CNet: Implementing a Deep Learning Framework from Scratch in C

CNet is a deep learning framework built from scratch using pure C with no external dependencies, designed to help developers understand the underlying principles of deep learning (such as tensor operations, automatic differentiation, computation graphs, etc.). This article will analyze its background, architecture, implementation details, practical applications, and learning value across different floors.

High-level abstractions in mature frameworks (PyTorch/TensorFlow) create barriers for learners to understand underlying principles. The CNet project fills this gap: implemented in pure C with no dependencies, it helps developers master core mechanisms like tensor operations, automatic differentiation, and computation graphs hands-on, and deeply understand concepts such as memory layout, gradient flow, and backpropagation from theory to practice.

CNet's core architecture consists of two main components:

1. Tensor Engine: N-dimensional tensors with flat memory + stride indexing (supports slicing/transposition without data copying), implementing 9 core operations (arithmetic, matrix multiplication, ReLU, etc.).
2. Automatic Differentiation System: Records computation processes based on DAG; each operation node includes input, type, and output. Backpropagation calls the backward method via function pointers and supports gradient accumulation (key for mini-batch training).

The tensor struct design balances functionality and efficiency, encapsulating fields like data, gradients, shape, and computation graph connections. Memory management strategies: reference counting (life cycle tracking), delayed allocation of gradient buffers (memory saving), and in-place operations (reducing copies). The computation graph uses post-order traversal to ensure backpropagation correctness.

Planned implementation of MNIST training process: batch data loading, supporting MLP (input 784 → hidden layer → output 10 with cross-entropy loss) and CNN architectures; optimizers include SGD and Adam. Additionally, Python bindings are provided via ctypes—core computations retain C's high performance while the Python side remains easy to use (sample code demonstrates tensor creation, forward/backward propagation).

Compilation: GCC compilation requires linking the math library (command: gcc -Wall -Wextra -g -o tensor tensor.c main.c -lm). Testing: Three modes (ops/ autograd/ all). Challenges and Solutions: Multi-dimensional indexing (stride mapping), gradient numerical stability (epsilon to prevent division by zero), memory leaks (reference counting + Valgrind detection).

Comparison: Advantages (zero dependencies, small code size, no GIL, high learning value); Disadvantages (limited functionality, no GPU acceleration, weak ecosystem). Learning Recommendations: Follow the order: tensor operations → automatic differentiation → computation graph → optimizer → MNIST training. Extension Directions: Convolution operations, GPU acceleration, more layer types (BatchNorm), model serialization, etc.

Although CNet cannot compete with industrial-grade frameworks, its educational value is unique: it demonstrates the minimal viable implementation of a deep learning framework, helping developers advance from "library users" to engineers who understand underlying mechanisms. It proves that pure C can also build a complete deep learning system, making it an excellent platform for in-depth learning.