Python & Ctypes populate an array of structures

Question

I have the following code:

lib.h

struct Node {
    int index;
    char name[10];
};
void getAllNodes(Node array[]);

libtest.py

from ctypes import *

class Node(Structure):
    _fields_ = [("index", c_uint), ("name", c_char*10)]

lib = CDLL("/path/to/lib.so")

nodeCount = 5
NodeArray = Node * nodeCount
getAllNodes = lib.getAllNodes     
pointers = NodeArray()
getAllNodes(pointers)

casted = cast(pointers, POINTER(NodeArray))
for x in range(0, nodeCount):
    node = cast(casted[x], POINTER(Node))
    print "Node idx: %s name: %s" % (node.contents.index, node.contents.name)

I must be close because the Node struct at index 0 has the correct values but the remainder are gibberish. What am I building incorrectly that's giving me the problem?

Eryk Sun · Accepted Answer · 2013-05-23 09:32:33Z

3

It isn't an array of pointers. It's an array of Node records, laid out contiguously in memory. As a parameter in a function call the array acts like a Node * (pointer to the first item). After the call you can simply iterate the array:

getAllNodes.argtypes = [POINTER(Node)]
getAllNodes.restype = None

nodeArray = (Node * nodeCount)()
getAllNodes(nodeArray)

For example:

>>> [node.name for node in nodeArray]
['node0', 'node1', 'node2', 'node3', 'node4']

answered May 23, 2013 at 9:32

Eryk Sun

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1 Comment

bitwit Over a year ago

Thank you. This works well. I'm not sure how I got so far off the beaten path. Cheers.

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