Re: [ctypes-users] [SPAM] CTYPES confusion - garbage returned from DLLFunction call??

[Mark T. <met...@gm...>]

"Lynton Grice" <lyn...@lo...> wrote in message 
news:25544.9695200243$124...@ne......
> Hi there,

[snip]


Please don't post in HTML.  Not everyone uses a reader that supports it.  It 
doesn't format well in a plain text reply, either.  I removed all the 
double- and triple-spacing it had.


> NOTE: I know the DLL is build up from native C file.....so I *assume* it 
> is
> a CDLL and not a WINDLL?


Not necessarily.  CDLL uses __cdecl calling convention (usually the 
default), but WINDLL uses the __stdcall calling convention.  Your DLL viewer 
likely can tell you.  Here's what mine says about a __stdcall function, 
using dumpbin.exe from Visual Studio 2008.

C:\WINDOWS\SYSTEM32>dumpbin /exports user32.dll | findstr SetWindowText
        647  286 0001F56B SetWindowTextA = _SetWindowTextA@8
        648  287 0001960E SetWindowTextW = _SetWindowTextW@8

Note the decorated name starts with _ and ends with @8.  That is __stdcall 
decoration.  __cdecl also starts with _ but won't have the @8.


> Before I mention the highlighted function above maybe a smaller example
> (within the SAME DLL) would explain my problem better:
>
> There is a simple function called "newMqString" that is defined as follows
> in the C API for the DLL (retrieved from the C header files I also have
> access to...):
>
> /**
> * To create a mqstring object with the String constant variable Provided
> */
> MQEXPORT mqstring newMqString(mqstring);
>
> And "mqstring" is defined in a file "mq_datatypes.h" as follows:
>
> typedef char * mqstring;
>
> So essentially it takes a STRING variable (char *) in my eyes and returns 
> a
> STRING (char *) variable...
>
> Now this is my first bit of confusion, I *think* the code below is OK for
> trying to just print the result after calling the "newMqString" function 
> BUT
> instead of seeing a STRING print out I get some arbitrary number come back
> like "9912208"
>
> import ctypes as c
> if __name__ == "__main__":
>      fmq = c.cdll.LoadLibrary("fmq-native-crtl.dll")
>      newMqString = fmq.newMqString
>      newMqString.argtypes = [c.c_char_p]
>      newMqString.retype = c.c_char_p
>      result = newMqString("Hello World")
>      print result
>
> This prints out something like: 9914096
> Why would I be getting back such garbage from the function call?


In the above code "retype" shoudl be "restype".  Since the default restype 
is integer you're seeing the c_char_p memory pointer as an integer value.


> Now the plot thickens.....I really need to know how to structure and call
> the following function in the DLL:
>
> This relates to the DLL function called "newInitialContext" highlighted
> above in the DLL Viewer..
>
> I have looked in the C header file called "initial_context.h" and this is
> the function definition:
>
> InitialContext newInitialContext(mqstring url, mqstring username, mqstring
> passwd);
>
> /**
> * @return a newly created InitialContext, NULL if an error occurs
> */
>
> So from my eyes this takes THREE STRING arguments (char *) and returns an
> "InitialContext" variable.
>
> I think the InitialContext is just a HASHTABLE type array....used for
> storing data etc.....
>
> struct _InitialContext;
> typedef struct _InitialContext* InitialContext;
>
> My question is how would I define the RETYPE for this function in the 
> Python
> code? And what would the basic structure of the call look like?
>
> Here is the basic code I have done.....I can debug over the line with no
> errors but it still returns garbage....
>
> newInitialContext = fmq.newInitialContext
> newInitialContext.argtypes = [c.c_char_p, c.c_char_p, c.c_char_p]
> newInitialContext = fmq.newInitialContext("http://localhost:1856",
> "anonymous", "anonymous")
>
> If I use the DEBUG probe to look at the value of "newInitialContext" I get
> the following:
>
> 9914297
>
> What is that?

This is also just a pointer value as an integer.  Since this is just an 
opaque data structure being used as  a handle value, use 
newInitialContext.restype = c_void_p.

-Mark