[Tutor] ctypes and spssio.dll

Albert-Jan Roskam fomcl at yahoo.com
Wed Mar 16 21:24:16 CET 2011


I'm still working on a program that uses a .dll to read SPSS system files.
It's getting somewhere already, but I'm still struggling with one thing. 
When using the C function spssGetVarNames I'm having trouble 
translating C arrays to Python lists. I want to translate an 
array that holds the variable names into a Python list. Same thing 
for variable types.

The documentation of the SPSSIO dll says the following:
int spssGetVarNames (int handle, int *numVars, char ***varNames, int **varTypes)
Parameter - Description
handle - Handle to the data file
numVars - Pointer to number of variables
varNames - Pointer to array of pointers to variable
varTypes - Pointer to array of variable types

In the code below, which can also be found on http://pastebin.com/d7d0hpyV, 
the equivalent Python function is called getVarNamesAndTypes(). This is 
the output I typically get:
retcode: 0
Varnames: ['\x80\xde\x10\x01P\xd8\x10\x01\xf0\xd0\x10\x01', None, None, None, 
None, None, None, None, None, None]
varTypes: [17885264, 0, 0, 0, 0, 0, 0, 0, 0, 0]

The first item of each list is clearly wrong, but what does it mean? If varType 
> 0 it is
supposed to be a string var of that length. And of course the varNames are 
mostly 'None'.

Probably, something is wrong with the way the C arrays are initialized, 
or with the two list comprehensions, but I can't find the solution.
varNames = [varNamesPtr[0][i] for i in range(numVars)] does not seem to work.

For those who are interested, the complete program can be found on 
http://pastebin.com/ff1b1Y9a (note that it's still work in progress)

Any hints? Thanks very much in advance!

import os, ctypes, datetime
# dll and py file should be in the same dir!
def loadSavFile(fn):
    os.environ["PATH"] += ";" + os.path.abspath(os.curdir)
    spssio = ctypes.windll.spssio32
    libc = ctypes.cdll.msvcrt
if os.path.exists(fn):
        fh = libc._fdopen(fn, "rb")
        fhPtr = ctypes.pointer(ctypes.c_int(fh))
        retcode = spssio.spssOpenRead(ctypes.c_char_p(fn), fhPtr)
return retcode, spssio, fh
        raise Exception, "File '%s' does not exist!" % fn
def getVarNamesAndTypes(fh, spssio):
    numVarsPtr = ctypes.pointer(ctypes.c_int())
    spssio.spssGetNumberofVariables(fh, numVarsPtr)
    numVars = numVarsPtr[0]

    varNamesArray = (ctypes.c_char_p * numVars)() 
    varNamesPtr = ctypes.pointer(varNamesArray)
    varTypesArray = (ctypes.c_int * numVars)()
    varTypesPtr = ctypes.pointer(varTypesArray)

    retcode = spssio.spssGetVarNames(fh, numVarsPtr, varNamesPtr, varTypesPtr)
    varNames = [varNamesPtr[0][i] for i in range(numVars)] # -- WRONG!!
    varTypes = [varTypesPtr[0][i] for i in range(numVars)]
return retcode, varNames, varTypes
savFileName = r"C:\Program Files\SPSS\Employee data.sav"
retcode, spssio, fh = loadSavFile(savFileName)
retcode, varNames, varTypes = getVarNamesAndTypes(fh, spssio)
print "retcode:", retcode
print "Varnames:", varNames


All right, but apart from the sanitation, the medicine, education, wine, public 
order, irrigation, roads, a fresh water system, and public health, what have the 
Romans ever done for us?

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