Скачать или смотреть How to Call a C+ + Function from Python and Get the Return Value

Learn how to seamlessly integrate `C+ + ` functions in your `Python` scripts by resolving common issues and optimizing your code for performance.
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Calling C+ + Functions from Python: A Practical Guide

Integrating C+ + libraries with Python can greatly enhance the performance of your applications, particularly if you need to leverage complex algorithms or existing C+ + code. However, errors can often arise, especially with function calls return types. In this post, we will walk through the steps to properly call a specific C+ + function from Python, examine the common pitfalls, and learn how to get the correct return values.

The Problem

You are trying to call a simple C+ + function from a Python script, but you encounter a frustrating AttributeError. Here is the C+ + function you wish to use:

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Your corresponding Python code attempts to access this function using the ctypes library:

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However, executing this code results in the following error:

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This indicates that the Python interpreter cannot find the getString function in the compiled C+ + shared library.

Understanding the Error

What Is Name Mangling?

C+ + compilers often apply name mangling to functions for supporting function overloading. This means that the function names are encoded to include information about their arguments and the namespace they belong to, which is why your getString function appears unreadable when accessed through Python.

Inspecting the Shared Library

You can use the readelf tool to inspect the contents of your .so file and confirm the mangled name of your function. The command would be:

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You should see output similar to:

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The mangled name _Z9getStringB5cxx11v indicates that the C+ + compiler has encoded information about the function, hinting why Python can't access it with the simple name getString.

Proposed Solutions

1. Mark the Function as extern

To make the function accessible, we can declare it with extern linkage. This makes it visible outside the current translation unit:

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2. Use extern "C"

However, to avoid name mangling altogether and allow Python to access the function using its original name, you can declare it as extern "C":

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3. Return a C-Style String Pointer

Additionally, when working with strings in Python, it’s more practical to return a C-style character pointer (char *). This approach allows for smoother memory management, as follows:

Updated C+ + Code:

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4. Update the Python Code

Lastly, modify your Python script to correctly interpret the returned string. You'll need to specify the return type of the C function in your Python code to c_char_p like this:

Updated Python Code:

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Conclusion

By following these guidelines, you should be able to successfully call your C+ + function from Python and obtain the desired return value without encountering any AttributeError. The most crucial aspects include understanding name mangling, properly using extern linkage, and managing memory between C+ + and Python effectively.

Don’t hesitate to modify these examples to suit your own functionality and integrate C+ + performance into your Python projects seamlessly!