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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。
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,
import platformfrom ctypes import *if platform.system() == 'Windows': libc = cdll.LoadLibrary('msvcrt.dll')elif platform.system() =='Linux': libc = cdll.LoadLibrary('libc.so.6') libc.printf('Hello ctypes!\n')
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,
import platformfrom 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')
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,
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,
import platformfrom 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) # 42libc.printf('%ld\n', 60000000) # 60000000#libc.printf('%f\n', 3.14) #>>> ctypes.ArgumentError#libc.printf('%f\n', c_float(3.14)) #>>> dont know why 0.000000libc.printf('%f\n', c_double(3.14)) # 3.140000
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,
>>> 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
from ctypes import *p = create_string_buffer(5) print sizeof(p) # 5print repr(p.raw) # '\x00\x00\x00\x00\x00'p.raw = 'Hi'print repr(p.raw) # 'Hi\x00\x00\x00'print repr(p.value) # 'Hi'
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,
import platformfrom 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 stringbottles = Bottles(42)libc.printf('%d bottles of beer\n', bottles)
输出,
42 bottles of beer
也可以为你的数据定义 _as_parameter 属性,如下,
import platformfrom 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 = 63libc.printf('%d bottles of beer\n', bottles)
输出,
63 bottles of beer
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,
import platformfrom 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)
输出,
String is "Hi", Int is 10, Double is 2.200000X, 2, 3.000000*** ERROR: argument 2:: wrong type
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 platformfrom 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_pprint '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 Nonelibc.strchr.argtypes = [c_char_p, c_char]print '4->', libc.strchr('abcdefghij', 'd') # Note, here not use c_char('w')
输出:
1-> 402913152-> defghij3-> None4-> defghij
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 platformfrom 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.valueprint 'f.val =', f.valueprint '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.valueprint 'after, f.val =', f.valueprint 'after, repr(s.value) =', repr(s.value)
输出,
i.val = 0f.val = 0.0repr(s.value) = ''after, i.val = 1after, f.val = 3.1400001049after, repr(s.value) = 'Hello'
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.yprint 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 = 10point.y = 20after, point.x = 0after, point.y = 5rc.upperleft.x = 0, rc.upperleft.y = 5rc.lowerright.x = 0, rc.lowerright.y = 0r.upperleft.x = 1, r.upperleft.y = 2r.lowerright.x = 3, r.lowerright.y = 4
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 = 1IB.y = 15-> IB2.x = 0-> IB2.y = 0
ctypes 提供了对 Array 的支持,
例如,
TenIntArrayType = c_int * 10ta = 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)
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.valuepi = pointer(i)pi[0] = 57print '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 = 42after, i.value = 57bool(null_ptr) = False
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 = 3bar.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] = 1bar.value[1] = 2bar.value[2] = 3*** ERROR: incompatible types, c_byte_Array_4 instance instead of LP_c_long instancenow, bar.value[0] = 0now, bar.value[1] = 0now, bar.value[2] = 0now, bar.value[3] = 0
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 * 5ia = 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
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 memoryprint '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 originalexcept 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) = 8len(short_array) = 4before resize, short_array is: 1 2 3 4ERROR: minimum size is 8after succeed resize to 32-byte, now sizeof(short_array) = 32after succeed resize to 32-byte, now len(short_array) = 4after reszie, short_array is: 1 2 3 4
class cell(Structure): passcell._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
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.xprint 'rc.a.y =', rc.a.yprint 'rc.b.x =', rc.b.xprint '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.xprint 'rc.a.y =', rc.a.yprint 'rc.b.x =', rc.b.xprint 'rc.b.y =', rc.b.yprint
输出,
rc.a.x = 1rc.a.y = 2rc.b.x = 3rc.b.y = 4after swap, bad result due to this is the pointer,rc.a.x = 3rc.a.y = 4rc.b.x = 3rc.b.y = 4
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// Windows need this compile direction for dll compilation, Linux no need #ifdef _MSC_VER #define DLL_EXPORT __declspec( dllexport ) #else #define DLL_EXPORT#endifDLL_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_rtvprint###################### 1D array get elem ###################### IntArray10Type = c_int * 10intArray = 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 * 3IntArray3Row3Col = IntArray3Col * 3arr2d = 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) = 2intArray[0] = 0intArray[1] = 1intArray[2] = 4intArray[3] = 9intArray[4] = 16intArray[5] = 25intArray[6] = 36intArray[7] = 49intArray[8] = 64intArray[9] = 81arr2d is:1 2 3 8 9 4 7 6 5
完。
https://www.cnblogs.com/gaowengang/p/7919219.html