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Abstract:

This transcript comprises a collection of blog posts by Chris Wellons from nullprogram.com, focusing on advanced C programming techniques, system-level development, and performance optimization. The articles cover diverse topics including the practical implications of new C23 language features for parameterized types, robust memory management strategies (arena allocators, dynamic heaps) for WebAssembly (Wasm) applications, and insights into Wasm ABI and runtime interactions. It also delves into low-level system programming, demonstrating how to implement robust raw OpenBSD system calls and create a highly optimized xxd utility. Furthermore, the collection provides a comprehensive guide to effective fuzz testing with AFL++, including sanitizer configurations, persistent mode usage, and test isolation. Finally, it details the implementation of custom hash tables and dynamic arrays in C, showcasing efficient data structures and security enhancements like hash hardening through ASLR.

Summary:

• Parameterized types in C using the new tag compatibility rule (June 26, 2025):

  • C23's new tag compatibility rule makes struct, union, and enum definitions compatible across different translation units if they share the same tag and fields.
  • This enables C macros to create "parameterized types" (e.g., Slice(T)) for data structures like dynamic arrays, similar to limited C++ templates.
  • The push macro for dynamic arrays can now leverage C23's typeof for robust alignment calculations.
  • This technique is primarily for type parameterization, not generic functions, and has minor limitations for nested parameterized types.

• WebAssembly: How to allocate your allocator (April 19, 2025):

  • Explores memory allocation in WebAssembly (Wasm), distinguishing between static and dynamic heaps, and exported vs. imported memory.
  • Static heaps (e.g., char heap[]) are zero-initialized by the Wasm runtime when exported, but can inflate module size.
  • Dynamic heaps utilize the memory.grow instruction (via __builtin_wasm_memory_grow) to expand memory during runtime, offering flexibility.
  • Importing memory leads to uninitialized linear memory, requiring explicit zeroing by toolchains, potentially creating large Wasm images.

• Lessons learned from my first dive into WebAssembly (April 04, 2025):

  • Provides practical advice for developing C applications for Wasm, emphasizing low-level control by coding directly against raw WASI interfaces to produce small binaries.
  • Details the Wasm module structure, ABI (e.g., Clang's __stack_pointer), and debugging challenges.
  • Recommends developing and debugging on native platforms before porting to Wasm due to immature Wasm tooling.
  • Discusses WASI system calls like fd_write and proc_exit, highlighting the complexity of path_open due to WASI's security model and preopened directories.
  • Advises using the --stack-first linker option for a safer stack memory layout.

• A more robust raw OpenBSD syscall demo (March 06, 2025):

  • Addresses the fragility of a raw OpenBSD system call demo, specifically concerning the pinsyscall security feature.
  • Demonstrates how to correctly implement entry points and syscall wrappers in assembly using extended inline assembly with proper register handling and noinline attributes.
  • Shows how to dynamically register syscall addresses with the linker using .openbsd.syscalls section and labels.
  • Provides recommended compilation flags (-static -nostdlib -no-pie) for robust static binaries.

• Robust Wavefront OBJ model parsing in C (March 02, 2025):

  • Presents a robust, partial parser for Wavefront OBJ 3D models in C, designed for safety and efficiency without hard-coded limits.
  • Employs an arena allocator for memory management and counted strings (Str struct) for safe string manipulation.
  • Features custom string utilities (cut, parseint, parsefloat) that are locale-independent and resilient to invalid inputs.
  • Uses a two-pass parsing approach to first count elements, then allocate and fill data in memory, followed by transformation for rendering.

• Meet the new xxd for w64devkit: rexxd (February 17, 2025):

  • Introduces rexxd, a rewritten xxd utility that significantly outperforms and is more robust than the original.
  • Highlights rexxd's speed improvements (10x-20x), full Unicode path support, and robust handling of 64-bit offsets.
  • Features a built-in strace-like -x option for debugging binary patches.
  • Discusses its portable platform layer (Plt struct) and custom buffered I/O, designed for efficiency and safety.

• Tips for more effective fuzz testing with AFL++ (February 05, 2025):

  • Provides a checklist for optimizing C/C++ fuzz testing with AFL++.
  • Emphasizes using Address Sanitizer (ASan) and Undefined Behavior Sanitizer (UBSan) with specific configurations for maximum bug detection.
  • Strongly recommends AFL++'s persistent mode for performance and including target source files directly in the fuzzer's translation unit to access static functions.
  • Stresses the importance of isolating fuzz tests by resetting global state and copying fuzz buffers to prevent read overflows.
  • Suggests using memory file descriptors (memfd_create) to back named paths for input, enabling parallel fuzzing and avoiding disk I/O.
  • Advises configuring targets with smaller fixed buffer sizes to trigger edge cases more quickly.

• Examples of quick hash tables and dynamic arrays in C (January 19, 2025):

  • Demonstrates implementing std::unordered_map and std::vector equivalents in C, using simple and efficient techniques.
  • The foundation is a custom bump allocator (Arena) and counted strings (Str).
  • Introduces two hash map implementations: a fixed-size Flat Hash Map (MSI) for bounded keys and a Hierarchical Hash Map (4-ary Trie) for unbounded keys, both built with the arena allocator.
  • Presents robust string concatenation functions and a generic push macro for dynamic arrays (slices).
  • Includes a "Hash hardening" bonus section, showing how to leverage Address Space Layout Randomization (ASLR) to seed hash functions and mitigate collision attacks.