Re: [ctypes-users] Re: Converting macro..

[Thomas H. <th...@py...>]

Andrew Markebo <and...@co...> writes:

> Ok follow up with source and so on, unfortunetaly I can't hand you the
> dll.. if you don't have labview.
>
> Running the call as:
> Cut_string(5,
>            LStrToLStrHandle(instr), \
>            LStrToLStrHandle(outstr) )
>
> gives 
>
>  LStrToLStrHandle(outstr) \
> WindowsError: exception: access violation reading 0x0000000F
>
> running it as:
>
> Cut_string(5,
>            pointer(LStrToLStrHandle(instr)), \
>            pointer(LStrToLStrHandle(outstr)) )
>
> popups a dialogue from LabView: "Error#3 memory.cpp, line 600 contact
> us..."
>
> Here my minimal example comes... 
>
> from ctypes import *
>
> class LStr(Structure) :
>     _fields_ = [ ("cnt", c_int), ("str", c_char_p) ]

That doesn't match the C definition of LStr that you gave before.  The C
version consists of one integer plus one character (one int plus an
array of one character, to be exact), the ctypes version contains an
integer plus a *pointer* to characters.

The former has 5 bytes, the latter has 8.
The ctypes version would look like this:

class LStr(Structure):
    _fields_ = [("cnt", c_int), ("str", c_char * 1)]

Normally, in C instances of structures like these (which are really
variable sized) are allocated with code like this:

LStr *p = (LStr *)malloc(sizeof(LStr) + count);

where <count> specifies the actual size of the character array.
If you wrote this:

LStr instring;

then the array would only have one character.
As I said, ctypes can't handle these variable sized structures (not very
well, at least).  So, again I would suggest you look up the code that I
posted using the struct module to build memory blocks containing these
beasts.

Then, assuming you have a 'LStr' function that creates them, you can:

instr = LStr(15, "abcdefghijklmno")
outstr = LStr(10, "..........")

pointer(instr) is then probably a LStrPtr.
And pointer(pointer(instr)) a LStrHandle.

Try calling the function like this:

P_instr = pointer(instr)
P_outstr = pointer(outstr)

Cut_string(5, byref(instr), byref(outstr))

Sorry, no more time.

Thomas

> LStrP=POINTER(LStr)  # <class 'ctypes.LP_LStr'>     
> LStrH=POINTER(LStrP) # <class 'ctypes.LP_LP_LStr'>  
>
> def LStrToLStrHandle(theLStr):
>     return LStrH(LStrP(theLStr))
> def LStrHandleToLStr(theHandle):
>     return theHandle.contents.contents
>
> instr=LStr(15, "abcdefghijklmno")
> outstr=LStr(10,"0123456789")
>
> Cut_string=windll.LoadLibrary("Cut_String.dll").Cut_string
> # void __stdcall Cut_string(int32 NewStringLength, LStrHandle *StringIn,
> #      LStrHandle *StringOut);
>
> Cut_string(5,
>            LStrToLStrHandle(instr), \
>            LStrToLStrHandle(outstr) \
>            )
>
>
>
>
>
> -- 
>  All people are equal, but some are more equal that the others
> 				    - Someone more equal
>  But we all are told that we differ??
> 				    - Me.. 
>
>
>
> -------------------------------------------------------
> SF email is sponsored by - The IT Product Guide
> Read honest & candid reviews on hundreds of IT Products from real users.
> Discover which products truly live up to the hype. Start reading now.
> http://ads.osdn.com/?ad_id=6595&alloc_id=14396&op=click
> _______________________________________________
> ctypes-users mailing list
> cty...@li...
> https://lists.sourceforge.net/lists/listinfo/ctypes-users