python ctypes, pass double pointer by reference

Question

The Problem

I'm trying to use a function in a c library with the following prototype:
int glip_get_backends(const char ***name, size_t *count);
The name argument here is the problem. It is a 2 dimmensional char array passed by reference. In C the function is used as follows:

const char** name;
size_t count;
glip_get_backends(&name, &count);
for (size_t i = 0; i < count; i++) {
 printf("- %s\n", name[i]);
}

Now I want to use this function from python using ctypes.

---

What I've tried

The most logical approach to me was doing this in python:

lglip = CDLL("libglip.so")
count = c_int(0)
backends = POINTER(POINTER(c_char))
lglip.glip_get_backends(byref(backends), byref(count))

which resulted in the error message

TypeError: byref() argument must be a ctypes instance, not '_ctypes.PyCPointerType'

---

The next approach was to use the POINTER() function three times and omitting the byref(), but that results in the following error:

ctypes.ArgumentError: argument 1: : Don't know how to convert parameter 1

---

Then I took inspiration from this question and came up with the following:

lglip = CDLL("libglip.so")
count = c_int(0)
backends = POINTER(POINTER(c_char))()
lglip.glip_get_backends(byref(backends), byref(count))
for i in range(0, count.value):
 print backends[i] 

Adding () after the definition of backends for some reason I don't fully comprehend has fixed the call, but the output I get is:

< ctypes.LP_c_char object at 0x7f4592af8710>
< ctypes.LP_c_char object at 0x7f4592af8710>

It seems I can't itterate over a 2 dimensional c array with one itterator in python, as it does not properly dereference it.

• Nebril: Did you try iterating over objects that are in backends[i]? This is a 2-dimensional array, so you may need to use nested loop for this. I am not familiar with ctypes usage, just using my intuition.
• youR.Fate: I can try that, but I don't know the ranges of the loops. That's why in C it just uses one iterator. Oh, I've just realized in C it uses %s, therefore printing until 0円. Working on it.
• Mark Tolonen: You had to add the () to backends because POINTER(POINTER(c_char)) is a type, similar to c_int vs c_int(0)...first is type, second is instance of type.

Answer 1

After realizing that in the C implementation %s is used to itterate over the substrings I came up with the following working solution:

lglip = CDLL("libglip.so")
count = c_int(0)
backends_c = POINTER(c_char_p)()
lglip.glip_get_backends(byref(backends_c), byref(count))
backends = []
for i in range(0, count.value):
 backends.append(backends_c[i])
print backends

Update: changed POINTER(POINTER(c_char))() to POINTER(c_char_p)(), as eryksun suggested. This way I can easily access the strings.

• Eryk Sun: c_char_p is a null-terminated string in ctypes (specifically this simple C type is a subclass of ctypes._SimpleCData defined with _type_ == 'z'). Use count = c_int(); backends = POINTER(c_char_p)() . That's a single pointer to a null-terminated string. Pass those by reference, as you're currently doing, for the function to fill in the initial pointer and array count. ctypes implements pointer arithmetic indexing, so backends[i] gets the i'th pointer in the array.
• youR.Fate: Thank you, I will try that out tomorrow and update the answer if it works as expected.
• youR.Fate: I had actually tried using c_char_p first, but then I still didn't have the parentheses at the end so I only got errors.
• youR.Fate: Yes, I had tried using backends = POINTER(c_char_p), but that didn't work since, as Mark Tolonen has also explained, that returns a type, not an instance. But I didn't know that was the problem so I switched to backends = POINTER(POINTER(c_char)) and eventually to backends = POINTER(POINTER(c_char))(). Using c_char_p again simply had not occured to me.

Answer 2

If backends members can use index method, you can use this code:

lglip = CDLL("libglip.so")
count = c_int(0)
backends = POINTER(POINTER(c_char))()
lglip.glip_get_backends(byref(backends), byref(count))
for i in range(0, count.value):
 print ''.join(backends[i][:backends[i].index("0")])

• youR.Fate: Nope, that only gives AttributeError: 'LP_c_char' object has no attribute 'index'.
• Nebril: Then I guess you will be good with your code. I cannot think of more elegant way that would use simple array-like syntax there.