James Colannino wrote:
Hey everyone. I'm having great difficulty debugging a function of mine
that adds more memory as needed when reading variable length lines.
When I run the code, it always fails after readline() calls addmemory()
for the second time (no matter how I change linebuf->buflen, or don't
change it...) If readline only has to call addmemory() once, no matter
what the size of memory being passed is, it works.[...]
I found the bug (for what it's worth.) It was very subtle, but it
caused catastrophic results at runtime. Notice line 6 below (I numbered
them.) This is the while loop in readline(). It says that it should
start counting down again from linebuf->buflen elements after the
reallocation. The problem is, half of those spaces are already filled,
so when I start counting down again from such a high number, forgetting
that half is already filled, I think I must blow away bookeeping
information stored by the memory allocator, thus, while the first call
to realloc() is successful, the second one isn't.
I changed that line of code to:
6: buflen = linebuf->buflen - bufcount;
That way, it makes sure to take into account those spaces which have
already been used. This code when compiled runs perfectly. Sorry for
the noise to the list. I honestly have been banging my head over this
one and really couldn't find a good solution (I was starting to suspect
something was up with glibc, which probably wasn't very bright :-P),
otherwise I wouldn'tve asked the question.
1: while ((c = getc(linebuf->fp)) != EOF) {
2:
3: if (buflen <= 2) { /* need more memory */
4: bufcount = bufpos - start;
5: start = addmemory(linebuf);
6: buflen = linebuf->buflen;
7: bufpos = start + bufcount;
8: }
9:
10: *(bufpos++) = c;
11: buflen -= 1;
12:
13: if (c == '\n') { /* we're done with one line */
14: *bufpos = '\0';
15: break;
16: }
17: }
James
--
My blog: http://www.crazydrclaw.com/
My homepage: http://james.colannino.org/
"Black holes are where God divided by zero." --Steven Wright
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