Styjun

This question follows this one.

I use CFFI to create a DLL and I call it from a C++ application. I was questioning myself to find how to release memory allocated by the DLL and I follow the idea mention by @metal in its answer.

Here is now my Python code:

import cffi

ffibuilder = cffi.FFI()

ffibuilder.embedding_api('''
 char* get_string();
 void free_char(char*);
''')

ffibuilder.set_source('my_plugin', '')

ffibuilder.embedding_init_code('''
 from my_plugin import ffi, lib

 @ffi.def_extern()
 def get_string():
 val = "string"
 return lib.strdup(val.encode())

 @ffi.def_extern()
 def free_char(ptr):
 lib.free(ptr)
''')

ffibuilder.cdef('''
 char *strdup(const char *);
 void free(void *ptr);
''')

ffibuilder.compile(target='my-plugin.*', verbose=True)

And my C++ code:

#include <iostream>
#include <windows.h>

typedef char* (__stdcall *get_string_t)();
typedef void (__stdcall *free_char_t)(char*);

int main()

 HINSTANCE hGetProcIDDLL = LoadLibrary("my-plugin.dll");

 if (!hGetProcIDDLL) 
 std::cout << "could not load the dynamic library" << std::endl;
 return -1;
 

 get_string_t get_string = (get_string_t)GetProcAddress(hGetProcIDDLL, "get_string");
 if (!get_string) 
 std::cout << "could not locate the function" << std::endl;
 return -1;
 

 free_char_t free_char = (free_char_t)GetProcAddress(hGetProcIDDLL, "free_char");
 if (!free_char) 
 std::cout << "could not locate the function" << std::endl;
 return -1;
 

 for(int i = 0; i < 25000000; i++)
 
 char* val = NULL;
 val = get_string();

 if(i % 10000 == 0)
 
 std::cout << "Value " << i << " = " << val << std::endl;
 

 if(val)
 free_char(val);
 

 std::cout << "End" << std::endl;

 return 0;

I get this result:

Value 0 = string
Value 10000 = string
Value 20000 = string
Value 30000 = string
Value 40000 = string
Value 50000 = string
Value 60000 = string
Value 70000 = string
Value 80000 = string
Value 90000 = string
Value 100000 = string
Value 110000 = string
Value 120000 = string
Value 130000 = string
Value 140000 = string
Value 150000 = string
Value 160000 = string
Value 170000 = string
Value 180000 = string
Value 190000 = string
Value 200000 = string
Value 210000 = string
Value 220000 = string
Value 230000 = string
Value 240000 = string
Value 250000 = string
From cffi callback <function get_string at 0x03470810>:
MemoryError: Stack overflow
From cffi callback <function get_string at 0x03470810>:

From cffi callback <function get_string at 0x03470810>:
From cffi callback <function get_string at 0x03470810>:
From cffi callback <function get_string at 0x03470810>:

What does this error mean? I don't have memory problem because I release memory with my new free_char function. By the way, if I remove the call to free_char I can make all the loops (but the memory is not release).

From documentation of cffi :

The recommended C compiler compatible with Python 2.7 is this one:
http://www.microsoft.com/en-us/download/details.aspx?id=44266

...

For Python 3.4 and beyond:
https://www.visualstudio.com/en-us/downloads/visual-studio-2015-ctp-vs

Then you should either downgrade python or upgrade visual-studio.