New features:

• Add EXTENT_ALLOC_FLAG_PINNED so custom extent-allocation hooks can mark non-reclaimable mappings, such as HugeTLB pages, for preferential reuse outside the decay and purge pipeline. Add the mallctl interfaces

  stats.pinned, stats.arenas.<i>.pinned,

  
  stats.arenas.<i>.extents.<j>.npinned,
  stats.arenas.<i>.extents.<j>.pinned_bytes, and
  stats.arenas.<i>.mutexes.extents_pinned.{counter} to report
  pinned-memory usage and mutex statistics. (@binliu19: be2de8c)
• Allow resuming per-CPU arena selection via thread.arena.
  (@Algunenano: 68c35f6)
• Better align the contents of human-readable and JSON malloc statistics.
  (@spredolac: 68fe1be, 30b1a41, 04aad97)
• Replace the runtime experimental_infallible_new option with the
  compile-time --enable-cxx-infallible-new option, enabling compiler-level optimizations and optimization in move constructors,

  and fix the new(std::nothrow) contract (@spredolac: 160ab9d, fe33667)

Incompatible changes:

• Adapt tcache fill and retention targets per bin to demand observed between GC events, replacing the fixed refill/flush policy. Remove

  seven legacy non-experimental controls: lg_tcache_nslots_mul,

  
  tcache_nslots_small_min, tcache_nslots_small_max,
  tcache_nslots_large, tcache_gc_delay_bytes,
  lg_tcache_flush_small_div, and lg_tcache_flush_large_div.
  Corresponding malloc_conf settings are silently ignored, matching
  opt.* mallctls return ENOENT, and tcache_ncached_max remains
  supported. (@spredolac: d13fe91)

Bug fixes:

• Preserve errno across free, free_sized, and free_aligned_sized,
  and across process_madvise-based page purging. (@spredolac: 3a77966,
  86f0582)
• Fix numeric overflow checks in size classes. (@spredolac: 6b24522)
• Accept NULL in free_sized() and free_aligned_sized() (C23
  correctness). (@bigbruno: 7ce8b91)
• Fix TSD lifecycle edge cases by 1) initializing thread-cache bins before marking the cache enabled, preventing reentrant bootstrap allocations from using uninitialized state, and 2) avoiding TSD recreation for late deallocations after thread teardown on generic-TSD platforms.

  (@fzakaria: 54f22c8, @spredolac: fb5499a)

• Use O_CLOEXEC when opening the THP sysfs file in init_thp_state.
  (@ibookstein: d070554)
• Fix duplicate opt.stats_print and opt.stats_print_opts fields in
  malloc_stats_print output. (@spredolac: dfe3a2e)
• Fix a potential deadlock during arena_reset. (@guangli-dai: 6957341)
• Fix a prof-sampling / guard-page interaction bug in the SAN. (@gctony:
  e36a0fa)

Optimizations and refactors:

• Modularize jemalloc's front end by extracting arena management, initialization, fork orchestration, and allocation dispatch from

  jemalloc.c; untangle tcache/arena ownership; and consolidate the

  
  internal header graph to eliminate circular dependencies. (@spredolac:
  ba1e2fe, 8874597, 9d75722, de9ad14, ...)
• Simplify ctl dispatch by refactoring arena helpers, internalizing an implementation-only control, replacing control-flow macros with typed

  helpers, and organizing ctl.c by subsystem. (@spredolac: 9e14346,

  
  901365d, 4e903a0, 5bc8d6e)
• Cap the base-block growth heuristic to avoid virtual-memory exhaustion
  under rare racy conditions. (@guangli-dai: 2f4db8c)
• Move background-thread lifecycle and state operations into the background-thread module, clarifying ownership independently of PAC/HPA

  callers. (@guangli-dai: f1f0792)

• Simplify the page-allocation boundary by replacing PAI vtable dispatch
  with direct PAC/HPA calls, removing the obsolete pai_t/pai.h abstraction, and moving deferred-work and decay orchestration out of the

  arena. (@guangli-dai: 1dfa6f7, 8edd101, d410f43)

• Refactor statistics collection and rendering into separate
  gather/emission stages and descriptor-driven tables. (@spredolac:
  184f304, 80c8fcb)
• Introduce an OS abstraction layer and move platform-dependent file/process I/O, time, synchronization, CPU, virtual-memory, atfork, error-handling, profiling, thread-yield, and configuration-access

  operations out of allocator core code. (@guangli-dai: c4158ac,

  
  c8e2e01, ...)

Portability improvements:

• Replace the std::__throw_bad_alloc call with standard C++ (#2900).
  (@lexprfuncall: 1a15fe3)
• Make arena_s use a flexible array member (bin_t all_bins[]) for C99
  or newer. (@grueninger: 300b58b)
• Fix rdtscp detection with --with-lg-vaddr. (@xinydev: e8a0d2b)
• Fix malloc_getcpu on macOS to read the current CPU number correctly.
  (@Algunenano: 5aabbc8)
• Use CLOCK_MONOTONIC for background-thread sleep to prevent clock-rollback stalls, and detect monotonic-condvar support at configure

  time. (@antonio2368: ebacec3, 8361239)

• Fix compilation warnings on macOS. (@gctony: abb0a8a)
• Fix thread-exit TSD cleanup on MinGW builds. (@gctony: 1e92317)
• Fix GCC 16 build warnings by removing -Wpedantic violations in macro and function syntax and explicitly NUL-terminating profiling thread-name

  copies to resolve -Wstringop-truncation. (@grueninger: 5acdcee,

  
  c22d929, @jasonangelov: 6a245e0)
• Parse PID-namespace symlinks without glibc-dependent strtok/atol,
  and return identifiers as uint64_t. (@guangli-dai: 278d90a)