Re: typedef union

"Nils Wagner" <[email protected]> wrote in message 
news:[email protected]...
> On Mon, 15 Mar 2010 20:36:33 -0700
>  "Mark Tolonen" <[email protected]> wrote:
>> Nils,
>>
>> Please reply to the mailing list instead of me directly.
>> Answers below.
>>
>>
>>
>> "Mark Tolonen" <[email protected]> wrote in
>>message
>> news:[email protected]...
>>>
>>> "Nils Wagner" <[email protected]> wrote in
>>>message
>>> news:[email protected]...
>>>> Hi all,
>>>>
>>>> I am newbie to ctypes.
>>>>
>>>> How can I "translate" the following C-code into Python ?
>>>> I don't know how to handle HEAD and memcpy ?
>>>> Any pointer would be appreciated.
>>>>
>>>> Thanks in advance.
>>>>
>>>> Nils
>>>>
>>>>
>>>>       typedef union {
>>>> int i;
>>>> float f;
>>>> char c[4];
>>>> } Word;
>>>>
>>>>       int
>>>> I,J,K,N,IDE,IA,IE,IERR,JUNIT,JUNCAT,NDATA,NREC,LREADY,ONE=1;
>>>>       Word BUF[100],HEAD[30];
>>>>
>>>>       for (I=5;I<LHEAD;I++)
>>>> HEAD[I].i = 0;
>>>>       HEAD[ 0].i = 1;
>>>>       HEAD[ 1].i = LHEAD + NDATA*7;
>>>>       HEAD[ 2].i = LHEAD;
>>>>       HEAD[ 3].i = NDATA;
>>>>       HEAD[ 4].i = 7;
>>>>       memcpy (HEAD[ 7].c,"DSIO",4);
>>>>       memcpy (HEAD[ 8].c,"TEST",4);
>>>>       memcpy (HEAD[ 9].c,"NPCO",4);
>>>>       memcpy (HEAD[10].c," ",4);
>>>>       memcpy (HEAD[11].c,"DSIO",4);
>>>>       HEAD[20].i = 1;
>>>>       HEAD[21].i = NDATA;
>>>>       STRING = "MM RAD";
>>>>       DSEINH(STRING,&HEAD[24].i,&ONE,
>>>> strlen(STRING));
>>>
>>> Here's a rough translation.  Note how the arrays are
>>>declared.  "c_char*4"
>>> is a 4-character array *type*.  (Word*100)() is a
>>>100-Word array
>>> *instance*.
>>> To assign to the char array, just use a maximum
>>>4-character string.
>>>
>>> import ctypes
>>>
>>> class Word(ctypes.Union):
>>>    _fields_ = [
>>>        ('i',ctypes.c_int),
>>>        ('f',ctypes.c_float),
>>>        ('c',ctypes.c_char*4)]
>>>
>>> I=J=K=N=IDE=IA=IE=IERR=JUNIT=JUNCAT=NDATA=NREC=LREADY=ONE=1
>>> BUF = (Word*100)()
>>> HEAD = (Word*30)()
>>>
>>> # not defined in C code
>>> LHEAD = 30
>>>
>>> for I in range(5,LHEAD):
>>>    HEAD[I].i = 0
>>>
>>> HEAD[ 0].i = 1;
>>> HEAD[ 1].i = LHEAD + NDATA*7
>>> HEAD[ 2].i = LHEAD
>>> HEAD[ 3].i = NDATA
>>> HEAD[ 4].i = 7
>>> HEAD[ 7].c = "DSIO"
>>> HEAD[ 8].c = "TEST"
>>> HEAD[ 9].c = "NPCO"
>>> HEAD[10].c = " "
>>> HEAD[11].c = "DSIO"
>>> HEAD[20].i = 1
>>> HEAD[21].i = NDATA
>>> STRING = "MM RAD"
>>> # Not enough info
>>> # DSEINH(STRING,&HEAD[24].i,&ONE,strlen(STRING));
>>
>>>From your private replies:
>>
>>> Thank you very much for your help.
>>> Here comes information concerning DSEINH.
>>>
>>> extern void dseinh      (char* const,int* const,
>>>                    const int* const,const size_t);
>>>
>>> How do I call dseinh from python (byref etc.) ?
>>
>>First load the dynamic library (.dll or .so) using CDLL.
>> See (untested)
>> example:
>>
>> dll = ctypes.CDLL('library.dll')
>> value1 = ctypes.c_uint()
>> value2 = ctypes.c_uint()
>> s = "string"
>> dll.dseinh(s,ctypes.byref(value1),ctypes.byref(value2),len(s)+1)
>>
>> Python strings are passed as null-terminated C strings,
>>so len(s) + 1
>> accounts for the null.  There is also a ctypes.sizeof()
>>function that can be
>> used with ctypes types and classes derived from
>>ctypes.Structure or
>> ctypes.Union.
>>
>> You can (and sometimes have to) declare the parameters
>>and return types of a
>> function.  See the Python ctypes documentation
>> (http://docs.python.org/library/ctypes.html).
>
> Why did you use ctypes.c_uint() and not ctypes.c_int() ?

typo.

> When shall I use byref() ?

When I function takes a pointer to "something", you need to create an 
instance of "something" and pass a reference to it.  So if a function takes 
an int* in order to return a value in it:

    void func(int* value);

You first create an c_int:

    v= ctypes.c_int()

Then call the function passing a reference:

   dll.func(ctypes.byref(v))

When the function returns, you get the Python integer result using:

    v.value

-Mark



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