tinycwrap

TinyCWrap is a lightweight helper around CFFI that:

• compiles one or more C sources into a shared library,
• auto-generates the cdef from your function/struct declarations,
• builds NumPy-friendly Python wrappers that take/return arrays and structs,
• optionally hot-reloads when the C file changes (inside IPython).

Quick start

pip install tinycwrap

Write a small C file (only non-static functions are exported):

/* kernels.c */
double dot(const double *x, const double *y, int len_x)
/* Return dot product between x and y */
{
    double acc = 0.0;
    for (int i = 0; i < len_x; ++i)
        acc += x[i] * y[i];
    return acc;
}

Wrap and call it from Python:

import numpy as np
from tinycwrap import CModule

cm = CModule("kernels.c")  # builds the shared library, creates wrappers

x = np.arange(5, dtype=np.float64)
y = np.ones_like(x)

cm.dot(x, y)           # -> 10.0, len_x auto-filled by convention
print(cm.dot.__doc__)  # docstring comes from the C comment

See examples/ and tests/ for more involved cases.

C coding conventions

TinyCWrap infers how to build wrappers from simple C conventions:

• Inputs vs outputs
  • const T *arg -> input NumPy array.
  • T *arg -> output/in-place array (prefer naming it out_* when possible).
  • T arg[N] in the signature -> fixed-size array (input if const, otherwise output).
  • Plain scalars (double, int, ...) -> Python scalars.
• Length parameters: integers named like len_x, n_x, or size_x are auto-filled from array x by convention. Otherwise pass them explicitly from Python or declare a contract (see below).
• Docstrings: the block comment immediately after the function header becomes the Python docstring.
• Structs: typedef struct { ... } Name; definitions in your headers/sources become Python classes with a .dtype and NumPy-backed storage.
• Compilation: extra sources can be passed (CModule("main.c", "helper.c")), and extra include dirs via include_dirs=[...]. Default compiler flags are -O3 -shared -fPIC -march=native -mtune=native plus the NumPy include path.

Contracts: tell the wrapper how to size things

Contracts are declared inside the doc comment with Contract: (or Contracts:). Separate multiple rules with semicolons. Supported forms:

• len_x=len(x) — mark len_x as the length of array x. If you omit len_x in Python, the wrapper fills it.
• shape(out)=n,m — allocate out with shape (n, m). You can define n,m=shape(a) to capture the shape of an input first.
• len(out)=len_x — allocate a 1D output array.
• postlen(out)=out_len — slice the output after the call using an integer pointer result out_len (useful when the C code writes less than the allocated length).
• own(return) — the returned pointer is owned by the caller; the wrapper will free it after copying to NumPy (requires a len(return)=... contract).
• expose(len_x) — opt out of naming-convention inference for len_x, keeping it as a required Python argument.

TinyCWrap also infers low-risk contracts from names:

• len_x, n_x, or size_x are treated as len(x) when x is an array argument.
• out_x is allocated with shape(x) when input array x exists.
• out_x is allocated with len_x, n_x, or size_x when no input x exists but such a length argument does.

Explicit contracts override inferred contracts.

Examples pulled from the test suite:

void mat_add(const double *a, const double *b, int n, int m, double *out)
/* Elementwise addition
   Contract: n,m=shape(a); shape(out)=n,m;
*/

void merge_sorted(const double *a, const double *b, int len_a, int len_b,
                  double *out, int *out_len)
/* Merge unique sorted values
   Contract: len_a=len(a); len_b=len(b);
             len(out)=len_a+len_b; postlen(out)=out_len;
*/

double *alloc_random_array(int *out_len)
/* Return a freshly malloc'ed array
   Contract: len(return)=out_len; own(return);
*/

Rules are parsed case-insensitively; whitespace does not matter.

Python wrapper behavior

• Automatic allocation: any out_* argument defaults to None in Python; TinyCWrap allocates it based on contracts, naming conventions, fixed array sizes, or by matching the shape of a related input (out_x matches x when no contract is present).
• Struct pointer outputs: non-const struct pointers are treated as in/out; when you pass an object/array explicitly, it is mutated in place and not returned. When you pass None, TinyCWrap allocates and returns the struct (or struct array).
• Optional length arguments: integer length parameters inferred from contracts or naming conventions default to None in the wrapper signature. If you pass them, they are cast to int; if not, the inferred expression is evaluated.
• Post contracts: when a contract uses postlen(...) or post shape(...), the wrapper slices/reshapes outputs after the C call using the values written by the C function.
• Scalar pointer outputs: pointers to integer-like types (e.g., int *out_len) are returned as plain Python integers alongside other outputs.
• Structs: for typedef struct declarations TinyCWrap generates a Python class:
  • fields are accessible as properties backed by a NumPy structured dtype (Name.dtype);
  • scalar struct outputs return Name objects;
  • struct-array outputs return compact NameArray wrappers, with field arrays as properties (points.x), scalar items as Name objects (points[0]), and Python construction as NameArray(array_like) or NameArray(x=..., y=...);
  • pointer fields paired with len_<field> automatically allocate NumPy arrays when you instantiate the struct with len_field or when you pass an array for that field;
  • _data holds the underlying structured array, Name.zeros(n) returns a raw array of that dtype, and NameArray.zeros(n) returns the typed wrapper.
• Reloading: inside IPython, pass reload=True (default) to auto-recompile before each cell if the C sources changed.

Worked examples

Two outputs and automatic lengths

void split_vectors(const double *inp, int len_inp,
                   double *out_even, double *out_odd)
/* Split even/odd elements
   Contract: len_inp=len(inp); len(out_even)=len_inp/2; len(out_odd)=len_inp/2;
*/

data = np.array([0, 1, 2, 3, 4, 5], dtype=np.float64)
out_even, out_odd = cm.split_vectors(data)  # lengths inferred, arrays allocated

Structs and struct arrays

typedef struct {
    double real;
    double imag;
} ComplexPair;

double complex_magnitude(const ComplexPair *z);

cp = cm.ComplexPair(real=3.0, imag=4.0)
cm.complex_magnitude(cp)  # -> 5.0

# struct arrays are NumPy dtypes; useful when C expects pointers to arrays of structs
pairs = cm.ComplexPair.zeros(2)
pairs["real"] = [1.0, 2.0]
pairs["imag"] = [0.0, -1.0]

Owned return value

double *alloc_random_array(int *out_len)
/* Contract: len(return)=out_len; own(return); */

arr, n = cm.alloc_random_array()  # returns NumPy array, frees the C buffer

Tips

• Keep function declarations (or headers) visible at the top level; TinyCWrap scans the C files and headers you pass.
• If a length cannot be inferred from a naming convention or contract, you must pass it explicitly from Python.
• Use cm.<func>.__source__ to inspect the wrapper code if something behaves unexpectedly.
• For debugging parsed signatures/contracts call cm._debug_specs().

That is all you need to start turning small C helpers into ergonomic Python callables.