CFFI Python C Interface Guide

CFFI Python C Interface Guide | Generated by AI

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What is CFFI?

CFFI (C Foreign Function Interface) is a Python library for calling C functions from Python code. It’s compatible with both CPython and PyPy, providing a safer and more portable alternative to libraries like ctypes. It supports two main modes: ABI (calling existing shared libraries) and API (compiling C code inline).

Installation

Install CFFI using pip:

pip install cffi

CFFI requires a C compiler (e.g., GCC on Linux, Visual Studio on Windows) to build modules.

Basic Usage Example

Here’s a step-by-step guide to a simple use case: calling a C function that adds two integers using API mode (recommended for new code).

Import and Set Up FFI:

from cffi import FFI
ffibuilder = FFI()

Define C Declarations: Specify the C function signatures in a string:
```
ffibuilder.cdef("""
    int add(int a, int b);
""")
```

Provide C Source Code: Include the C implementation:

ffibuilder.set_source("_example",
    """
    int add(int a, int b) {
        return a + b;
    }
    """)

Compile the Module: Run this script once to build the C extension:
```
if __name__ == "__main__":
    ffibuilder.compile(verbose=True)
```
This generates a compiled module (e.g., _example.cpython-39-x86_64-linux-gnu.so).

Use the Compiled Module: In your Python code, import and call the function:

from _example import lib
result = lib.add(5, 3)
print(result)  # Output: 8

Key Concepts

FFI Object: The main interface, created with FFI(). Use cdef() for declarations and set_source() for code.
Declarations: Tell Python about C types, structs, functions, etc. Strings must match C syntax exactly.
Type Conversion: CFFI handles basic types (int, float, pointers) automatically. Use arrays, structs, or callbacks for complexity.
Error Handling: Exceptions like CDefError occur for invalid C definitions. C runtime errors (e.g., via errno) can be checked with ffi.errno.
Memory Management: Use ffi.new() for C structs/arrays, and ensure proper deallocation to avoid leaks.

Modes: ABI vs. API

ABI Mode (for existing libraries): Load a shared library (e.g., ffi.dlopen("mylib.so")) and call functions directly. Example:
```
from cffi import FFI
ffi = FFI()
ffi.cdef("int add(int a, int b);")
lib = ffi.dlopen("/path/to/libmylib.so")
result = lib.add(5, 3)
```
This is faster for prebuilt libs but less portable—you need the exact binary.
API Mode (recommended): Compile C code inline, as in the example above. It’s safer, optimizes better, and works across platforms.

Advanced Features

Structs and Unions: Define complex types, e.g., cdef("struct Point { int x, y; }").
Arrays and Pointers: Use ffi.new("int[10]") for arrays or ffi.cast("int*", ptr) for casts.
Callbacks: Pass Python functions to C using ffi.callback().
Integration with Packages: Many libraries (e.g., cryptography, NumPy) use CFFI for performance.
Performance Tips: Compilation takes time; use inplace builds or cffi packages for distribution. Avoid heavy Python-C transitions in loops.

For full documentation, see the official CFFI docs. This covers the essentials—experiment with the examples to adapt to your needs. If you have a specific use case, provide more details for tailored advice.

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