Question

I’m trying to redo the Misaka module in ctypes, but when I try to use bufputs I get an error (see end of 2nd code sample). When I pass the pointer to the function I use pointer(b). That doesn’t work and byref(b) doesn’t work either.

This is the function signature:

/* bufputs • appends a NUL-terminated string to a buffer */
void
bufputs(struct buf *, const char*);

This is my code:

>>> from ctypes import *
>>> sundown = cdll.LoadLibrary('./libsundown.so.1')
>>> sundown
<CDLL './libsundown.so.1', handle 1e2f190 at 1bea0d0>
>>> # OUT: <CDLL './libsundown.so.1', handle 2840d80 at 2797290>
>>> class buf(Structure):
...     _fields_ = [
...         ('data', c_char_p),
...         ('size', c_size_t),
...         ('asize', c_size_t),
...         ('unit', c_size_t),
...         ('ref', c_int)]
... 
>>> sundown.bufnew.argtypes = [c_size_t]
>>> sundown.bufnew.restype = buf
>>> b = sundown.bufnew(c_size_t(1024))
>>> sundown.bufputs.argtypes = [POINTER(buf), c_char_p]
>>> s = c_char_p('this is a test')
>>> sundown.bufputs(pointer(b), s)
python2: malloc.c:3574: mremap_chunk: Assertion `((size + offset) & (mp_.pagesize-1)) == 0' failed.
Aborted

I can’t figure out what I did wrong.

Answer 1

The OP’s solution, originally posted in the question

class buf(Structure):
    _fields_ = [
        ('data', c_char_p),
        ('size', c_size_t),
        ('asize', c_size_t),
        ('unit', c_size_t),
        ('ref', c_int)
    ]
buf_p = POINTER(buf)

sundown.bufnew.argtypes = [c_size_t]
sundown.bufnew.restype = buf_p
sundown.bufgrow.argtypes = [buf_p, c_size_t]
sundown.bufputs.argtypes = [buf_p, c_char_p]

ib = buf()

# ctypes does this internally:
# memset(byref(ib), 0x0, sizeof(buf))

text = 'this is some text'
ib.data = text
ib.size = len(text)

ob = sundown.bufnew(128)
sundown.bufgrow(ob, int(math.ceil(ib.size * 1.4)))

sundown.bufputs(ob, 'test')