Cython – Export my lib to a .so file and use in a C program: Symbols not found?

Have you ever tried to export your Cython library to a shared object file (.so) and use it in a C program, only to be greeted with the frustrating error message “symbols not found”? Don’t worry, you’re not alone! In this article, we’ll delve into the world of Cython and C, and provide a step-by-step guide on how to export your Cython library and use it seamlessly in a C program, sans the “symbols not found” error.

What is Cython?

Cython is a superset of the Python programming language that allows you to write Python code that can be compiled to C code, making it an attractive option for performance-critical applications. It’s often used to create bindings for C libraries, wrap existing C code, or write performance-critical parts of a Python application in a language that’s more efficient than Python.

Why export to a .so file?

Exporting your Cython library to a shared object file (.so) allows you to use it in C programs, making it a crucial step in integrating your Cython code with C code. Shared object files are essentially libraries that can be dynamically linked to a program, making it possible to use your Cython code in a C program without having to recompile the entire program.

The Problem: Symbols not found

When you try to use your Cython library in a C program, you might encounter the “symbols not found” error. This error occurs when the linker is unable to find the symbols (functions or variables) defined in your Cython library. There are several reasons why this error might occur, including:

• Incorrect compilation flags
• Missing or incorrect export statements
• Incompatible data types
• Invalid or missing header files

Step-by-Step Guide to Exporting your Cython Library to a .so file

Let’s create a simple Cython library, compile it to a .so file, and use it in a C program. We’ll cover the following steps:

1. cythonize your Cython code
2. Compile the Cython code to C code
3. Compile the C code to a .so file
4. Write a C program to use the .so file
5. Compile the C program and link it with the .so file

Step 1: Cythonize your Cython code

Create a new file called mylib.pyx with the following Cython code:

cpdef int add(int a, int b):
    return a + b

This code defines a simple function add that takes two integers and returns their sum.

Step 2: Compile the Cython code to C code

Use the cython command to compile the Cython code to C code:

cython -3 mylib.pyx

This will generate a file called mylib.c.

Step 3: Compile the C code to a .so file

Use the following command to compile the C code to a .so file:

gcc -shared -fPIC -o mylib.so mylib.c

This will generate a file called mylib.so.

Step 4: Write a C program to use the .so file

Create a new file called main.c with the following C code:

#include <stdio.h>
#include "mylib.h"

int main() {
    int result = add(2, 3);
    printf("Result: %d\n", result);
    return 0;
}

This code includes the header file mylib.h (which we’ll create next), calls the add function from the Cython library, and prints the result.

Step 5: Create a header file for the Cython library

Create a new file called mylib.h with the following contents:

int add(int a, int b);*/
;

This header file declares the add function, which is defined in the Cython library.

Step 6: Compile the C program and link it with the .so file

Use the following command to compile the C program and link it with the .so file:

gcc -o main main.c -L. -lmylib

This will generate an executable file called main.

Troubleshooting Common Issues

If you’re still encountering issues, here are some common troubleshooting tips:

Incompatible Data Types

Make sure that the data types used in your Cython code and C code are compatible. In our example, we used in both the Cython code and C code.

Missing or Incorrect Export Statements

Verify that you’ve included the correct export statements in your Cython code. In our example, we used the cpdef statement to define the add function.

Invalid or Missing Header Files

Ensure that you’ve created a valid header file (mylib.h) that declares the functions or variables defined in your Cython library.

Conclusion

Exporting your Cython library to a .so file and using it in a C program can be a bit tricky, but with the right steps and troubleshooting tips, you can overcome the “symbols not found” error and seamlessly integrate your Cython code with C code. Remember to:

• Cythonize your Cython code
• Compile the Cython code to C code
• Compile the C code to a .so file
• Write a C program to use the .so file
• Compile the C program and link it with the .so file
• Troubleshoot common issues, such as incompatible data types, missing or incorrect export statements, and invalid or missing header files

By following these steps and tips, you’ll be well on your way to harnessing the power of Cython and C to create high-performance applications.

Keyword	Description
Cython	A superset of the Python programming language that allows you to write Python code that can be compiled to C code.
.so file	A shared object file that can be dynamically linked to a program, making it possible to use your Cython code in a C program without having to recompile the entire program.
Symbols not found error	An error that occurs when the linker is unable to find the symbols (functions or variables) defined in your Cython library.

Now, go forth and conquer the world of Cython and C! If you have any questions or need further assistance, feel free to ask in the comments below.

Frequently Asked Question

Cython to the rescue! But, wait, what’s this? Exporting your Cython library to a .so file and using it in a C program, only to encounter those pesky “symbols not found” errors? Fear not, dear developer, for we’ve got the solutions to your most pressing questions!