On 31/01/12 18:46, Bart Van Assche wrote:
> On Tue, Jan 31, 2012 at 4:30 PM, Alban Rochel
> <[email protected]> wrote:
>> I work on a Linux application which does an extensive use of net-snmp to
>> monitor and control a bunch of external equipments, all synchronised on
>> a ptp or ntp clock. The "fun" part is that the clock is not always
>> available, and that the system can be completely out of synch at startup
>> (~1h early or late) and catch up later.
>>
>> We have noticed that the application sometimes froze at this point, and
>> investigating a little, I have found that the culprit was the use of
>> timeval structures set by gettimeofday in snmp_sess_select_info (release
>> 5.3.3) or snmp_sess_select_info2_flags (release 5.7.1), which provided
>> wrong timeout values in this case (computing deltas with timeval structs
>> obtained with gettimeofday before and after the time change).
>>
>> I understand that my case is quite specific, but as a suggestion, I
>> would propose to use a really monotonic clock to compute the actual
>> timeout, e.g. using clock_gettime with the CLOCK_MONOTONIC argument on
>> Unix systems. Of course, in the end, it may not be as simple as it seems!
>>
>> How to reproduce:
>> - Write a basic client sending requests in a loop
>> - Change system time regularly (e.g. add or remove 10 minutes, I haven't
>> bothered determining which way raised the issue)
>> - Occasionnally, the client freezes for 10 minutes.
>>
>> Tested on RHEL 5.3 x64, with net-snmp 5.3.3 and 5.7.1.
>>
>> Tell me if you think I should file a bug report. Thanks in advance for
>> your suggestions.
> This is what I found by googling for Net-SNMP, gettimeofday and
> CLOCK_MONOTONIC:
> http://sourceforge.net/tracker/index.php?func=detail&aid=3164831&group_id=12694&atid=112694.
>
> Bart.
Hello all,
FYI, I have fixed this problem by replacing all the calls to
gettimeofday() by calls to mygettimeofay():
int mygettimeofday(struct timeval *tv, struct timezone * unused)
{
struct timespec ts;
int res = clock_gettime(CLOCK_MONOTONIC, &ts);
if(res == 0)
{
tv->tv_sec = ts.tv_sec;
tv->tv_usec = ts.tv_nsec/1000L;
}
return res;
}
It seems to do the trick (more testing needed, but so far, so good), but
I have no idea whether it's portable or not. It requires linking to librt.
Alban
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