RFR: 8254270: linux 32 bit build doesn't compile libjdwp/log_messages.c

[Thomas Stuefe]

On Thu, 5 Nov 2020 18:22:56 GMT, Thomas Stuefe <stuefe at openjdk.org> wrote:

>>> I don't think this is the case. If you assume the arguments are not null terminated, then there is no limit to how long the string could be, where-as the error messages are very specific with the (incorrectly) calculated range of potential overflow.
>> 
>> I must be missing what you're suggesting here?
>
> Hi,
> 
> I think the compiler is correct. The 3 in "%.3d" of the second argument is precision, not scale. Which is the *minimum* numbers of digits to be printed.
> 
> So we get:
> 
>     (void)snprintf(tbuf, ltbuf,
>                    "%s.%.3d %s", timestamp_date_time,
>                    (int)(millisecs), timestamp_timezone);
> 
> with timestamp_date_time = DT_SIZE = 20 and timestamp_timezone = TZ_SIZE = 56
> 
> and the second argument prints at least three digits but possibly more if the argument is longer. MAX_INT is 2147483647. So length could be 10.
> 
> 56 + 20 + 10 = 86.

... so the problem would be that the compiler does not believe us that millisecs will be always <1000. And there is no way to truncate output for numerical format specifiers.

One solution could be to first print the millisecs to a buffer large enough to hold MAX_INT. And then print that buffer as a string, which can be truncated with precision.

char tmp[10 + 1];
snprintf(tmp, sizeof(tmp), "%d", millisecs);
snprintf(tbuf, ltbuf, "%s.%.3s %s", timestamp_date_time, tmp, timestamp_timezone);

That may be enough to shut the compiler up.

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PR: https://git.openjdk.java.net/jdk/pull/1067