Python 调用 C/C++ 函数

2019.11.04

Python Call C/C++ functions

Packages

import ctypes

Methods

Compile of C/C++ function to dynamic library

For C

gcc functionsName.c -fPIC -shared -o libName.so

For C++

g++ functionsName.cpp -fPIC -shared -o libName.so

Important: the dynamic library of C++ MUST be compiled as C style, the solution as below:

macro definition:

#define CEXT extern "C"
// then add the macro keywords before function name:
CEXT void helloWorld(){
...
}

Import

import ctypes

functionBranch = ctypes.cdll.LoadLibrary("./libName.so")

Declare the data type of return value and parameters

functionBranch.funcName.restype = ctypes.c_float

Optional Type:

ctypes type	C type	Python Type
c_char	char	1-character string
c_wchar	wchar_t	1-character unicode string
c_byte	char	int/long
c_ubyte	unsigned char	int/long
c_bool	bool	bool
c_short	short	int/long
c_ushort	unsigned short	int/long
c_int	int	int/long
c_uint	unsigned int	int/long
c_long	long	int/long
c_ulong	unsigned long	int/long
c_longlong	__int64 or longlong	int/long
c_ulonglong	unsigned __int64 or unsigned long long	int/long
c_float	float	float
c_double	double	float
c_longdouble	long double float	float
c_char_p	char *	string or None
c_wchar_p	wchar_t *	unicode or None
c_void_p	void *	int/long or None

For pointer type: add _p after the type name, such as c_int_p of int*.

Declaration of C-type arrays / Matrix as Argument

An effective way is to import numpy.ctypeslib as npct and use the ndpointer function to declare a pointer:

import ctypes
import numpy as np
import numpy.ctypeslib as npct

radiats = ctypes.cdll.LoadLibrary("./libradiats.so")
radiats.dc_radiat.argtypes = [ctypes.c_float, ctypes.c_float, ctypes.c_float, npct.ndpointer(dtype = ctypes.c_float, ndim = 2, flags="C_CONTIGUOUS")]

aa = np.zeros([3, 3], dtype = ctypes.c_float)
strike = 20
dip = 30
rake = 0
radiats.dc_radiat(strike,dip,rake,aa)
print(aa)

Also, the declaration of argument is shown above. The ndpointer function declared a 2-D pointer with the data type ctypes.c_float

To Be Noticed

The bit width of float for C/C++ programming language is 32, but for Python, not only the float but also np.float,it is 64. So REMEMBER to set the same data type when declaring the restype or argtypes, especially when the matrix transmission existed in the argments.

Python C/C++

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Dongsh