Snippets of C

A collection of small, self-contained C programs. Each file demonstrates a focused systems-programming concept using minimal, technical, production-style code.

The collection is designed as a reference for developers who work with:

• low-level Linux/POSIX APIs
• concurrency primitives
• memory management
• filesystem correctness
• networking and IPC
• kernel interfaces
• secure and defensive programming patterns
• JIT compilation and raw machine-code execution

---

Topics

Concurrency & Synchronization

• futex wait/wake, mutex, latch
• atomics and memory ordering
• seqlock
• sense-reversing barrier, eventfd barrier
• SPSC and MPMC lock-free ring buffers
• reader-writer lock (fair)
• Treiber stack with epoch-based reclamation
• RCU-style linked list
• CRDT PN-counter
• thread pool
• fiber / ucontext cooperative scheduler
• POSIX robust mutex (owner-death recovery)
• POSIX message queues with priority ordering

Memory Management

• mmap arenas with guard pages
• huge pages (MAP_HUGETLB)
• memfd sealing
• sliding-window memory mapping
• reference-counted objects
• LIFO scratch-region allocator
• userfaultfd demand-paging
• buddy system allocator (power-of-2, split/coalesce)

Filesystem & Storage

• atomic file replacement (write + rename)
• directory walking via openat
• log-structured record store with compaction
• key-value store with write-ahead journal
• filesystem transactions with rollback
• file-descriptor cache
• ring-buffer logger in shared memory

Linux Kernel Interfaces

• pidfd
• userfaultfd
• fanotify
• inotify
• epoll (edge-triggered event loop with timerfd)
• eventfd
• io_uring
• kqueue (BSD/macOS)
• BPF socket counter
• perf event monitoring
• vDSO time
• syscall tracing via ptrace

Networking & IPC

• edge-triggered TCP server with epoll
• UNIX socket credential passing (SCM_CREDENTIALS)
• file descriptor passing over UNIX sockets (SCM_RIGHTS)
• POSIX shared memory SPSC ring
• zero-copy file serving with sendfile
• zero-copy pipe pumping with splice
• token-bucket rate limiter

Security & Sandboxing

• strict seccomp and seccomp-BPF filters
• OpenBSD pledge / unveil
• Landlock LSM filesystem restriction (Linux ≥ 5.13)
• user namespace isolation (userns_spawn, userns_unshare)
• O_PATH capability-style access
• cgroup v2 memory limit enforcement

Signals, Timers & Async

• alternate signal stack for crash handlers
• hierarchical timer wheel
• POSIX AIO
• signalfd / self-pipe fallback
• accurate periodic timer with timerfd and missed-tick detection

Cryptography & Randomness

• HKDF key derivation via OpenSSL EVP
• constant-time byte comparison
• cryptographically secure token generation

JIT & Machine Code

• runtime machine-code generation: mmap (RW) → write bytes → mprotect (RX) → call
• W^X page discipline
• extracting compiled functions as raw byte blobs (objcopy, nm, xxd -i)
• self-contained syscall-only payload with no libc, no relocations (sysinfo_jit)
• generic JIT host loader (jit_host)

---

Build

Build all snippets:

make

Build a single snippet:

make futex_wait_wake

List all detected targets:

make list

Clean binaries:

make clean

The Makefile inspects each source file and links only the libraries it needs:

Condition	Extra flag
Uses pthreads	-pthread
Uses POSIX AIO or shm_open	-lrt
Includes <liburing.h>	-luring
Includes <openssl/...>	-lcrypto

JIT pipeline

For snippets in the JIT & Machine Code category, the build flow differs:

# 1. compile to object file — do NOT link
gcc -O2 -fno-stack-protector -std=c11 -c payload.c -o payload.o

# 2. must print nothing — any output means broken relocations
objdump -r payload.o

# 3. extract the raw .text blob
objcopy -O binary --only-section=.text payload.o payload.bin

# 4. find the entry-point offset inside the blob
nm payload.o | grep ' T run'

# 5. generate a C array to embed in the host
xxd -i payload.bin

# standalone test (no host needed)
gcc -O2 -fno-stack-protector -DSTANDALONE -nostartfiles \
    -std=c11 payload.c -o payload_test && ./payload_test

Notes

• Most snippets are Linux-only due to specific kernel syscalls.
• kqueue_timer and sandbox_pledge_unveil are BSD/macOS-only.
• JIT snippets additionally require x86-64.
• Each file is intentionally isolated: no shared headers, no inter-file dependencies.
• Designed for experimentation, auditing, and reference — every syscall is checked, every resource released.

License

MIT