[TUHS] Did realloc ever zero the new memory?

Norman Wilson norman at oclsc.org
Sat Sep 12 01:41:23 AEST 2015

I've used realloc a lot, and never run into bugs.  Maybe I've just
been lucky, or maybe it's that I probably didn't use it that much
until the latter 1980s, and then more with pukka Doug malloc code
than with the stuff floating around elsewhere.

Never mind that sometime around 1995 I found a subtle bug in the
pukka Doug malloc (not realloc): arena blew up badly when presented
with a certain pattern of alternating small and large allocs and frees,
produced by a pukka Brian awk regression test.  I had a lot of (genuine)
fun tracking that down, writing some low-level tools to extract the
malloc and free calls and sizes and then a simulator in (what else?)
awk to test different fixes.  Oh, for the days when UNIX was that

I've never heard before of a belief that the new memory belonging
to realloc is always cleared, except in conjunction with the utterly-
mistaken belief that that's true of malloc as well.  I don't think it
was ever promised to be true, though it was probably true by accident
just often enough (just as often as with malloc) to fool the careless.

Norman Wilson
Toronto ON

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