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Inline C

Effortlessly write inline C functions in Python source code

# coding: inlinec
from inlinec import inlinec

def Q_rsqrt(number):
    float Q_rsqrt( float number )
        long i;
        float x2, y;
        const float threehalfs = 1.5F;

        x2 = number * 0.5F;
        y  = number;
        i  = * ( long * ) &y;                       // evil floating point bit level hacking
        i  = 0x5f3759df - ( i >> 1 );               // what the fuck? 
        y  = * ( float * ) &i;
        y  = y * ( threehalfs - ( x2 * y * y ) );   // 1st iteration

        return y;


Inlinec supports gnu-specific c extensions, so you're likely to have reasonable success #includeing glibc headers.

Inspired by Pyxl

How does this work?

Python has a mechanism for creating custom codecs, which given an input token stream, produce an output token stream. Inlinec consumes the entire token stream, runs a fault-tolerant parser on it (parso), finds which function nodes are annotated with an @inlinec decorator, creates a ctypes wrapper for the content of the function, and replaces the function body with a call to the ctypes wrapper. The import for the wrapper is lifted to the top of the file. Once this transformation has been made, the source code is re-tokenized and the Python interpreter only sees the transformed source. So a function like this:

def test():
    void test() {
        printf("Hello, world");

Gets turned into:

from test_8281231239129310 import lib as test_8281231239129310_lib, ffii as test_8281231239129310_ffi

def test():
    return test_8281231239129310_lib.test()

In theory, this allows inline c functions to be called with a one-time compilation overhead and the same performance characteristics as ctypes -- the underlying FFI library.


Note: This is just a proof of concept

  • Passing pointers to C functions does not currently work (aside from strings)
  • Shells out to gcc -E to preprocess the source code
  • Compilation is not cached and takes a long time every run
  • Compilation pollutes the current directory with .so, .o, .c files
  • The source file is parsed multiple times unnecessarily
  • Many more


Inlinec requires a C compiler to be installed on the system (tested with GCC and Clang), as well as the python development libraries to be installed (python3-dev). To play around with it in a container you can use the provided Dockerfile, just run docker build, exec into a shell in the container, and you have a working installation of inlinec.

> docker build -t inlinec . && docker run -it inlinec bash