Python/Ctypes - Segmentation Fault on Access to Pointer (to Array)

Question

This is similar except I do not get a single value and instead segfault on [0] or .contents

How can I access the elements in 'array' from Python?

In C I have something like

#pragma pack(push, 1)
struct Data {
    int i;
    int *array;
};
#pragma pack(pop)

typedef struct {
    bool (* const get)   (struct Data*, const char *source);
    void (* const print) (struct Data*);
} DataFn;
extern DataFn const DATAFUNC;

In Python I have something like

class Data(ctypes.Structure):
    _fields_ = [
        ("i", ctypes.c_int),
        ("array",   ctypes.POINTER(ctypes.c_int))
    ]

class DATA_FN(ctypes.Structure):
    _fields_ = [
        ("get"   , ctypes.CFUNCTYPE(
                                          ctypes.c_bool,
                                          ctypes.POINTER(Data),
                                          ctypes.c_char_p)),
        ("print" , ctypes.CFUNCTYPE(None, 
                                          ctypes.POINTER(Data)))
    ]

From python I can do something like ..

    with open("xx0c.exx", "rb") as f:
        fdata = f.read()
        dfn.get(ctypes.byref(data),f fdata)
        dfn.print(ctypes.byref(data))
        print(data)
        print(data.i)                #7, correct
        #print(data.array.contents)  #segfault
        #print(data.array)           #segfault
        ptr = ctypes.pointer(data)
        print(ptr.contents.array[0]) #segfault

C function called from python will print .. which is correct

i = 7

array = 0x6ffff360034

array 0 = 20

array 1 = 27a40

array 2 = 127e20

array 3 = 128ae0

array 4 = 47e850

array 5 = 57ec30

array 6 = 580230

I can see the array in python isn't anywhere near the array in C.

Sample C code as asked. These are static, but they can be accessed through DATAFUNC. The C programs that use these are fine.

static bool getdata(struct Data *d, const char *filedata) {
    bool good = false;
    if (d != NULL && filedata != NULL) {
        d -> i       = (int32_t)filedata[0];
        d -> array   = (uint32_t*)(filedata + sizeof(int32_t));
        if ( d->i > 0 ) {
          good = true;
        }
    }
    return good;
}

static void printdata(struct Data *d) {
    if (d == NULL) {
        return;
    }
    printf("i = %i\n", d -> i);
    printf("array = %p\n", d -> array);
    unsigned int i;
    for (i = 0; i < d -> i; i++) {
        printf("array %i = %x\n", i, d -> array[i]);
    }
}

Can you please show your C code as well? It could be that the error is there. — Some programmer dude
– Some programmer dude, Commented Apr 24, 2018 at 13:31
I'm willing to, but as a warning it's working correctly for C programs that include it (it's also specific for a binary file format). — Keeways
– Keeways, Commented Apr 24, 2018 at 13:35
Are you reading the file inside a Python function? Because then the variable file will be local to that function, and its lifetime ends when the function end, making your pointer to the data invalid. — Some programmer dude
– Some programmer dude, Commented Apr 24, 2018 at 13:55
Yes it's read in the function. The portions posted here are technically together in the script, but segmented for formatting in the post & to show outputs for each line. — Keeways
– Keeways, Commented Apr 24, 2018 at 13:56
Then that's most likely your problem. Dynamically allocate memory to hold a copy of the data, and add functions to release this data when it's not needed anymore. — Some programmer dude
– Some programmer dude, Commented Apr 24, 2018 at 14:03

Keeways · Accepted Answer · 2018-04-25 02:02:44Z

1

I've resolved the issue by adding pack to the python struct like as follows.

class Data(ctypes.Structure):
    _pack_   = 1
    _fields_ = [
        ("i", ctypes.c_int),
        ("array",   ctypes.POINTER(ctypes.c_int))
    ]

This fixes everything and gives the output of .. once a loop was added to iterate through arrays.

number of offsets = 7
offsets = 0x6ffff360034
offset 0 = 20
offset 1 = 27a40
offset 2 = 127e20
offset 3 = 128ae0
offset 4 = 47e850
offset 5 = 57ec30
offset 6 = 580230
<__main__.Data object at 0x6ffffd5f400>
7
<__main__.LP_c_int object at 0x6ffffd5f488>
0x20
0x27a40
0x127e20
0x128ae0
0x47e850
0x57ec30
0x580230

answered Apr 25, 2018 at 2:02

Keeways

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