我试图在项目中使用ctypes模块.我正在创建一个动态分配的“ max_entries”对数组,一旦该数组用完,我将创建一个新的大小为(1.5 * max_entries)的数组,并将内容从旧数组复制到新数组.

不幸的是,当我尝试访问此new_array的内容时,出现了“空指针访问”异常.相应的C代码似乎可以正常工作. (请参见下面的代码.)

我想知道我是否缺少有关ctypes模块工作方式的信息.任何帮助将不胜感激. (不确定这是否是适合我的问题的邮件列表.)

/谢谢！

#!/usr/bin/env python

from ctypes import *

import math

import random

class PAIR(Structure):

_fields_ = [("a", c_long),

("b", c_long)]

class MY_ARR(Structure):

_fields_ = [("no_entries", c_longlong),

("max_entries", c_longlong),

("entries", POINTER(POINTER(PAIR)))

]

def extendArray(x):

print "Extending Array"

print "Before: %d/%d" % (x.no_entries, x.max_entries)

old_arr = x.entries

# Create a new array

new_max_entries = int(math.ceil(1.5 * x.max_entries))

x.entries = (POINTER(PAIR) * new_max_entries)()

# Copy the entries from the old array to the new array

for i in range(x.no_entries):

x.entries[i] = old_arr[i]

x.max_entries = new_max_entries

print "After: %d/%d" % (x.no_entries, x.max_entries)

return x

def printPair(x):

print x.contents.a, x.contents.b

def printArray(x):

print "Printing %d/%d Entries" % (x.no_entries, x.max_entries)

for i in range(x.no_entries):

printPair(x.entries[i])

if __name__ == "__main__":

x = MY_ARR(0, 10, (POINTER(PAIR) * 10)())

for i in range(100):

if x.no_entries == x.max_entries:

print "

Printing Before Extension"

printArray(x)

extendArray(x)

print "

Printing After Extension"

printArray(x)

my_pair = PAIR(i, random.randint(0, 100))

x.entries[x.no_entries] = pointer(my_pair)

x.no_entries += 1

printPair(x.entries[i])

printArray(x)

现在,不幸的是,当我尝试运行此代码时,出现“空指针访问”异常：

$python TestExtension.py

0 40

1 40

2 11

3 36

4 82

5 73

6 93

7 100

8 75

9 80

Printing Before Extension

Printing 10/10 Entries

0 40

1 40

2 11

3 36

4 82

5 73

6 93

7 100

8 75

9 80

Extending Array

Before: 10/10

After: 10/15

Printing After Extension

Printing 10/15 Entries

Traceback (most recent call last):

File "TestExtension.py", line 55, in

printArray(x)

File "TestExtension.py", line 42, in printArray

printPair(x.entries[i])

File "TestExtension.py", line 37, in printPair

print x.contents.a, x.contents.b

ValueError: NULL pointer access

相应的C代码可以完美地工作：

#include

#include

#include

typedef struct {

long a;

long b;

} pair;

typedef struct {

long long no_entries;

long long max_entries;

pair **entries;

} my_arr;

my_arr *extend_array(my_arr *x) {

int i;

pair **old_entries = x->entries;

long long new_max_entries = ceil(1.5 * x->max_entries);

printf("Extending Array

");

printf("Before: %lld/%lld

", x->no_entries, x->max_entries);

x->entries = malloc(sizeof(pair *) * new_max_entries);

for (i = 0; i < 100; ++i) {

x->entries[i] = old_entries[i];

}

x->max_entries = new_max_entries;

free(old_entries);

printf("After: %lld/%lld

", x->no_entries, x->max_entries);

return x;

}

void print_pair(pair *p) {

printf("%ld%ld

", p->a, p->b);

}

void print_array(my_arr *x) {

int i;

printf("Printing %lld/%lld entries

", x->no_entries, x->max_entries);

for (i = 0; i < x->no_entries; ++i) {

print_pair(x->entries[i]);

}

}

int main(int argc, char *argv[])

{

int i;

my_arr x = {

0,

10,

malloc(sizeof(pair *) * 10)

};

for (i = 0; i < 100; ++i) {

if (x.no_entries == x.max_entries) {

extend_array(&x);

}

pair *my_pair = malloc(sizeof(pair));

my_pair->a = i;

my_pair->b = rand() % 100;

x.entries[x.no_entries++] = my_pair;

print_pair(x.entries[i]);

}

print_array(&x);

return 0;

}