The following code works:

test.py:

import ctypes
lib = ctypes.CDLL("./libtest.so")
string_buffers = [ctypes.create_string_buffer(8) for i in range(4)]
pointers = (ctypes.c_char_p*4)(*map(ctypes.addressof, string_buffers))
lib.test(pointers)
results = [s.value for s in string_buffers]
print results

test.c (compiled to libtest.so with gcc test.c -o libtest.so -shared -fPIC):

#include <string.h>
void test(char **strings) {
    strcpy(strings[0],"this");
    strcpy(strings[1],"is");
    strcpy(strings[2],"a");
    strcpy(strings[3],"test!");
}

As Aya said, you should make sure there is room for the terminating zero. But I think your main problem was that the string buffer was garbage collected or something similar, as there was no direct reference to it anymore. Or something else is causing trouble in the creation process of the string buffers when no references are stored for them. For example this results in four times the same address instead of different addresses:

import ctypes
pointers = [ctypes.addressof(ctypes.create_string_buffer(8)) for i in range(4)]
print pointers

Pass a byte string as the error message suggests. ctypes will complain if it is too long:

import ctypes

class MyStructure(ctypes.Structure):
    _fields_ = [("a", ctypes.c_char * 32)]

mystruct = MyStructure(b'Hi')
print(mystruct.a)
mystruct = MyStructure(b'Hi'*16)
print(mystruct.a)
mystruct = MyStructure(b'Hi'*17)
print(mystruct.a)

Result:

b'Hi'
b'HiHiHiHiHiHiHiHiHiHiHiHiHiHiHiHi'
Traceback (most recent call last):
  File "D:\dev\Python36\Lib\site-packages\pythonwin\pywin\framework\scriptutils.py", line 326, in RunScript
    exec(codeObject, __main__.__dict__)
  File "C:\Users\metolone\Desktop\x.py", line 10, in <module>
    mystruct = MyStructure(b'Hi'*17)
ValueError: bytes too long (34, maximum length 32)