Other primitives

A grab-bag of smaller, self-contained data structures and utilities used across the codebase. Most are < 5 KB of source.

Generic structures

Primitive	Source	Purpose
Linked list	src/adlist.c, src/adlist.h	Doubly-linked list. Used for client lists, blocked client queues, deferred reply chunks.
Mstr	src/mstr.c, src/mstr.h	Variable-length micro-strings used by entry.c (hash field+value with optional TTL).
Entry	src/entry.c, src/entry.h	Field/value pair used by hash field expiration.
Fenwick tree	src/fwtree.c, src/fwtree.h	Binary indexed tree used for hot-key tracking and a few percentile estimations.
Sparkline	src/sparkline.c, src/sparkline.h	ASCII sparkline used by LATENCY GRAPH.
Vector	src/vector.c, src/vector.h	Generic dynamic array.
Pqsort	src/pqsort.c, src/pqsort.h	Partial qsort — sort just the K smallest/largest elements, used by SORT LIMIT.
Sort	src/sort.c	The SORT and SORT_RO command implementation.

Time and randomness

Primitive	Source	Purpose
Monotonic time	src/monotonic.c, src/monotonic.h	Wraps clock_gettime(CLOCK_MONOTONIC) with a fast getter cached per cron tick.
Local time	src/localtime.c	Async-signal-safe gmtime_r/localtime_r for the crash handler.
Random	src/rand.c, src/rand.h, src/mt19937-64.c	Pseudo-random generators. mt19937 is used where reproducibility matters; rand is used for general randomness.

Crypto / hashing

Primitive	Source	Purpose
SHA1	src/sha1.c, src/sha1.h	Used by SCRIPT LOAD to hash scripts and by some auth paths.
SHA256	src/sha256.c, src/sha256.h	Used to hash ACL passwords.
SipHash	src/siphash.c	The default hash for dict. Seeded at startup.
CRC64	src/crc64.c, src/crc64.h, src/crcspeed.c, src/crccombine.c	Checksum for RDB files.
CRC16	src/crc16.c, src/crc16_slottable.h	Cluster slot computation (CRC16(key) % 16384).

Compression

Primitive	Source	Purpose
LZF	src/lzf_c.c, src/lzf_d.c, src/lzf.h, src/lzfP.h	Compresses RDB strings (rdbcompression yes) and middle quicklist nodes.

Numbers

Primitive	Source	Purpose
Fast float strtod	src/fast_float_strtod.c, src/fast_float_strtod.h	A heavily-optimised float parser used by command-argument parsing (ZADD scores, SET EX, …). Replaces the slower libc strtod.
Util	src/util.c, src/util.h	String parsing (string2ll, string2ld, ld2string, getRandomBytes, glob match).
Strl	src/strl.c	strlcpy/strlcat portability shim.
Endianconv	src/endianconv.c, src/endianconv.h	Endianness helpers for RDB cross-platform compatibility.

Misc subsystems

Primitive	Source	Purpose
Hot keys	src/hotkeys.c, src/keymeta.h	Tracks the most-accessed keys via reservoir sampling, surfaced via OBJECT FREQ and --hotkeys.
Estore	src/estore.c, src/estore.h	Per-DB expiration store wrapping ebuckets. See Expiration.
Keymeta	src/keymeta.c, src/keymeta.h	Per-key metadata: minimum hash-field TTL, plus extension points.
Hyperloglog	src/hyperloglog.c	Probabilistic cardinality counter.
GCRA	src/gcra.c	Generic Cell Rate Algorithm. Token-bucket primitive used internally and exposed via GCRA* commands.
Geohash	src/geohash.c, src/geohash_helper.c, src/geo.c	Geospatial encoding (52-bit interleaved coordinates) plus radius/box queries.
Bio	src/bio.c, src/bio.h	Background IO threads (close fd, fsync, lazy free). See Memory management.
Threads manager	src/threads_mngr.c, src/threads_mngr.h	Cross-thread stack capture. Used by the watchdog and DEBUG STACK-TRACE.
Memtest	src/memtest.c	RAM consistency test invoked by the crash handler and --test-memory.
Setproctitle	src/setproctitle.c	Portable setproctitle(3) replacement.
Setcpuaffinity	src/setcpuaffinity.c	Portable affinity setter for server-cpulist / bio-cpulist etc.
Syscheck	src/syscheck.c, src/syscheck.h	Startup checks for the OS (transparent huge pages, swap, atomic ops). Surfaces warnings in the log.
Childinfo	src/childinfo.c	Pipe-based child→parent communication for COW accounting.
Memory prefetch	src/memory_prefetch.c, src/memory_prefetch.h	Software prefetching used in dict lookups to hide L1/L2 latency.
Logreqres	src/logreqres.c	When built with LOG_REQ_RES, captures every request/reply pair to JSON for schema validation.

These primitives are deliberately small and self-contained. Most can be grokked in a single read of their .c and .h.

Where to start

• Add a hashing function — pattern after siphash.c. Register on a dictType.
• Add a benchmark utility — redis-benchmark.c is the right host for synthetic load tests.
• Add a system check — src/syscheck.c runs at startup and logs warnings; extend the table.

Related pages

• Persistence — uses CRC64, LZF.
• Cluster — uses CRC16.
• Memory management — uses bio threads.