[TUHS] Did realloc ever zero the new memory?

Doug McIlroy doug at cs.dartmouth.edu
Sun Sep 13 10:32:57 AEST 2015

> Did any Unix or Unix like OS ever zero fill on realloc?

> On zero fill, I doubt many did that.  Many really early on when memory
> was small.

This sparks rminiscence. When I wrote an allocation strategy somewhat
more sophisticated than the original alloc(), I introduced realloc() and
changed the error return from -1 to the honest pointer value 0. The
latter change compelled a new name; "malloc" has been with us ever since.

To keep the per-byte cost of allocation low, malloc stuck with alloc's
nonzeroing policy. The minimal extra code to handle calls that triggered
sbrk had the startling property that five passes through the arena might
be required in some cases--not exactly scalable to giant virtual address

It's odd that the later introduction of calloc() as a zeroing malloc()
has never been complemented by a similar variant of realloc().

> Am I the only one that remembers realloc() being buggy on some systems?

I've never met a particular realloc() bug, but realloc does inherit the
portability bug that Posix baked into malloc(). Rob Pike and I
requested that malloc(0) be required to return a pointer distinct from
any live pointer. Posix instead allowed an undefined choice between
that behavior and an error return, confounding it with the out-of-memory
indication. Maybe it's time to right the wrong and retire "malloc".
The name "alloc" might be recycled for it. It could also clear memory
and obsolete calloc().


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