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Talk:Longest string challenge: Difference between revisions
→bounds checking in C, take 2
Line 348:
::::: What do you think of the version I added? --[[User:Rdm|Rdm]] 21:02, 17 August 2011 (UTC)
:::::: <code>if (!longer(bufend, line))</code> isn't really checking for accumulator overflow, is it? Also, if you use <code>memset</code> to mark unused buffer region, they should to be called every time the accumulator is reset (i.e. found a new longest line), or it would be possible that there's a null byte in the tail region of <code>buf</code> before the reset, but accumulated strings haven't reached there yet, and next <code>if (!longer(bufend, line))</code> can give a false positive on overflow. (I think) --[[User:Ledrug|Ledrug]] 21:24, 17 August 2011 (UTC)
:::::EDIT yes that does happen. Modified the code to this: (LINE_MAX is your 1000000, your buffer would have been <code>#define ACCU_MAX (11*LINE_MAX + 1)</code>, but the point is the same:<lang c>#define LINE_MAX 6
#define ACCU_MAX (4*LINE_MAX+1)
int main() {
char line[LINE_MAX];
char buf[ACCU_MAX];
char *linend= &line[LINE_MAX - 1];
char *bufend= &buf[ACCU_MAX - LINE_MAX - 1];
char *last = buf;
char *next = buf;
memset(line, 1, LINE_MAX);
memset(buf, 1, ACCU_MAX);
buf[0]= buf[ACCU_MAX - 1]= 0;
while (fgets(line, LINE_MAX, stdin)) {
if (!*linend) exit(1);
if (longer(last, line)) continue;
if (!longer(bufend, line)) exit(1);
if (longer(line, last)) next = buf;
last = next;
strcpy(next, line);
while (*next) next = &next[1];
}</lang> and it aborts on the input <code>echo "aa\nbb\ncc\ndd\nee\nff\naaa" | ./a.out</code> but works if you remove any of the doubles. --[[User:Ledrug|Ledrug]] 21:28, 17 August 2011 (UTC)
== Boring solution v. restrictions ==
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