C++ function in python return number instead of string

Question

C++ that creates string:

#include <iostream>
#include <string>

using namespace std;

extern "C" {
    string getchars() {
        string output = "";
        char letters[4] = { 'A', 'G', 'T', 'C' };
        for (int i = 0; i < 10000000; i++) {
            output += letters[rand() % 4];
        }
        return output.c_str();
    }

}

Python code that should return string generated in C++:

from ctypes import cdll
lib = cdll.LoadLibrary('mylib.so')
print(lib.getchars())

And it returns different numbers like 18806352, What should I do to return normal string? In C++ This functions works well

string getchars should be char *getchars, since that's what c_str() returns. — Barmar
– Barmar, Commented Jun 12, 2020 at 7:39
The string returned by c_str() becomes invalid when the std::string is destroyed. Since output is destroyed when the function returns, you can't return output.c_str(). — Barmar
– Barmar, Commented Jun 12, 2020 at 7:41
I think you need allocate a dynamic C string and copy output.c_str() to it, then return that pointer. — Barmar
– Barmar, Commented Jun 12, 2020 at 7:43
@Barmar I have no idea how to do it, I'd be grateful if you write a bit of code how could I do it and why — WhoAmI
– WhoAmI, Commented Jun 12, 2020 at 7:45

afkid · Accepted Answer · 2020-06-12 12:52:52Z

2

Have a look at this discussion. This issue seems a lot similar with How to return char ** from C++ and fill it in a list in Python using ctypes?

It appears you need to specify the return type of the fuction in cdll. So you need to define the result type before calling the method.

I think this should work. Otherwise have a closer look to: https://docs.python.org/2/library/ctypes.html

[EDITED]

C++ implementation of file test.cpp (implementation of @Barmar):

#include <iostream>
#include <string>
#include <cstring>

using namespace std;

extern "C" char * getchars() {
    string output = "";
    char letters[4] = { 'A', 'G', 'T', 'C' };
    for (int i = 0; i < 10000000; i++) {
        output += letters[rand() % 4];
    }
    char *return_string = new char[output.length() + 1];
    strcpy(return_string, output.c_str());
    return return_string;
}

Compilation:

g++ -std=c++11 -Wall -Wextra -pedantic -c -fPIC test.cpp -o test.o
g++ -shared test.o -o test.so

Python file test.py:

from ctypes import cdll, c_char_p
lib = cdll.LoadLibrary('./test.so')
lib.getchars.restype = c_char_p
result = lib.getchars()
print(result)

Python version 3.7.4

This should work.

edited Jun 12, 2020 at 12:52

answered Jun 12, 2020 at 11:33

afkid

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9 Comments

WhoAmI Over a year ago

now my lib.getchars() (after print(...)) prints me <__main__.LP_c_char_p object at 0x00FD2760>, how can I get the value?

afkid Over a year ago

Try using lib.getchars().value

WhoAmI Over a year ago

lib.getchars().value and lib.getchars().values doesnt work

afkid Over a year ago

As this is a python object it has a type and a value.

afkid Over a year ago

Be aware that you might need to allocate the data via a malloc or something. Because otherwise even if you return .c_str() at the end, the string object is already destroyed. So you need to allocate this data to give it to python and free it in python.

|

Barmar · Accepted Answer · 2020-06-12 07:51:18Z

0

You need to return a C string, not a C++ std::string. You'll need to make a dynamic copy of the c_str() so it will have lifetime past the function that created it.

extern "C" {
    char * getchars() {
        string output = "";
        char letters[4] = { 'A', 'G', 'T', 'C' };
        for (int i = 0; i < 10000000; i++) {
            output += letters[rand() % 4];
        }
        char *return_string = new char[output.length() + 1];
        strcpy(return_string, output.c_str());
        return return_string;
    }
}

edited Jun 12, 2020 at 7:51

answered Jun 12, 2020 at 7:49

Barmar

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5 Comments

WhoAmI Over a year ago

'strdup' was not declared in this scope

Barmar Over a year ago

It's two lines of trivial code to emulate, I added it to the answer.

WhoAmI Over a year ago

did what you've wrote, included #include <string.h> #include <stdlib.h>, .so object created, but in python returns number anyway.. Did you tried it yourself? Maybe it will make some sence in the case it work for you or I missed sth

463035818_is_not_an_ai Over a year ago

@WhoAmI i suppose python still prints the value of the pointer instead of the string

Barmar Over a year ago

I assumed that ctypes would do the appropriate conversion to Python string.

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