Invalid data read by strcol(), possible memory corruption

[Juhász P. <pet...@gm...>]

Dear gnuplot list members,

The following data file + gnuplot script produce incorrect behavior
with a recent git build, indicating possible memory corruption while
reading strings from the data file:

# -- data file ---
1;1;
1;2;
1aa;3;1.1
1;4;
1;5;
1;2;
1;3;
1;4;
1;5;

# -- script ----
set dataf sep ';'
plot 'foo' u 0:2:(strcol(3)) w labels point


On my machine this produces a plot where all points after the third
have the same "1.1" label. When you move the plot around (in an
interactive terminal, e.g. with an arrow key), all points get the same
label.

Interestingly, everything seems to work as expected if you remove the
"a"s from the data file. Other, even weirder effects can be coaxed out
of gnuplot if you add some characters to other lines than the third.

This seems like a bug. I think the explicit non-standard separator and
the field being the last one are both necessary conditions.

Some background and explanation:

This report is actually related to an earlier discussion on this list,
where I complained that lines containing missing values are silently
dropped, and that column(i) seemed different from $i. Today I updated
to the current development version, and noticed that my workaround to
the missing values using column() no longer worked. So I experimented
with some things, and at first I've thought that strcol() might be able
to help me, as it didn't seem to be bothered by missing fields, but
then I noticed the weirdness described above.

As for fixing this bug, I think it is important to preserve strcol()'s
ability to just return the empty string in case of missing data - after
all, it is advertised to return the content of column X as a string. If
the column is empty, then by definition its contents are the empty
string.

best regards,
Peter Juhasz