Unix V4 1973 sources reveal linear sbrk allocator and explicit map-based malloc in dmr/malloc.c
V4 Unix allocators from 1973 separated linear heap growth from map-based region management, providing explicit control absent in later single-heap designs. The recovered code demonstrates production use of sbrk extension and typed maps for core and swap. These patterns prefigure modern arena allocators used in performance-critical software.
The recovered V4 tape yielded two allocators. c20.c implemented alloc by aligning requests, extending the heap via sbrk, and advancing lasta without free or arena naming. Callers omitted error checks because exhaustion triggered immediate exit. dmr/malloc.c defined struct map with m_size and m_addr, passing explicit regions to malloc and mfree so that coremap and swapmap remained separate address spaces.
These interfaces decoupled allocation from a single implicit heap. Modern arena or pool allocators replicate the map pattern to bound fragmentation and enable bulk release. The 1973 design eliminated per-call NULL tests and pointer invalidation on extension because sbrk preserved existing mappings.
Operationally the approach reduced runtime checks in the C compiler pipeline and swap management. Contemporary libraries that expose region handles achieve similar predictability for long-lived servers and embedded systems where general-purpose malloc introduces measurable tail latency.
Full V4 sources now allow direct comparison of these primitives against later malloc implementations in V6 and V7, documenting the shift from explicit maps to the single-heap model that dominates current C runtimes.
Sources (3)
- [1]Napcakes V4 malloc analysis(https://napcakes.nekoweb.org/posts/malloc/)
- [2]Unix Heritage Society V4 tape recovery(https://www.tuhs.org)
- [3]Dennis Ritchie V4 directory listing(https://github.com/dspinellis/unix-history-repo)