MALLOC_PERTURB_: everyone should set this envvar

[John Reiser]

>     # http://udrepper.livejournal.com/11429.html
>     export MALLOC_PERTURB_=$(($RANDOM % 255 + 1))

For supportability (and to help remind you to turn it off before
performance tests), then also consider adding a line such as:
     echo 1>&2 MALLOC_PERTURB_=$MALLOC_PERTURB_  " # $HOME/.bash_profile"

Also inexpensive:
     export MALLOC_CHECK_=1  # 0: off; 1: announce on stderr; 2: abort
     echo 1>&2 MALLOC_CHECK_=$MALLOC_CHECK_  " # $HOME/.bash_profile"
which handles double free(), single-byte overruns, etc.

For "always on" detection of overlapping memcpy() at low cost,
then consider  http://bitwagon.com/glibc-memlap/glibc-memlap.html .

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