Re: typedef union

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() ?
When shall I use byref() ?

Nils


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