ctypes-users

[ctypes-users] Dereferencing a pointer

[Bryan D R. <bdr...@ju...>]

I am currently working with pyFMOD (python extensions to FMOD, a sound
library) to add callback functionality to the python library.  Several
functions in FMOD/pyFMOD return void pointers, notably the callback
functions

FOUND_STREAMCALLBACK
Callback used with user streams.

signed char FOUND_STREAMCALLBACK(
FOUND_STREAM *stream,
void *buff,
int len,
int param
);

FOUND_DSPCALLBACK
Callback definition for DSP units.

void *FSOUND_DSPCALLBACK(
void *originalbuffer,
void *newbuffer,
int length,
int param
);
Here is my callback code which is located in pyFMOD
#Bryan's Callback Code:
_FSOUND_DSPCALLBACK = CFUNCTYPE(c_void_p, pointer(c_int), pointer(c_int),
c_int, c_int)
_FSOUND_STREAMCALLBACK = CFUNCTYPE(c_byte, c_int, c_void_p, c_int, c_int)
_FSOUND_ALLOCCALLBACK = CFUNCTYPE(c_void_p, c_uint)
_FSOUND_REALLOCCALLBACK = CFUNCTYPE(c_void_p, c_void_p, c_uint)
_FSOUND_READCALLBACK = CFUNCTYPE(c_int, c_void_p, c_int, c_uint)
_FSOUND_TELLCALLBACK = CFUNCTYPE(c_int, c_uint)

#The next function had a void return type.  I'm not sure how to do that
in ctypes.
#If this doesn't work, I'll try posting to the mailing list.
_FSOUND_FREECALLBACK = CFUNCTYPE(c_void_p, c_void_p)
#The next function should return void as well.
_FSOUND_CLOSECALLBACK = CFUNCTYPE(c_void_p, c_uint)
#This function uses a signed character for input 3, the last one.
#Not sure if c_byte is the correct type or not.
_FSOUND_SEEKCALLBACK = CFUNCTYPE(c_int, c_uint, c_int, c_byte)
#This function used a signed character as well.
_FSOUND_OPENCALLBACK = CFUNCTYPE(c_uint, c_char_p)

This is the code I am trying to run and get the error with
import pyFMOD
from ctypes import *
def myDSP_Callback(originalbuffer, newbuffer, length, param):
    print "timmy"
    originalbuffer.contents

pyFMOD.FSOUND_Init(44100, 32, 0);
s = pyFMOD.FSOUND_Stream_OpenFile(".\\forkbomb.ogg", pyFMOD.FSOUND_2D, 0)
a = pyFMOD.FSOUND_Stream_CreateDSP(s, myDSP_Callback, 301, 0)
pyFMOD.FSOUND_DSP_SetActive(a, True)
pyFMOD.FSOUND_Stream_Play(0, s)
    
print "started"
for i in range(1000000):
    pass
print "stopped"
pyFMOD.FSOUND_Stream_Stop(s)

This is the error I get
Traceback (most recent call last):
  File "C:\Source\Python\pyDanceR\fmodTest.py", line 1, in ?
    import pyFMOD
  File "C:\PYTHON23\lib\site-packages\pyFMOD.py", line 6, in ?
    _FSOUND_DSPCALLBACK = CFUNCTYPE(c_void_p, pointer(c_int),
pointer(c_int), c_int, c_int)
  File "C:\PYTHON23\Lib\site-packages\ctypes\__init__.py", line 206, in
pointer
    return POINTER(type(inst))(inst)
  File "C:\PYTHON23\Lib\site-packages\ctypes\__init__.py", line 189, in
POINTER
    {'_type_': cls})
TypeError: _type_ must have storage info

How do I acess and modify the pointer original buffer?  I know it's
length(from a function) and size(It is set to 32 or 16 bit depending on
the processor.).

[ctypes-users] Re: Dereferencing a pointer

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

Bryan D Rosander <bdr...@ju...> writes:

> I am currently working with pyFMOD (python extensions to FMOD, a sound
> library) to add callback functionality to the python library.  Several
> functions in FMOD/pyFMOD return void pointers, notably the callback
> functions
[...]

> #The next function had a void return type.  I'm not sure how to do that
> in ctypes.

With ctypes CVS, you could use None as return type.  Doesn't work in
0.6.3, but you can probably use c_int instead, and ignore the result.

> This is the error I get
> Traceback (most recent call last):
>   File "C:\Source\Python\pyDanceR\fmodTest.py", line 1, in ?
>     import pyFMOD
>   File "C:\PYTHON23\lib\site-packages\pyFMOD.py", line 6, in ?
>     _FSOUND_DSPCALLBACK = CFUNCTYPE(c_void_p, pointer(c_int),
> pointer(c_int), c_int, c_int)
>   File "C:\PYTHON23\Lib\site-packages\ctypes\__init__.py", line 206, in
> pointer
>     return POINTER(type(inst))(inst)
>   File "C:\PYTHON23\Lib\site-packages\ctypes\__init__.py", line 189, in
> POINTER
>     {'_type_': cls})
> TypeError: _type_ must have storage info

Somewhat similar problem as another poster had: pointer() is a function
creating instances of ctypes' types.  So, pointer(c_int) throws an
error, you should use pointer(c_int(32)) to create an instance of a
pointer pointing to the integer 32.

But, in the CFUNCTYPE call, you need ctypes' types, not ctypes'
instances.  That's what POINTER creates:

_FSOUND_DSPCALLBACK = CFUNCTYPE(c_void_p, POINTER(c_int), c_int, c_int)

Hope that helps,

Thomas

[ctypes-users] Dereferencing a slice of pointer (was: Re: Dereferencing a pointer)

[Holger D. <H.D...@gm...>]

On a related note:

I am currently trying to wrap a function which allocates a buffer for
me; something like
,----
| int foo(char **out_buffer, size_t *out_buffer_length);
`----

The data in the buffer can be binary, i.e. it may contain a 0-byte,
which is why we get a length returned as well.

Because of the 0-byte, I cannot use something like 

,----
| out_buffer = c_char_p()
| out_buffer_length = c_int()
| success = the_lib.foo(byref(out_buffer), byref(out_buffer_length))
`----

What I currently have is:
,----
| out_buffer = POINTER(c_char)()
| out_buffer_length = c_int()
| success = the_lib.foo(pointer(out_buffer), byref(out_buffer_length))
| out_buffer = "".join([out_buffer[i] for i in range(out_buffer_length)])
`----
and this works.

Two questions:
a) Could this be done better?

b) Why doesn't the pointer metaclass support slicing?  I.e. this could
   have simplified my last line to
   ,----
   | out_buffer = "".join(out_buffer[:out_buffer_length])
   `----
   I had a quick glance at the code and it is just missing -- is this
   intentional or just not yet done?


Thanks,
        Holger

Re: [ctypes-users] Dereferencing a slice of pointer

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

Holger Duerer <H.D...@gm...> writes:

> I am currently trying to wrap a function which allocates a buffer for
> me; something like
> ,----
> | int foo(char **out_buffer, size_t *out_buffer_length);
> `----
>
> The data in the buffer can be binary, i.e. it may contain a 0-byte,
> which is why we get a length returned as well.
>
> Because of the 0-byte, I cannot use something like 
>
> ,----
> | out_buffer = c_char_p()
> | out_buffer_length = c_int()
> | success = the_lib.foo(byref(out_buffer), byref(out_buffer_length))
> `----
>
> What I currently have is:
> ,----
> | out_buffer = POINTER(c_char)()
> | out_buffer_length = c_int()
> | success = the_lib.foo(pointer(out_buffer), byref(out_buffer_length))
> | out_buffer = "".join([out_buffer[i] for i in range(out_buffer_length)])
> `----
> and this works.
>
> Two questions:
> a) Could this be done better?

No, it's the 'correct' way to do it, but let me make two remarks.
You could use byref(out_buffer) in the the_lib.foo call instead of
pointer(out_buffer), this will be slightly faster.
And it seems in your code the pointer can never be freed because you
overwrite the out_buffer variable. Maybe you were just simplifying...

> b) Why doesn't the pointer metaclass support slicing?  I.e. this could
>    have simplified my last line to
>    ,----
>    | out_buffer = "".join(out_buffer[:out_buffer_length])
>    `----
>    I had a quick glance at the code and it is just missing -- is this
>    intentional or just not yet done?

I probably had a mental block (Brett vor'm Kopf) because I always
thought about slicing support for arrays instead of pointers first.
And I couldn't decide if it would be better to return a new ctypes array
instance as the slice, or a list of array elements.
The array instance would be much more complicated, because this would
have to be an instance of a probably not yet existing class.

For pointer slicing, it is simpler because an array would not make any
sense.  So I think the list of elements is what needs to be returned,
for pointers *and* for arrays.  Same for slice assignment, of course.

Oh, and for arrays or pointers of c_chars it could be specialized to
return a string instead of a list of characters instead, so that you
could write

   ,----
   | out_buffer = out_buffer[:out_buffer_length]
   `----

instead of the join above.
Sounds good?

Thanks,

Thomas