使用ctypes进行python和c的混合编程

青旅半醒 2022-02-14 03:42 222阅读 0赞

参考：[https://www.cnblogs.com/gaowengang/p/7919219.html][https_www.cnblogs.com_gaowengang_p_7919219.html]

Python 和 C 的混合编程工具有很多，这里介绍 Python 标准库自带的 ctypes 模块的使用方法。

*  **初识**

Python 的 ctypes 要使用 C 函数，需要先将 C 编译成动态链接库的形式，即 Windows 下的 .dll 文件，或者 Linux 下的 .so 文件。先来看一下 ctypes 怎么使用 C 标准库。

Windows 系统下的 C 标准库动态链接文件为 msvcrt.dll (一般在目录 C:\\Windows\\System32 和 C:\\Windows\\SysWOW64 下分别对应 32-bit 和 64-bit，使用时不用刻意区分，Python 会选择合适的)

Linux 系统下的 C 标准库动态链接文件为 libc.so.6 (以 64-bit Ubuntu 系统为例， 在目录 /lib/x86\_64-linux-gnu 下)

例如，以下代码片段导入 C 标准库，并使用 printf 函数打印一条消息，

![复制代码][copycode.gif]

另外导入dll文件，还有其它方式如下，详细解释请参阅 ctypes module 相关文档，

![复制代码][copycode.gif]

import platform
    from ctypes import *
    
    if platform.system() == 'Windows':
        libc = cdll.LoadLibrary('msvcrt.dll')
        #libc = windll.LoadLibrary('msvcrt.dll')  # Windows only
        #libc = oledll.LoadLibrary('msvcrt.dll')  # Windows only
        #libc = pydll.LoadLibrary('msvcrt.dll')
      
        #libc = CDLL('msvcrt.dll')
        #libc = WinDLL('msvcrt.dll')  # Windows only
        #libc = OleDLL('msvcrt.dll')  # Windows only
        #libc = PyDLL('msvcrt.dll')
    elif platform.system() =='Linux':
        libc = cdll.LoadLibrary('libc.so.6')
        #libc = pydll.LoadLibrary('libc.so.6')
    
        #libc = CDLL('libc.so.6')
        #libc = PyDLL('libc.so.6')
        
    libc.printf('Hello ctypes!\n')

![复制代码][copycode.gif]

*  **ctypes 数据类型**

ctypes 作为 Python 和 C 联系的桥梁，它定义了专有的数据类型来衔接这两种编程语言。如下表，

![986259-20171129140831581-1373706197.png][]

注：Python 中的类型，除了 None，int， long， Byte String，Unicode String 作为 C 函数的参数默认提供转换外，其它类型都必须显式提供转换。

**None**：对应 C 中的 NULL

**int**, **long**： 对应 C 中的 int，具体实现时会根据机器字长自动适配。

**Byte String**：对应 C 中的一个字符串指针 char \* ，指向一块内存区域。

**Unicode String** ：对应 C 中一个宽字符串指针 wchar\_t \*，指向一块内存区域。

例如，

![复制代码][copycode.gif]

import platform
    from ctypes import *
    
    if platform.system() == 'Windows':
        libc = cdll.LoadLibrary('msvcrt.dll')
    elif platform.system() == 'Linux':
        libc = cdll.LoadLibrary('libc.so.6')
    
    libc.printf('%s\n', 'here!')        # here!
    libc.printf('%S\n', u'there!')      # there!
    libc.printf('%d\n', 42)             # 42
    libc.printf('%ld\n', 60000000)      # 60000000
    
    #libc.printf('%f\n', 3.14)          #>>> ctypes.ArgumentError
    #libc.printf('%f\n', c_float(3.14)) #>>> dont know why 0.000000
    libc.printf('%f\n', c_double(3.14)) # 3.140000

![复制代码][copycode.gif]

*  **创建可变的 string buffer**

Python 默认的 string 是不可变的，所以不能传递 string 到一个 C 函数去改变它的内容，所以需要使用 create\_string\_buffer，对应 Unicode 字符串，要使用 create\_unicode\_buffer,

定义和用法如下，

![复制代码][copycode.gif]

>>> help(create_string_buffer)
    Help on function create_string_buffer in module ctypes:
    
    create_string_buffer(init, size=None)
        create_string_buffer(aString) -> character array
        create_string_buffer(anInteger) -> character array
        create_string_buffer(aString, anInteger) -> character array

![复制代码][copycode.gif]

from ctypes import *
    
    p = create_string_buffer(5)  
    print sizeof(p)     # 5
    print repr(p.raw)   # '\x00\x00\x00\x00\x00'
    p.raw = 'Hi'
    print repr(p.raw)   # 'Hi\x00\x00\x00'
    print repr(p.value) # 'Hi'

![复制代码][copycode.gif]

*  **传递自定义参数类型到 C 函数**

ctypes 允许你创建自定义参数类型，它会自动去搜索自定义数据的 \_as\_parameter 属性，将其作为 C 函数的参数，例如，

![复制代码][copycode.gif]

import platform
    from ctypes import *
    
    if platform.system() == 'Windows':
        libc = cdll.LoadLibrary('msvcrt.dll')
    elif platform.system() == 'Linux':
        libc = cdll.LoadLibrary('libc.so.6')
        
    class Bottles(object):
        def __init__(self, number):
            self._as_parameter_ = number  # here only accept integer, string, unicode string
    bottles = Bottles(42)
    libc.printf('%d bottles of beer\n', bottles)

![复制代码][copycode.gif]

输出，

42 bottles of beer

也可以为你的数据定义 \_as\_parameter 属性，如下，

![复制代码][copycode.gif]

import platform
    from ctypes import *
    
    if platform.system() == 'Windows':
        libc = cdll.LoadLibrary('msvcrt.dll')
    elif platform.system() == 'Linux':
        libc = cdll.LoadLibrary('libc.so.6')
        
    class Bottles(object):
        def __init__(self):
            self._as_parameter_ = None  # only accept integer, string, unicode string
         
        @property
        def aspram(self):
            return self._as_parameter_
         
        @aspram.setter
        def aspram(self, number):
            self._as_parameter_ = number
             
    bottles = Bottles()
    bottles.aspram = 63
    libc.printf('%d bottles of beer\n', bottles)

![复制代码][copycode.gif]

输出，

63 bottles of beer

*  **指定 C 函数的参数类型**

可以指定要调用 C 函数的参数类型，如果传入参数不符合指定的类型，则 ctypes 会尝试转换，如果转换不成功，则抛 ArgumentError，例如，

![复制代码][copycode.gif]

import platform
    from ctypes import *
    
    if platform.system() == 'Windows':
        libc = cdll.LoadLibrary('msvcrt.dll')
    elif platform.system() == 'Linux':
        libc = cdll.LoadLibrary('libc.so.6')
        
    libc.printf.argtypes = [c_char_p, c_char_p, c_int, c_double]
        
    libc.printf('String is "%s", Int is %d, Double is %f\n', 'Hi', 10, 2.2)
    libc.printf('%s, %d, %f\n', 'X', 2, 3)
    try:
        libc.printf("%d %d %d", 1, 2, 3)
    except ArgumentError, e:
        print "*** ERROR: %s" % str(e)

![复制代码][copycode.gif]

输出，

String is "Hi", Int is 10, Double is 2.200000
    X, 2, 3.000000
    *** ERROR: argument 2: <type 'exceptions.TypeError'>: wrong type

*  **指定 C 函数的返回值类型**

如果不指定 C 函数的返回值， ctypes 默认返回 int 类型，如果要返回特定类型，需要指定返回类型 restype，

例如，

import platform
    from ctypes import *
    
    if platform.system() == 'Windows':
        libc = cdll.LoadLibrary('msvcrt.dll')
    elif platform.system() == 'Linux':
        libc = cdll.LoadLibrary('libc.so.6')
        
    print '1->', libc.strchr('abcdefghij', c_char('d'))
         
    libc.strchr.restype = c_char_p
    
    print '2->', libc.strchr('abcdefghij', c_char('d'))
    print '3->', libc.strchr('abcdefghij', 'd')  # Note, here C function strchr not know what 'd' mean, so rerurn None
    
    libc.strchr.argtypes = [c_char_p, c_char]
    print '4->', libc.strchr('abcdefghij', 'd')  # Note, here not use c_char('w')

输出：

1-> 40291315
    2-> defghij
    3-> None
    4-> defghij

*  **按引用传递参数**

有些情况下，需要 C 函数修改传入的参数，或者参数过大不适合传值，需要按引用传递，ctypes 提供关键字 byref() 处理这种情况，

例如，

import platform
    from ctypes import *
    
    if platform.system() == 'Windows':
        libc = cdll.LoadLibrary('msvcrt.dll')
    elif platform.system() == 'Linux':
        libc = cdll.LoadLibrary('libc.so.6')
        
    i = c_int()
    f = c_float()
    s = create_string_buffer('\000' * 32)
    print 'i.val =', i.value
    print 'f.val =', f.value
    print 'repr(s.value) =', repr(s.value)
    libc.sscanf('1 3.14 Hello', '%d %f %s', byref(i), byref(f), s)
    print 'after, i.val =', i.value
    print 'after, f.val =', f.value
    print 'after, repr(s.value) =', repr(s.value)

输出，

i.val = 0
    f.val = 0.0
    repr(s.value) = ''
    after, i.val = 1
    after, f.val = 3.1400001049
    after, repr(s.value) = 'Hello'

*  **使用结构体**

ctypes 支持结构体的使用，从 Structure 类派生，数据放在 \_fields\_ 中，

例如，

class Point(Structure):
        _fields_ = [('x', c_int), ('y', c_int)]
            
    point = Point(10, 20)
    print 'point.x =', point.x 
    print 'point.y =', point.y
        
    point = Point(y=5)
    print 'after, point.x =', point.x 
    print 'after, point.y =', point.y
    print
        
    class Rect(Structure):
        _fields_ = [('upperleft', Point), ('lowerright', Point)]
            
    rc = Rect(point)
    print 'rc.upperleft.x = %d, rc.upperleft.y = %d' % (rc.upperleft.x, rc.upperleft.y)
    print 'rc.lowerright.x = %d, rc.lowerright.y = %d' % (rc.lowerright.x, rc.lowerright.y)
            
    r = Rect(Point(1, 2), Point(3, 4))
    print 'r.upperleft.x = %d, r.upperleft.y = %d' % (r.upperleft.x, r.upperleft.y)
    print 'r.lowerright.x = %d, r.lowerright.y = %d' % (r.lowerright.x, r.lowerright.y)

输出，

point.x = 10
    point.y = 20
    after, point.x = 0
    after, point.y = 5
    
    rc.upperleft.x = 0, rc.upperleft.y = 5
    rc.lowerright.x = 0, rc.lowerright.y = 0
    r.upperleft.x = 1, r.upperleft.y = 2
    r.lowerright.x = 3, r.lowerright.y = 4

*  **位域**

ctypes 提供了对位域的支持，

例如，

class IntBit(Structure):
        _fields_ = [('x', c_uint, 2), ('y', c_uint, 4)]
        
    IB = IntBit(1, 15)
    print 'IB.x = %d' % IB.x 
    print 'IB.y = %d' % IB.y
        
    IB2 = IntBit(4, 16)
    print '-> IB2.x = %d' % IB2.x 
    print '-> IB2.y = %d' % IB2.y

输出，

IB.x = 1
    IB.y = 15
    -> IB2.x = 0
    -> IB2.y = 0

*  **数组**

ctypes 提供了对 Array 的支持，

例如，

TenIntArrayType = c_int * 10
    ta = TenIntArrayType(1, 2, 3, 4, 5, 6, 7, 8, 9, 10)
    for item in ta:
        print item,
    print
        
    class PointEx(Structure):
        _fields_ = [('x', c_int), ('y', c_int)]
            
    class MyStruct(Structure):
        _fields_ = [('a', c_int), ('b', c_int), ('pointex_array', PointEx * 4)]
            
    ms = MyStruct(4, 5, ((1,1), (2,2), (3,3), (4,4)))
    for item in ms.pointex_array:
        print '(item.x, item.y) = (%d, %d)' % (item.x, item.y) 
    print

输出，

1 2 3 4 5 6 7 8 9 10
    (item.x, item.y) = (1, 1)
    (item.x, item.y) = (2, 2)
    (item.x, item.y) = (3, 3)
    (item.x, item.y) = (4, 4)

*  **指针**

ctypes 使用关键字 pointer 提供了对指针的支持，注意指针解引用使用 \[0\]，

例如，

i = c_int(42)
    print 'before, i.value =', i.value
    pi = pointer(i)
    pi[0] = 57
    print 'after, i.value =', i.value
        
    # create NULL pointer, also can use this way, but recommend use 'pointer' not 'POINTER'
    null_ptr = POINTER(c_int)()
    print 'bool(null_ptr) =', bool(null_ptr)

输出，

before, i.value = 42
    after, i.value = 57
    bool(null_ptr) = False

*  **类型转换**

ctypes 提供了类型转换方法 cast(),

例如，

class Bar(Structure):
        _fields_ = [('count', c_int), ('value', POINTER(c_int))]
            
    bar = Bar()
    bar.count = 3
    bar.value = (c_int * 3)(1, 2, 3)
    for idx in range(bar.count):
        print 'bar.value[%d] = %d' % (idx, bar.value[idx])
        
    ## use cast to convert 
    try:
        bar.value = (c_byte * 4)()
    except TypeError, e:
        print '*** ERROR: %s' % str(e)
            
    bar.value = cast((c_byte * 4)(), POINTER(c_int))
    for idx in range(4):
        print 'now, bar.value[%d] = %d' % (idx, bar.value[idx])
    print

输出，

bar.value[0] = 1
    bar.value[1] = 2
    bar.value[2] = 3
    *** ERROR: incompatible types, c_byte_Array_4 instance instead of LP_c_long instance
    now, bar.value[0] = 0
    now, bar.value[1] = 0
    now, bar.value[2] = 0
    now, bar.value[3] = 0

*  **回调函数**

ctypes 通过 CFUNCTYPE 支持回调函数，

例如，

import platform 
    from ctypes import *
    
    if platform.system() == 'Windows':
        libc = cdll.LoadLibrary('msvcrt.dll')
    elif platform.system() == 'Linux':
        libc = cdll.LoadLibrary('libc.so.6')
        
    IntArray5 = c_int * 5
    ia = IntArray5(5, 1, 7, 33, 99)
        
    # CFUNCTYPE(restype, *argtypes, **kw)
    CmpFuncType = CFUNCTYPE(c_int, POINTER(c_int), POINTER(c_int))
        
    def py_cmp_func(a, b):
        if a[0] > b[0]:
            return 1
        elif a[0] < b[0]:
            return -1
        else:
            return 0
            
    cmpfunc = CmpFuncType(py_cmp_func)
        
    print 'before sort, the ia list is: ',
    for item in ia:
        print item,
            
    # void qsort (void* base, size_t num, size_t size, int (*compar)(const void*,const void*));
    libc.qsort(ia, len(ia), sizeof(c_int), cmpfunc)
        
    print '\nafter sort, the ia list is: ',
    for item in ia:
        print item,
    print

输出，

before sort, the ia list is:  5 1 7 33 99 
    after sort, the ia list is:  1 5 7 33 99

*  **Resize Space**

ctypes 提供了 resize 变量占用空间的方法 resize()，注意，只能增大，不能小于原始空间，

例如，

short_array = (c_short * 4)(1, 2, 3, 4)
    print 'sizeof(short_array) =', sizeof(short_array)  # get 8, means short_array take 8-byte memory
    print 'len(short_array) =', len(short_array)
    print 'before resize, short_array is: ',
    for idx in range(len(short_array)):
        print short_array[idx],
    print
        
    try:
        resize(short_array, 4)  # resize short_array to 4-byte, raise error, due to cannot resize smaller than original
    except ValueError, e:
        print 'ERROR: %s' % str(e)
            
    resize(short_array, 32)
    print 'after succeed resize to 32-byte, now sizeof(short_array) =', sizeof(short_array)
    print 'after succeed resize to 32-byte, now len(short_array) =', len(short_array)
    print 'after reszie, short_array is: ',
    for idx in range(len(short_array)):
        print short_array[idx],

输出，

sizeof(short_array) = 8
    len(short_array) = 4
    before resize, short_array is:  1 2 3 4
    ERROR: minimum size is 8
    after succeed resize to 32-byte, now sizeof(short_array) = 32
    after succeed resize to 32-byte, now len(short_array) = 4
    after reszie, short_array is:  1 2 3 4

*  **Other**

class cell(Structure):
        pass
    cell._fields_ = [('name', c_char_p), ('next', POINTER(cell))]
        
    c1 = cell()
    c2 = cell()
    c1.name = 'foo'
    c2.name = 'bar'
    c1.next = pointer(c2)
    c2.next = pointer(c1) 
        
    p = c1
    for i in range(10):
        print p.name,
        p = p.next[0]

输出，

foo bar foo bar foo bar foo bar foo bar

*  **坑**

ctypes 相对于其它工具，使用起来有点繁琐，而且有很多坑，需要小心谨慎，

例如，

class POINT(Structure):
        _fields_ = [('x', c_int), ('y', c_int)]
            
    class RECT(Structure):
        _fields_ = [('a', POINT), ('b', POINT)]
            
    p1 = POINT(1, 2)
    p2 = POINT(3, 4)
    rc = RECT(p1, p2)
        
    print 'rc.a.x =', rc.a.x
    print 'rc.a.y =', rc.a.y
    print 'rc.b.x =', rc.b.x
    print 'rc.b.y =', rc.b.y
        
    rc.a, rc.b = rc.b, rc.a
        
    print 'after swap, bad result due to this is the pointer,'
    print 'rc.a.x =', rc.a.x
    print 'rc.a.y =', rc.a.y
    print 'rc.b.x =', rc.b.x
    print 'rc.b.y =', rc.b.y
    print

输出，

rc.a.x = 1
    rc.a.y = 2
    rc.b.x = 3
    rc.b.y = 4
    after swap, bad result due to this is the pointer,
    rc.a.x = 3
    rc.a.y = 4
    rc.b.x = 3
    rc.b.y = 4

*  ** Load 自定义 DLL **

以 C 函数文件 needforspeed.c 为例，

//----------------------------------------------------------------------------
    // Purpose: this c module is used to speed up the Python program, should be 
    //          compiled into dll, and then load into Python module with ctypes
    //          method.
    //
    // Compile Methods:
    //
    //    ======================
    //    Windows: use MSVC, x64
    //    ======================
    //
    //    C:\Program Files (x86)\Microsoft Visual Studio 12.0\VC\bin\amd64> cl /LD needforspeed.c /o nfs.dll
    //
    //    ======
    //    Linux:
    //    ======
    //     
    //    $ gcc -fPIC -shared needforspeed.c -o nfs.so
    //----------------------------------------------------------------------------
    
    #include <stdio.h>
    
    // Windows need this compile direction for dll compilation, Linux no need 
    #ifdef _MSC_VER
        #define DLL_EXPORT __declspec( dllexport ) 
    #else
        #define DLL_EXPORT
    #endif
    
    DLL_EXPORT void hello_world(void) {
        printf("Hello world!\n");
    }
    
    DLL_EXPORT int mod(int m, int n) {
        return m % n;
    }
    
    DLL_EXPORT int get_array_elem(int arr[], int idx) {
        return arr[idx];
    }
    
    DLL_EXPORT int get_array2D_elem(int arr[][3], int row, int col) {
        return arr[row][col];
    }

在 Windows 下编译为 nfs.dll, 在 Linux 下编译为 nfs.so，Python 中调用如下，

import platform 
    from ctypes import *
    
    if platform.system() == 'Windows':
        mylib = cdll.LoadLibrary('./nfs.dll')
    elif platform.system() == 'Linux':
        mylib = cdll.LoadLibrary('./nfs.so')
     
    mylib.hello_world()
    print
     
    mod_rtv = mylib.mod(c_int(10), c_int(4))
    print 'mod_rtv(10 % 4) =', mod_rtv
    print
    
    #####################
    # 1D array get elem #
    #####################
     
    IntArray10Type = c_int * 10
    intArray = IntArray10Type()
    for idx in range(10):
        intArray[idx] = idx**2
     
    for idx in range(10):
        print 'intArray[%d] = %d' % (idx, mylib.get_array_elem(intArray, idx))
    print
    
    #####################
    # 2D array get elem #
    #####################
    
    IntArray3Col = c_int * 3
    IntArray3Row3Col = IntArray3Col * 3
    arr2d = IntArray3Row3Col(IntArray3Col(1, 2, 3), IntArray3Col(8, 9, 4), IntArray3Col(7, 6, 5))
    
    print 'arr2d is:'
    for r in range(3):
        for c in range(3):
            print '%d ' % mylib.get_array2D_elem(arr2d, r, c),
        print

输出，

Hello world!
    
    mod_rtv(10 % 4) = 2
    
    intArray[0] = 0
    intArray[1] = 1
    intArray[2] = 4
    intArray[3] = 9
    intArray[4] = 16
    intArray[5] = 25
    intArray[6] = 36
    intArray[7] = 49
    intArray[8] = 64
    intArray[9] = 81
    
    arr2d is:
    1  2  3 
    8  9  4 
    7  6  5

[https_www.cnblogs.com_gaowengang_p_7919219.html]: https://www.cnblogs.com/gaowengang/p/7919219.html
[copycode.gif]: /images/20220214/d6c954a4613c49629e380715bcfd0852.png
[986259-20171129140831581-1373706197.png]: /images/20220214/03213e24d3d04b00872fcdcd6d3fce99.png