ERROR:
print("Entries %d" % ptcp_table[0].dwNumEntries)
for i in range(0, ptcp_table[0].dwNumEntries):
print(ptcp_table[0].table[i])
#ip_addr.S_un.S_addr = ptcp_table[0].table[i].dwLocalAddr
#ip_addr_string = inet_nota(ip_addr)
#print(ip_addr_string)
#print(string_at(ip_addr_string))
It crashes when trying to access dwLocalAddr apart of table[i].
ptcp_table[0].table[i].dwLocalAddr
However it doesn't crash when just printing ptcp_table[0].table[i].
I've tried printing and accessing other fields; but Python just crashes.
Here are my struct definitions:
class MIB_TCPROW2(Structure):
_fields_ = [
("dwState", c_ulong),
("dwLocalAddr", c_ulong),
("dwLocalPort", c_ulong),
("dwRemoteAddr", c_ulong),
("dwRemotePort", c_ulong),
("dwOwningPid", c_ulong),
("dwOffloadState", c_int)
]
class MIB_TCPTABLE2(Structure):
_fields_ = [
("dwNumEntries", c_ulong),
("table", POINTER(MIB_TCPROW2))
]
Definition of GetTcpTable2:
GetTcpTable2 = windll.iphlpapi.GetTcpTable2
GetTcpTable2.argtypes = [POINTER(MIB_TCPTABLE2), POINTER(c_ulong), c_char]
GetTcpTable2.restype = c_ulong
I have a small hunch that in the definition of the MIB_TCPTABLE2 struct; the documentation says that table is an array of MIB_TCPROW2 size ANY_SIZE; and further inspection is that ANY_SIZE is 1 from checking the iphlpapi.h file. And I know that the size of POINTER(MIB_TCPROW2) doesn't equal the size of MIB_TCPROW2.

Answer 1

I looked into other ctypes questions revolving around variable length fields inside of a struct, and came to an answer which suggested to use a factory method to generate the class definitions.
def MIB_TCPTABLE2_FACTORY(size):
class MIB_TCPTABLE2(Structure):
_fields_ = [
("dwNumEntries", c_ulong),
("table", MIB_TCPROW2 * size)
]
return MIB_TCPTABLE2
I can use this knowing the size returned from GetTcpTable2 to create a new type. And then all I have to do is change the argtypes of GetTcpTable2 to accept a void *.
GetTcpTable2.argtypes = [c_void_p, POINTER(c_ulong), c_char]

Answer 2

This is how I solved it. I first got the required size by passing in the following arguments:
ret = windll.iphlpapi.GetTcpTable2(None, byref(tcp_table_size), True)
Notice None is the first argument, which is equivalent to NULL when passing it into ctypes windows function. Then I defined the MIB_TCPTABLE2 class and inside it, I passed in the size returned by the first call to GetTcpTable2:
class MIB_TCPTABLE2(Structure):
_fields_ = [
("dwNumEntries", c_ulong),
("table", MIB_TCPROW2 * tcp_table_size.value),
]
Next, I created an instance of the structure and called GetTcpTable2 again passing in the newly created structure:
tcp_table = MIB_TCPTABL2()
ret = windll.iphlpapi.GetTcpTable2(byref(tcp_table), byref(tcp_table_size), True)
Here's the sample code:
from ctypes import *
import socket
import struct
NO_ERROR = 0
ERROR_INSUFFICIENT_BUFFER = 122
TcpConnectionOffloadStateInHost = 0
TcpConnectionOffloadStateOffloading = 1
TcpConnectionOffloadStateOffloaded = 2
TcpConnectionOffloadStateUploading = 3
TcpConnectionOffloadStateMax = 4
class MIB_TCPROW2(Structure):
_fields_ = [
("dwState", c_ulong),
("dwLocalAddr", c_ulong),
("dwLocalPort", c_ulong),
("dwRemoteAddr", c_ulong),
("dwRemotePort", c_ulong),
("dwOwningPid", c_ulong),
("dwOffloadState", c_ulong),
]
def main():
windll.iphlpapi.GetTcpTable2.argtypes = [c_void_p, POINTER(c_ulong), c_bool]
tcp_table_size = c_ulong()
ret = windll.iphlpapi.GetTcpTable2(None, byref(tcp_table_size), True)
if ret == ERROR_INSUFFICIENT_BUFFER:
class MIB_TCPTABLE2(Structure):
_fields_ = [
("dwNumEntries", c_ulong),
("table", MIB_TCPROW2 * tcp_table_size.value),
]
tcp_table = MIB_TCPTABLE2()
ret = windll.iphlpapi.GetTcpTable2(byref(tcp_table), byref(tcp_table_size), True)
if ret != NO_ERROR:
print("ERROR: GetTcpTable2() failed, error = " + str(ret))
else:
for i in range(tcp_table.dwNumEntries):
dest_ip = socket.inet_ntoa(struct.pack('<L', tcp_table.table[i].dwRemoteAddr))
print("PID: " + str(tcp_table.table[i].dwOwningPid) + ", DEST IP: " + dest_ip)
if __name__ == "__main__":
main()