How to pass char pointer to C++ API from python?

Question

I am trying to call the following C++ method from my python code:

TESS_API TessResultRenderer* TESS_CALL TessTextRendererCreate(const char* outputbase)
{
    return new TessTextRenderer(outputbase);
}

I'm having difficulty with how to pass the pointer to the method:

Is following the right way?

textRenderer = self.tesseract.TessTextRendererCreate(ctypes.c_char)

or should I be doing:

outputbase = ctypes.c_char * 512
textRenderer = self.tesseract.TessTextRendererCreate(ctypes.pointer(outputbase))

Doing above gives me error:

TypeError: _type_ must have storage info

Binary Birch Tree · Accepted Answer · 2016-04-27 01:33:27Z

4

You should be passing in a string.

For example:

self.tesseract.TessTextRendererCreate('/path/to/output/file/without/extension')

Here's a generalized example with a mock API. In lib.cc:

#include <iostream>

extern "C" {
  const char * foo (const char * input) {
    std::cout <<
      "The function 'foo' was called with the following "
      "input argument: '" << input << "'" << std::endl;

    return input;
  }
}

Compile the shared library using:

clang++ -fPIC -shared lib.cc -o lib.so

Then, in Python:

>>> from ctypes import cdll, c_char_p
>>> lib = cdll.LoadLibrary('./lib.so')
>>> lib.foo.restype = c_char_p
>>> result = lib.foo('Hello world!')
The function 'foo' was called with the following input argument: 'Hello world!'
>>> result
'Hello world!'

edited Apr 27, 2016 at 1:33

answered Apr 26, 2016 at 13:16

Binary Birch Tree

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2 Comments

Anthony Over a year ago

Passing in a string worked for me! however, the calls to different methods is failing. I asked a related question and was wondering if you can help stackoverflow.com/questions/36871072/…

Eryk Sun Over a year ago

Note that a string should only be directly passed for const char *. If it's not const, use ctypes.create_string_buffer('initial value, or use an integer size'). Python strings are semantically immutable, and CPython depends on this to allow interning various strings in code objects and attribute names. If an interned string gets mutated, it's not just a matter of data corruption but also breaks attribute access and dict lookup if the string is a dict key.

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