Python calling C's SDK has a return value that does not match expectations and segmentation fault

1. The SDK return value is not an INT type 1.1 login function call

def Login (IP, port, username, password, device_info, Error_code):
"""
Llong Client_login (
Char *pchdvrip, WORD Wdvrport,
Char *pchusername, Char *pchpassword,
Lpnet_deviceinfo lpdeviceinfo, int *error = 0);
:p Aram IP:
:p Aram Port:
:p Aram Username:
:p Aram Password:
:p Aram Device_info:
:p Aram Error_code:
: Return : Llong
"""
Ip_buffer = C_buffer (IP)
# ip_buffer.encode (' UTF8 ')
# user_id = C_longlong (0)
user_id = Sdk._dll. Client_login (ByRef (Ip_buffer), port, username, password, byref (device_info), ByRef (Error_code))
return user_id # C_longlong (user_id). Value

1.2 Invalid ID

User ID as a handle, passed in to other SDK functions, error, invalid handle. View a negative value appears. So the suspicion is type mismatch

1.3 Set return type 1.3.1 Error reason

Check the Web, and read the following document, Python called C's SDK, the default is to return the int type, according to the login C version of the function definition, the return is Llong type

15.17.1.8. Return types

By default functions is assumed to return the C int type. Other return types can is specified by setting the restype attribute of the function object.

Here's a more advanced example, it uses strchr the function, which expects a string pointer and a char, and returns a Poin ter to a string:

>>> STRCHR = libc.strchr>>> strchr ("ABCdef", Ord ("D"))  8059983>>> Strchr.restype = c_ Char_p   # c_char_p is a pointer to a string>>> STRCHR ("ABCdef", Ord ("D")) ' Def ' >>> print strchr ("ABCD EF ", Ord (" X ")) none>>>

1.3.2 Modification

Set the return value of the SDK function to C_longlong, solve the problem

Sdk._dll. Client_login.restype = C_longlong

2. Segmentation fault of large probability appearing in callback function scene

On the internet to find a lap, the general two possibilities: memory out of bounds or read and write illegal, there is also a function call stack too deep.

2.1 Read and Write lock

The code itself added condition read-write lock, BUF is also in the writing time allocation, multiple debugging, should not be this place because of the problem. The print log is also independent of read and write operations.

Write

index = UserData  # c_uint (userdata). Value
_buf_cond.acquire ()
# Time.sleep (0.2)

# Copy pictures to memory
# _pic_buf.buf = PBuf  C_char and C_byte conversion
Try
    temp = [Pbuf[i] for I in xrange (0, Revlen)]
    _buf_list[index].buf = Struct.pack ('%db '% Revlen, *temp)
    # serial Number
    _BUF_LIST[INDEX].SN = C_ulong (cmdserial). Value
    _buf_list[index].id = Index
    _buf_list[index].size = C_uint (Revlen). Value
    _buf_list[index].ext = ' jpeg '  # encode_dict.get (encodetype, ' jpeg ')
Except Exception, E:
    Logger.error (' Cache exception:%s '% str (e))
Finally
    _buf_cond.notify ()
    _buf_cond.release ()

Read

_buf_cond.acquire ()

_buf_cond.wait (timeout=15.0)
# Wait for 200ms to access the data again
# Time.sleep (0.2)
if _buf_list[self.index].sn = = snap. Cmdserial and _buf_list[self.index].id = = Self.index:
    Self.save_picture (_buf_list[self.index].buf, _buf_list[self.index].ext)
    Self.info (' Success for channel%d, ip:%s,port:%s '% (channel, SELF.IP, Self.port))
    Pass
_buf_cond.release ()

2.2 Reduce stack Call hierarchy

Since this SDK was introduced, a callback function was used. Therefore, the callback function definition hierarchy is reduced.

2.2.1 Before modification

Pass-through function to base class, Cfunctype instantiation function in base class

defined in base class

Self.callback = Cfunctype (c_void_p, C_longlong, POINTER (c_byte), C_uint, C_uint, C_ulong, C_ulonglong)

def set_callback (self, Save_after_recv_pic, index):
    Self.dll.CLIENT_SetSnapRevCallBack (Self._callback (save_after_recv_pic), index)

Subclasses, _save_after_recv_pic are also defined in subclasses as Staticmethod

def _set_callback (self):
    Try
        If 0 <= Self.index < _buf_size:

            Self.set_callback (Self._save_after_recv_pic, Self.index)   # function call hierarchy too deep, often reported segmentation fault
            Return True
        Else
            Self.error (' Set UserData parameter to save callback function error:%d '% self.index)
            Return False
    Except Exception, E:
        Self.error (' Set save callback function failed,%s '% str (e))
        Return False

2.2.2 Problem resolution after modification

Direct instantiation of callback functions in subclasses

Self.capture_callback = Self.callback (self._save_after_recv_pic)

Directly registering a callback function in a subclass

def _set_callback (self):
    Try
        If 0 <= Self.index < _buf_size:
            Self.dll.CLIENT_SetSnapRevCallBack (Self.capture_callback, Self.index)
            # Self.set_callback (Self._save_after_recv_pic, Self.index)   # function call hierarchy too deep, often reported segmentation fault
            Return True
        Else
            Self.error (' Set UserData parameter to save callback function error:%d '% self.index)
            Return False
    Except Exception, E:
        Self.error (' Set save callback function failed,%s '% str (e))
        Return False

Python calling C's SDK has a return value that does not match expectations and segmentation fault