NLE (NetHack Learning Environment) — Per-Env Refactor

Modified fork of NLE 0.9.0 / NetHack 3.6.6 for use with PufferLib's native C vecenv.

What changed

Stock NLE uses process-global state (NEARDATA globals, static locals, shared DLB file descriptors), which limits it to one env per process. Multi-env training requires either subprocess isolation or dlopen-per-copy — both slow and memory-heavy.

This fork migrates all mutable state into a per-env nle_ctx_t struct (~75KB), enabling thousands of independent NetHack instances in a single process with zero mutexes:

• Global-to-local migration: All NEARDATA/static globals → nle_ctx_t fields accessed via current_nle_ctx pointer (initial-exec TLS)
• Function-local statics: Per-file state structs for 15+ files with persistent locals (RNG, trap handling, display, save/restore)
• Thread-safe DLB: pread() replaces lseek()+read() for concurrent data-file access
• Per-env arena allocator: Bump allocator for level data, eliminating malloc contention
• Direct linkage: Single shared libnethack.so, no dlopen overhead

The C API (nle_start, nle_step, nle_end) is unchanged. Callers set current_nle_ctx before each call to select which env instance to operate on.

Build

make -C src/build nethack -j8

Produces src/build/libnethack.so and src/build/dat/ (game data files).

Usage with PufferLib

This repo is cloned automatically by PufferLib's build.sh when building the nethack environment. You don't need to clone it manually.

# In PufferLib:
bash build.sh nethack    # clones this repo into vendor/nle/, builds libnethack.so

Performance

With PufferLib's OMP-parallel vecenv:

Envs	Threads	Training SPS
64	1	9,200
4096	1	31,300
4096	4	136,300

License

NetHack is distributed under the NetHack General Public License. NLE additions are under NLE's license.