● Covered by 1 source · 1 reportLow impact1 neutral
Analysis of Early Unix V4 malloc() Implementation Reveals Linear Allocator
Key points
- Unix V4 (1973) `malloc()` was a linear allocator.
- Found in the C compiler's object code improver.
- Used `sbrk()` to extend memory, no explicit freeing.
- Exited on allocation failure, no NULL checks.
Unix V4 Memory Allocation
The `malloc()` implementation in the fourth edition of Unix, released in November 1973, has been examined. This early version of Unix, where the majority of the kernel was written in C, included a basic memory allocation routine within its C compiler sources.
The specific code for `alloc()` was located in `/usr/c/c20.c:814`, part of what was described as a primitive optimizer or "C object code improver."
Linear Allocator Design
The `alloc()` routine functioned as a linear allocator. It aligned the requested size, checked if the current memory region was sufficient, and if not, attempted to extend the heap by 2000 units using the `sbrk()` system call. If `sbrk()` failed, the program would exit.
This design meant that callers did not need to check for NULL return values, as the system would terminate on allocation failure. Memory was not explicitly freed, which was a common practice in systems of that era where programs often ran to completion without complex memory deallocation needs.
Historical Context
The discovery of a tape containing Unix V4 last year, previously thought lost, provided access to these historical sources. The simplicity and directness of this early `malloc()` implementation offer insight into the foundational memory management techniques used in the nascent stages of Unix development, predating modern complex memory allocators and arena-based designs.
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Reporting from
An analysis of the `malloc()` implementation in Unix V4 (November 1973 release) reveals a linear allocator within the C compiler's object code improver. This early memory management routine allocated memory sequentially and extended the heap using `sbrk()` without explicit freeing, reflecting the system's design constraints and priorities at the time.