How do I pass a multiline string from Python to C using ctypes?

Question

I have the following simple C function:

void pyentry(const char *config)
{
    printf("%s\n",config);
    fflush(stdout);
}

My ctypes definition is as follows:

libc = ct.CDLL("./amr.so")
entry = wrap_function(libc, 'pyentry', ct.POINTER(Checkpoint), [ct.c_wchar_p])
json = "this is a test"
start = entry(json)

Where wrap_function is simply a wrapper for more easily defining ctypes access to C functions:

def wrap_function(lib, funcname, restype, argtypes):
    func = lib.__getattr__(funcname)
    func.restype = restype
    func.argtypes = argtypes
    return func

I have compiled as a shared library and I am trying to call it, but in C it is only printing the first character of the string I send in. I am assuming this is because I have the wrong argument tpyes in my ctypes definition, but I am not having any luck figuring out out the right one.

Can someone tell me why my C function is only seeing the first character in the passed string?

I managed to get it working using ct.POINTER(ct.c_char) as the type. Thanks for the hint. I do not know why I didn't start with that. — KBriggs
– KBriggs, Commented Feb 23, 2021 at 20:32
pyentry is void, so .restype shoud be None. .argtypes shoudl be [ct.c_char_p]. getattr(lib,funcname) is more Pythonic. start = entry(json) should just be entry(json). — Mark Tolonen
– Mark Tolonen, Commented Feb 23, 2021 at 21:44

ghchoi · Accepted Answer · 2021-02-24 16:36:24Z

1

Try:

entry = wrap_function(libc, 'pyentry', None, [ct.POINTER(ct.c_char)])
json = "this is a test".encode('utf-8')

pyentry takes const char* and returns void. So argtypes and restype could be [ct.POINTER(ct.c_char)] and None.

And char* points a sequence of bytes, not Python string. So json should be converted to bytes.

edited Feb 24, 2021 at 16:36

answered Feb 24, 2021 at 16:30

ghchoi

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