CODEWITHSUNDEEP
pythonctypes
JasonEleventeen
JasonEleventeen

Reputation: 343

Python double pointer

I'm trying to get the values from a pointer to a float array, but it returns as c_void_p in python

The C code

double v;
const void *data;  
pa_stream_peek(s, &data, &length);  
v = ((const float*) data)[length / sizeof(float) -1];

Python so far

import ctypes
null_ptr = ctypes.c_void_p()
pa_stream_peek(stream, null_ptr, ctypes.c_ulong(length))

The issue being the null_ptr has an int value (memory address?) but there is no way to read the array?!

Upvotes: 4

Views: 13749

Answers (4)

Nathan Kitchen
Nathan Kitchen

Reputation: 4867

When you pass pointer arguments without using ctypes.pointer or ctypes.byref, their contents simply get set to the integer value of the memory address (i.e., the pointer bits). These arguments should be passed with byref (or pointer, but byref has less overhead):

data = ctypes.pointer(ctypes.c_float())
nbytes = ctypes.c_sizeof()
pa_stream_peek(s, byref(data), byref(nbytes))
nfloats = nbytes.value / ctypes.sizeof(c_float)
v = data[nfloats - 1]

Upvotes: 0

tom10
tom10

Reputation: 69182

To use ctypes in a way that mimics your C code, I would suggest (and I'm out-of-practice and this is untested):

vdata = ctypes.c_void_p()
length = ctypes.c_ulong(0)
pa_stream_peek(stream, ctypes.byref(vdata), ctypes.byref(length))
fdata = ctypes.cast(vdata, POINTER(float))

Upvotes: 1

igkuk7
igkuk7

Reputation: 339

You'll also probably want to be passing the null_ptr using byref, e.g.

pa_stream_peek(stream, ctypes.byref(null_ptr), ctypes.c_ulong(length))

Upvotes: 0

Unknown
Unknown

Reputation: 46773

My ctypes is rusty, but I believe you want POINTER(c_float) instead of c_void_p.

So try this:

null_ptr = POINTER(c_float)()
pa_stream_peek(stream, null_ptr, ctypes.c_ulong(length))
null_ptr[0]
null_ptr[5] # etc

Upvotes: 3

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