Access array of c-structs using Python ctypes

Question

I have a C-function that allocates memory at the address passed to and is accessed via Python. The pointer contents does contain an array of structs in the C code, but I am unable to get ctypes to access the array properly beyond the 0th element. How can I get the proper memory offset to be able to access the non-zero elements? Python's ctypes.memset is complaining about TypeErrors if I try to use their ctypes.memset function.

typedef struct td_Group
{
    unsigned int group_id;
    char groupname[256];
    char date_created[32];
    char date_modified[32];
    unsigned int user_modified;
    unsigned int user_created;
} Group;

int getGroups(LIBmanager * handler, Group ** unallocatedPointer);

############# python code below: 
class Group(Structure):
    _fields_ = [("group_id", c_uint),
                ("groupname", c_char*256),
                ("date_created", c_char*32),
                ("date_modified", c_char*32),
                ("user_modified", c_uint),
                ("user_created", c_uint)]


myGroups = c_void_p()
count = libnativetest.getGroups( nativePointer, byref(myGroups) )
casted = cast( myGroups, POINTER(Group*count) )
for x in range(0,count):
    theGroup = cast( casted[x], POINTER(Group) )
    # this only works for the first entry in the array:
    print "~~~~~~~~~~" + theGroup.contents.groupname

Related: Access c_char_p_Array_256 in Python using ctypes

What error are you getting? Please add the entire traceback. — yak
– yak, Commented Nov 30, 2011 at 0:55
@yak: When one is playing around with ctypes, tracebacks are a privilege not always available. If you step out of the line, the norm is the Python interpreter ending up in a segfault — jsbueno
– jsbueno, Commented Nov 30, 2011 at 2:09

verideskstar · Accepted Answer · 2011-11-30 15:01:33Z

4

D Hess put me in the right direction; The solution was:

GroupArray = POINTER(Group * count)
group_array = GroupArray.from_address(addressof(myGroups))
for x in range(0,count):
    print "~~~~~~~~~~" + group_array.contents[x].groupname

answered Nov 30, 2011 at 15:01

verideskstar

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Comments

David K. Hess · Accepted Answer · 2011-11-30 16:45:53Z

3

First create a new type which is an array of Group:

GroupArray = Group * count

Then create an instance of a GroupArray this way:

group_array = GroupArray.from_address(myGroups.value)

Then your loop would work like this:

for x in range(0,count):
    print "~~~~~~~~~~" + group_array[x].groupname

edited Nov 30, 2011 at 16:45

answered Nov 30, 2011 at 1:46

David K. Hess

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2 Comments

verideskstar Over a year ago

Unfortunately, that is printing garbage characters. Note that myGroups is a pointer to the first element of the array (a Group*), not a Group**. The call to libnativetest.getGroups is passing the address of myGroups, which is making it a Group**.

David K. Hess Over a year ago

Oops, you are correct. That's a bug - try the fix I put in. Use myGroups.value instead of addressof(myGroups).

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