CODEWITHSUNDEEP
pythonc++pointersctypes
Jakub Hachlowski
Jakub Hachlowski

Reputation: 1

python ctypes and C++ pointers

C++ funtion:

DLLENTRY int VTS_API
    SetParamValue( const char *paramName, void *paramValue ) //will return zero(0) in the case of an error
  {
    rtwCAPI_ModelMappingInfo* mmi = &(rtmGetDataMapInfo(PSA_StandAlone_M).mmi);
    int idx = getParamIdx( paramName );
    if (idx<0) {
      return 0;                        //Error
    }

    int retval = capi_ModifyModelParameter( mmi, idx, paramValue );
    if (retval == 1 ) {
      ParamUpdateConst();
    }

    return retval;
  }

and my Python code:

import os
from ctypes import *

print(os.getcwd())
os.chdir(r"C:\MY_SECRET_DIR")
print(os.getcwd())

PSAdll=cdll.LoadLibrary(os.getcwd()+"\PSA_StandAlone_1.dll")

setParam=PSAdll.SetParamValue
setParam.restype=c_int

setParam.argtypes=[c_char_p, c_void_p]

z=setParam(b"LDOGenSpdSetpoint", int(20) )

returns

z=setParam(b"LDO_PSA_GenSpdSetpoint01", int(20) )
WindowsError: exception: access violation reading 0x00000020

Any idea what can help? I already tried POINTER and byref(), but I am getting the same output

Upvotes: 0

Views: 560

Answers (1)

CristiFati
CristiFati

Reputation: 41116

SetParamValue expects a pointer (void* or c_void_p) as the 2nd argument, but you're passing an int.
The function will interpret it as a pointer (memory address), and when attempting to dereference it (to get its content), it will segfault (Access Violation), as the process doesn't have permissions on that address.

To fix the problem, pass the proper arguments to the function:

z = setParam(b"LDOGenSpdSetpoint", pointer(c_int(20)))

You can find more details on [Python 3]: ctypes - A foreign function library for Python.

Upvotes: 1

Related Questions