You can either create a c_float instance and initialize with a pointer to that instance, or create a c_float array and pass it, which in ctypes imitates a decay to a pointer to its first element.

Note that ctypes.pointer() creates pointers to existing instances of ctypes objects while ctypes.POINTER() creates pointer types.

test.c - for testing

#ifdef _WIN32
#   define API __declspec(dllexport)
#else
#   define API
#endif

typedef struct Input {
    float* number1;
    float  number2;
} Input;

API void expose_init(Input* input) {
    printf("%f %f\n",*input->number1, input->number2);
}

test.py

import ctypes

class input_struct (ctypes.Structure):
    _fields_ = (('number1', ctypes.POINTER(ctypes.c_float)),
                ('number2', ctypes.c_float))

c_instance = ctypes.CDLL('./test')
init_func = c_instance.expose_init
# Good habit to fully define arguments and return type
init_func.argtypes = ctypes.POINTER(input_struct),
init_func.restype = None

inp_str_ptr = input_struct()
num = ctypes.c_float(20)     # instance of c_float, similar to C "float num = 20;"
inp_str_ptr.number1 = ctypes.pointer(num) # similar to C "inp_str_ptr.number1 = #"
inp_str_ptr.number2 = 100

c_instance.expose_init(ctypes.byref(inp_str_ptr))

# similar to C "float arr[1] = {30}; inp_str_ptr = arr;"
inp_str_ptr.number1 = (ctypes.c_float * 1)(30)
c_instance.expose_init(ctypes.byref(inp_str_ptr))

Output:

20.000000 100.000000
30.000000 100.000000

Dealing with pointers and arrays is explained in [Python.Docs]: ctypes - Type conversions.

I prepared a dummy example for you.

main00.c:

#if defined(_WIN32)
#  define DECLSPEC_DLLEXPORT __declspec(dllexport)
#else
#  define DECLSPEC_DLLEXPORT
#endif


static int kSize = 5;


DECLSPEC_DLLEXPORT int size() {
    return kSize;
}


DECLSPEC_DLLEXPORT int function(int dummy, float *data1, float *data2) {
    for (int i = 0; i < kSize; i++) {
        data1[i] = dummy * i;
        data2[i] = -dummy * (i + 1);
    }
    return 0;
}

code00.py:

#!/usr/bin/env python

import sys
import ctypes as ct


c_float_p = ct.POINTER(ct.c_float)


def main(*argv):
    dll = ct.CDLL("./dll00.so")

    size = dll.size
    size.argtypes = []
    size.restype = ct.c_int


    function = dll.function
    function.argtypes = [ct.c_int, c_float_p, c_float_p]
    function.restype = ct.c_int

    sz = size()
    print(sz)

    data1 = (ct.c_float * sz)()
    data2 = (ct.c_float * sz)()

    res = function(1, ct.cast(data1, c_float_p), ct.cast(data2, c_float_p))
    for i in range(sz):
        print(data1[i], data2[i])


if __name__ == "__main__":
    print("Python {0:s} {1:d}bit on {2:s}\n".format(" ".join(item.strip() for item in sys.version.split("\n")), 64 if sys.maxsize > 0x100000000 else 32, sys.platform))
    rc = main(*sys.argv[1:])
    print("\nDone.")
    sys.exit(rc)

Notes:

• The C part tries to mimic what your .dll does (or at least what I understood):
  • size - gets the arrays sizes
  • function - populates the arrays (till their size - assuming that they were properly allocated by the caller)
• Python part is straightforward:
  • Load the .dll
  • Define argtypes and restype (in your code it's restype's) for the 2 functions (for size_func not necessary)
  • Get the lengths
  • Initialize the arrays
  • Pass them to function_func using ctypes.cast

Output (on Lnx, as building the C code is much simpler, but works on Win as well):

[cfati@cfati-ubtu16x64-0:~/Work/Dev/StackOverflow/q050043861]> gcc -shared -o dll00.so main00.c
[cfati@cfati-ubtu16x64-0:~/Work/Dev/StackOverflow/q050043861]> python3 code00.py
Python 3.8.5 (default, Jan 27 2021, 15:41:15) [GCC 9.3.0] 64bit on linux

5
0.0 -1.0
1.0 -2.0
2.0 -3.0
3.0 -4.0
4.0 -5.0

Not fully tested, but I think it's something along this line:

buffer_size = 720 * 288 * ctypes.sizeof(ctypes.c_float)
rgb_buffer = ctypes.create_string_buffer(buffer_size) 
ctypes.memmove(rgb_buffer, getRgbBuffer(), buffer_size)

Key is the ctypes.memmove() function. From the ctypes documentation:

memmove(dst, src, count)
Same as the standard C memmove library function: copies count bytes from src to dst. dst and src must be integers or ctypes instances that can be converted to pointers.

After the above snippet is run, rgb_buffer.value will return the content up until the first '\0'. To get all bytes as a python string, you can slice the whole thing: buffer_contents = rgb_buffer[:].

Here is a minimal example to pass C float as a parameter using numpy. It is an exercise for the OP to apply it to their function. Note that the C function must assume a fixed size or be passed the length of the array in some form.

Also note that the dtype is declared as ct.c_float since Python float is typically 64-bit and C float is typically 32-bit. Make sure the type sizes agree.

test.c

#include <stdio.h>

__declspec(dllexport) // for Windows
void func(float* p, size_t size) {
    for(size_t i = 0; i < size; ++i)
        printf("p[%zu] = %f\n", i, p[i]);
}

test.py

import ctypes as ct
import numpy as np

dll = ct.CDLL('./test')
# The helper function "ndpointer" can declare the expected type
# and either number of dimensions expected or the shape of the
# numpy array.  ctypes will then require that array and type check
# the parameter. 
dll.func.argtypes = np.ctypeslib.ndpointer(dtype=ct.c_float, ndim=1),
dll.func.restype = None

x_spots = np.arange(-1, 2, 1, dtype=ct.c_float)
dll.func(x_spots, len(x_spots))

Output:

p[0] = -1.000000
p[1] = 0.000000
p[2] = 1.000000

The problem was that I missed restype to setup:

lib.test_ret_float.restype = c_float

That solves the problem

The function ctypes.POINTER takes an argument that is a ctypes type. When you use it in the line

_lib.solve.argtypes = (ctypes.c_double, ctypes.c_double, ctypes.c_double, ctypes.POINTER(ctypes.c_double), ctypes.POINTER(ctypes.c_double))

that's exactly what you're doing, since ctypes.c_double is a type.

When later you call it like this:

ctypes.POINTER(p)

the argument isn't a type but an instance of a type - an already initialized variable of type ctypes.c_double.

All you need to do is to create a pointer to the already initialized variables p and q. Just replace ctypes.POINTER(p) with ctypes.byref(p).

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