C / C ++ from Python

I wrote about how to call Python from C in a previous article, now let's talk how to do the opposite and call C / C ++ from Python. Once I began to write about this, we will reveal the whole topic to the end. Moreover, there’s nothing complicated here either.

C

Everything is simple here, python can call C functions without any problems.

test.c:

#include "test.h" int a = 5; double b = 5.12345; char c = 'X'; int func_ret_int(int val) { printf("get func_ret_int: %d\n", val); return val; } double func_ret_double(double val) { printf("get func_ret_double: %f\n", val); return val; } char * func_ret_str(char *val) { printf("get func_ret_str: %s\n", val); return val; } char func_many_args(int val1, double val2, char val3, short val4) { printf("get func_many_args: int - %d, double - %f, char - %c, short - %d\n", val1, val2, val3, val4); return val3; }
      
      

        
        
        
      

    
        
        
        
      
      

        
        
        
      

    
     

test.h:

 #ifndef _TEST_H_ #define _TEST_H_ #ifdef __cplusplus extern "C" { #endif #include <stdio.h> #include <string.h> #include <unistd.h> int func_ret_int(int val); double func_ret_double(double val); char *func_ret_str(char *val); char func_many_args(int val1, double val2, char val3, short val4) #ifdef __cplusplus } #endif #endif /* _TEST_H_ */
      
      

        
        
        
      

    
        
        
        
      
      

        
        
        
      

    
     

How to compile:

 gcc -fPIC -shared -o libtest.so test.c
      
      

        
        
        
      

    
        
        
        
      
      

        
        
        
      

    
     

The source compiles into a dynamic library and is ready for battle.

We pass to python. The example shows how to pass arguments to a function, get the result of a function, and also how to get and change the values ​​of global variables.

main.py:

 #!/usr/bin/python3 #-*- coding: utf-8 -*- import ctypes #   test = ctypes.CDLL('./objs/libtest.so') ## #    ## # ,    int test.func_ret_int.restype = ctypes.c_int # ,     int test.func_ret_int.argtypes = [ctypes.c_int, ] # ,    double test.func_ret_double.restype = ctypes.c_double # ,     double test.func_ret_double.argtypes = [ctypes.c_double] # ,    char * test.func_ret_str.restype = ctypes.c_char_p # ,     char * test.func_ret_str.argtypes = [ctypes.POINTER(ctypes.c_char), ] # ,    char test.func_many_args.restype = ctypes.c_char # ,     int, double. char, short test.func_many_args.argtypes = [ctypes.c_int, ctypes.c_double, ctypes.c_char, ctypes.c_short] print('ret func_ret_int: ', test.func_ret_int(101)) print('ret func_ret_double: ', test.func_ret_double(12.123456789)) #      ,       . print('ret func_ret_str: ', test.func_ret_str('Hello!'.encode('utf-8')).decode("utf-8") ) print('ret func_many_args: ', test.func_many_args(15, 18.1617, 'X'.encode('utf-8'), 32000).decode("utf-8")) print() ## #    ## # ,    int a = ctypes.c_int.in_dll(test, "a") print('ret a: ', a.value) #   . a.value = 22 a = ctypes.c_int.in_dll(test, "a") print('ret a: ', a.value) # ,    double b = ctypes.c_double.in_dll(test, "b") print('ret b: ', b.value) # ,    char c = ctypes.c_char.in_dll(test, "c") print('ret c: ', c.value.decode("utf-8"))
      
      

        
        
        
      

    
        
        
        
      
      

        
        
        
      

    
     

All possible data types and their designations can be found in the python documentation .

Work with structures

C - structure declaration in test.h:

 typedef struct test_st_s test_st_t; struct test_st_s { int val1; double val2; char val3; };
      
      

        
        
        
      

    
        
        
        
      
      

        
        
        
      

    
     

Function for working with our structure:

 test_st_t * func_ret_struct(test_st_t *test_st) { if (test_st) { printf("C get test_st: val1 - %d, val2 - %f, val3 - %c\n", test_st->val1, test_st->val2, test_st->val3); } return test_st; }
      
      

        
        
        
      

    
        
        
        
      
      

        
        
        
      

    
     

Python:

 import sys import struct #    Python   C class test_st_t(ctypes.Structure): _fields_ = [('val1', ctypes.c_int), ('val2', ctypes.c_double), ('val3', ctypes.c_char)] # ,    test_st_t * test.func_ret_struct.restype = ctypes.POINTER(test_st_t) # ,     void * test.func_ret_struct.argtypes = [ctypes.c_void_p] #   test_st = test_st_t(19, 3.5, 'Z'.encode('utf-8')) # Python None == Null C ret = test.func_ret_struct(None) print('ret func_ret_struct: ', ret) #   None,      ret = test.func_ret_struct(ctypes.byref(test_st)) #    C print('ret val1 = {}\nret val2 = {}\nret val3 = {}'.format(ret.contents.val1, ret.contents.val2, ret.contents.val3.decode("utf-8")))
      
      

        
        
        
      

    
        
        
        
      
      

        
        
        
      

    
     

C ++

It’s a bit more complicated here because ctypes can only work with C functions. This is not a problem for us, just C will bind the C ++ code.

C ++ class methods and C bindings:

 #include "test.hpp" /* *   */ std::string test::ret_str(std::string val) { std::cout << "get ret_str: " << val << std::endl; return val; } int test::ret_int(int val) { std::cout << "get ret_int: " << val << std::endl; return val; } double test::ret_double(double val) { std::cout << "get ret_double: " << val << std::endl; return val; } /* *  C    C++ */ //   test,     . test *test_new() { return new test(); } //   test. void test_del(test *test) { delete test; } /* *   . */ //    ret_str char *test_ret_str(test *test, char *val) { // char *  std::string std::string str = test->ret_str(std::string(val)); // std::string  char * char *ret = new char[str.length() + 1]; strcpy(ret, str.c_str()); return ret; } //    ret_int int test_ret_int(test *test, int val) { return test->ret_int(val); } //    ret_double double test_ret_double(test *test, double val) { return test->ret_double(val); } /* *   . */ //    a int test_get_a(test *test) { return test->a; } //    b double test_get_b(test *test) { return test->b; } //    c char test_get_c(test *test) { return test->c; }
      
      

        
        
        
      

    
        
        
        
      
      

        
        
        
      

    
     

But there is one caveat, the binding must be declared as extern C. So that the ++ compiler does not overload the binding function names. If he does, then we will not be able to work with our functions through ctypes.

test.hpp:

 #include <iostream> #include <string.h> class test { public: int a = 5; double b = 5.12345; char c = 'X'; std::string ret_str(std::string val); int ret_int(int val); double ret_double(double val); }; #ifdef __cplusplus extern "C" { #endif test *test_new(); void test_del(test *test); char *test_ret_str(test *test, char *val); int test_ret_int(test *test, int val); double test_ret_double(test *test, double val); int test_get_a(test *test); double test_get_b(test *test); char test_get_c(test *test); #ifdef __cplusplus } #endif
      
      

        
        
        
      

    
        
        
        
      
      

        
        
        
      

    
     

How to compile:

g ++ -fPIC -shared -o libtestpp.so test.cpp

With python, everything is just as simple.

 #   testpp = ctypes.CDLL('./objs/libtestpp.so') # ,     testpp.test_new.restype = ctypes.c_void_p #   test test = testpp.test_new() ## #    ## # ,    char * testpp.test_ret_str.restype = ctypes.c_char_p # ,     void *  char * testpp.test_ret_str.argtypes = [ctypes.c_void_p, ctypes.c_char_p] # ,    int testpp.test_ret_int.restype = ctypes.c_int # ,     void *  int testpp.test_ret_int.argtypes = [ctypes.c_void_p, ctypes.c_int] # ,    double testpp.test_ret_double.restype = ctypes.c_double # ,     void *  double testpp.test_ret_double.argtypes = [ctypes.c_void_p, ctypes.c_double] print('  :') #   1-       print('ret test_ret_str: ', testpp.test_ret_str(test, 'Hello!'.encode('utf-8')).decode("utf-8")) print('ret test_ret_int: ', testpp.test_ret_int(test, 123)) print('ret test_ret_double: ', testpp.test_ret_double(test, 9.87654321)) ## #    ## # ,    int testpp.test_get_a.restype = ctypes.c_int # ,    double testpp.test_get_b.restype = ctypes.c_double # ,    char testpp.test_get_c.restype = ctypes.c_char print('\n  :') print('ret test_get_a: ', testpp.test_get_a(test)) print('ret test_get_b: ', testpp.test_get_b(test)) print('ret test_get_c: ', testpp.test_get_c(test).decode("utf-8")) #   testpp.test_del(test)
      
      

        
        
        
      

    
        
        
        
      
      

        
        
        
      

    
     

The code tried to comment clearly, to write less here)

I hope it will be useful.

Thanks

DollaR84 for his help.

References

Source codes for examples

Previous Python article from C