Re: typedef union

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).

In regards to your other email:

> Is the following loop in C

>

> do {

>    IDE = IDE + 1;

> } while (ID != KYNPCO)

>

> equivalent to

>

> while ID != KYNPCO:

>     IDE += 1

>

> in python ?



No, if ID == KYNPCO, the first version would still increment IDE; the 2nd 
would not.



Python doesn't have do/while.  Either:



IDE += 1

while ID != KYNPCO:

    IDE += 1



or:



while True:

    IDE += 1

    if ID == KYNPCO:

        break



Would give the correct result.



-Mark





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