Interesting. I could reproduce this, it is quite strange. It seems like a bug in BSD awk. I noticed gawk and mawk do not display this behavior, but BSD awk does...

An alternative way of doing it does not display the behavior:

Code:

awk 'BEGIN{for(i=1;i<=100000000;i++) for(j=1;j<=10;j++) s=sprintf("a%d",j) }'

but

Code:

awk 'BEGIN{for(i=1;i<=100000000;i++) for(j=1;j<=10;j++) s=sprintf("a%s",j) }'

does!

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Exploring this further, the problem seems to be in the number-to-string conversion:

Problem:

Code:

awk 'BEGIN{for(i=1;i<=100000000;i++) for(j=1;j<=10;j++) s=2 "a" }'

No Problem

Code:

awk 'BEGIN{for(i=1;i<=100000000;i++) for(j=1;j<=10;j++) s="2" "a" }'

--

Code:

$ awk --version
awk version 20070501
$ uname -rs
Darwin 16.0.0

MacOS Sierra 10.12