ctypes.cast() is used to convert one ctype instance to another ctype datatype. You don't need it To convert it to python string. Just use ".value" to get it in python string.

>>> s = "Hello, World"
>>> c_s = c_char_p(s)
>>> print c_s
c_char_p('Hello, World')
>>> print c_s.value
Hello, World

More info here

As already mentioned in comments, although perhaps not as explicitly as some might require, the .value attribute converts from ctypes.c_char_p to a Python string.

This code round trips a Python string to a ctypes.c_char_p and then back to a Python string:

>>> p = c_char_p("Hello, World")
>>> p
c_char_p(140323010068388)
>>> type(p)
<class 'ctypes.c_char_p'>
>>> s = p.value
>>> s
'Hello, World'
>>> type(s)
<type 'str'>

Thanks to @ErykSun the solution:

Python code

string1 = "my string 1"
string2 = "my string 2"

# create byte objects from the strings
b_string1 = string1.encode('utf-8')
b_string2 = string2.encode('utf-8')

# send strings to c function
my_c_function.argtypes = [ctypes.c_char_p, ctypes.c_char_p]
my_c_function(b_string1, b_string2)

from ctypes import *

charptr = POINTER(c_char)

test = CDLL('test.so')
test.initializetest.argtypes = []
test.initializetest.restype = charptr
test.searchtest.argtypes = [charptr]
test.searchtest.restype = c_int

buf = test.initializetest()
test.searchtest(buf)
print cast(buf, c_char_p).value
# TODO Release the "buf" memory or it will leak.

EDIT

Initially I used c_char_p to pass the buffer between the functions but c_char_p is like a const pointer. If used as a restype, you will actually get a Python str back. So for initializetest it will create a string from the allocated memory (by copying data) and throw the pointer away.

Now we're creating a new type, a POINTER to c_char. This is then used in both functions.

For Python, this type points to a single char so we have to cast it to get the whole string after searchtest is done. We cast to c_char_p because we just want to read the value so a const pointer is OK.

As a side note, this illustrates the disastrous effect of using c_char_p with functions that modify the array (as searchtest above does):

>>> libc.memset.argtypes = [c_char_p, c_int, c_int]
>>> foo = 'Python'
>>> foo
'Python'
>>> libc.memset(foo, ord('x'), 3)
44808532
>>> foo
'xxxhon'
>>>

Note how we've managed to change an immutable Python string!

The argtypes setup line isn't even needed because ctypes assumes c_char_p if Python str is used as argument.

There are a number of problems:

1. Define .argtypes for your functions. It will catch errors passing incorrect parameters. Add the line below and note it is plural and is a tuple of argument types. The comma makes a 1-tuple:

python_p_printme.argtypes = c_char_p,

2. Once you make that change, you'll get an error because this code:

   c_sends = pointer(c_char_p(CHAR.encode('utf-8')

is actually sending a C char** (a pointer to a c_char_p). Once you've set the argtypes properly, you can just call the function with a byte string and it will work. Your function becomes:

def printmeP(CHAR):
    print("In Print function")
    print(CHAR)
    python_p_printme(CHAR.encode())
    print("DONE function - Python")

3. There is one more subtle problem. While the program may appear to work at this point Python strings are immutable so if the function being called requires a mutable string, you must allocate a mutable buffer using either create_unicode_buffer (for c_wchar_p) or create_string_buffer (for c_char_p); otherwise, the strcat in your C code is going to corrupt the Python string.

Here's a full example:

test.cpp

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

// For Windows compatibility
#ifdef _WIN32
#   define API __declspec(dllexport)
#else
#   define API
#endif

// For C++ compiler compatibility
#ifdef __cplusplus
extern "C" {
#endif

API void alter_me(char* param, size_t length) {
    // truncates if additional info exceeds length
    strncat_s(param, length, " adding some info", length - 1);
}

#ifdef __cplusplus
}
#endif

test.py

from ctypes import *

lib = CDLL('./test')
alter_me = lib.alter_me
alter_me.argtypes = c_char_p,c_size_t
alter_me.restype = None

data = create_string_buffer(b'test',size=10)
alter_me(data,sizeof(data))
print(data.value)

data = create_string_buffer(b'test',size=50)
alter_me(data,sizeof(data))
print(data.value)

Output:

b'test addi'
b'test adding some info'

Note you do not need to use create_string_buffer if the C function does not alter the buffer, such as if the C parameter is const char*. Then you could just call printme(b'test string').

c_char_p is a subclass of _SimpleCData, with _type_ == 'z'. The __init__ method calls the type's setfunc, which for simple type 'z' is z_set.

In Python 2, the z_set function (2.7.7) is written to handle both str and unicode strings. Prior to Python 3, str is an 8-bit string. CPython 2.x str internally uses a C null-terminated string (i.e. an array of bytes terminated by \0), for which z_set can call PyString_AS_STRING (i.e. get a pointer to the internal buffer of the str object). A unicode string needs to first be encoded to a byte string. z_set handles this encoding automatically and keeps a reference to the encoded string in the _objects attribute.

>>> c = u'spam'
>>> a = c_char_p(c)
>>> a._objects
'spam'
>>> type(a._objects)
<type 'str'>

On Windows, the default ctypes string encoding is 'mbcs', with error handling set to 'ignore'. On all other platforms the default encoding is 'ascii', with 'strict' error handling. To modify the default, call ctypes.set_conversion_mode. For example, set_conversion_mode('utf-8', 'strict').

In Python 3, the z_set function (3.4.1) does not automatically convert str (now Unicode) to bytes. The paradigm shifted in Python 3 to strictly divide character strings from binary data. The ctypes default conversions were removed, as was the function set_conversion_mode. You have to pass c_char_p a bytes object (e.g. b'spam' or 'spam'.encode('utf-8')). In CPython 3.x, z_set calls the C-API function PyBytes_AsString to get a pointer to the internal buffer of the bytes object.

Note that if the C function modifies the string, then you need to instead use create_string_buffer to create a c_char array. Look for a parameter to be typed as const to know that it's safe to use c_char_p.

As you said, use a POINTER(c_char) to get a pointer to the array of binary data. To put that together into a string, you can just take a slice of it, since array indexing works as expected with ctypes pointers:

clibblah = ctypes.cdll.LoadLibrary('clibblah.dylib')
get_pixels = clibblah.get_pixels
get_pixels.restype = ctypes.POINTER(ctypes.c_char)

pixels = get_pixels()
num_pixels = clibblah.get_pixel_length()

# Slice the ctypes array into a Python string
a = pixels[:num_pixels]