[systemd-devel] odd seek_tail behaviour

[Sascha Kattelmann]

Hi,

here is a related "bug" report: https://bugs.freedesktop.org/show_bug.cgi?id=64614

Doing a "next" after "seek_tail" ends up in some unexpected behaviour. Just do a "previous" after seeking the tail and everything works fine.
The problem is symmetrical: Same goes for "previous" after "seek_head". You need to do a "next" immediately after "seek_head".

I hope that will help,
Sascha

----- Original Message -----
From: "Daurnimator" <quae at daurnimator.com>
To: "systemd Mailing List" <systemd-devel at lists.freedesktop.org>
Sent: Monday, October 13, 2014 7:27:22 AM
Subject: [systemd-devel] odd seek_tail behaviour

Hi All, 

I was trying to write a program that tailed the journal, but found that sd_journal_seek_tail() didn't work as expected. 
That is: that it would seek to the last/most recent thing in the journal, and I could tail things from there. 

I whipped up a quick demonstration program, that shows that messages I 'next' through, are before the 'cutoff': 

#include <stdio.h> 
#include <assert.h> 
#include "systemd/sd-journal.h" 

int main() { 
sd_journal* j; 
assert(sd_journal_open(&j, 0)==0); 
uint64_t from, to; 
assert(sd_journal_get_cutoff_realtime_usec(j, &from, &to)==1); 
printf("CUTOFF = %llu\n", to); 

printf("TAIL = %d\n", sd_journal_seek_tail(j)); 

for (int i=0; i<10; i++) { 
printf("NEXT = %lld\n", sd_journal_next(j)); 
assert(sd_journal_get_realtime_usec(j, &from)==0); 
printf("FOUND from %llu\n", from); 
} 
return 0; 
} 

Compiled with: gcc -std=c99 tail_logs.c -l systemd 

Example output: 

CUTOFF = 1413177397982789 
TAIL = 0 
NEXT = 1 
FOUND from 1400437372012374 
NEXT = 1 
FOUND from 1400438753925868 
NEXT = 1 
FOUND from 1400438753926192 
NEXT = 1 
FOUND from 1400438753926257 
NEXT = 1 
FOUND from 1400438753926289 
NEXT = 1 
FOUND from 1400438753926309 
NEXT = 1 
FOUND from 1400438753926330 
NEXT = 1 
FOUND from 1400438753926353 
NEXT = 1 
FOUND from 1400438753926373 
NEXT = 1 
FOUND from 1400438753926395 


Is this behaviour expected? I'm using systemd 216. 

Regards, 
Daurn. 


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