Alessandro Vesely writes:
On Fri 30/Jan/2015 11:30:25 +0100 Szépe Viktor wrote: > Idézem/Quoting Alessandro Vesely <[email protected]>: > >> Every now and then log lines from a filter through stderr to syslog get >> intermixed. For example, this came as a single line: >> >> Jan 29 13:49:35 wmail courierfilter: >> zdkimfilter[31367]:INFO:zdkimfilter[31364]:drop >> msg,id=00000000005DC056.0000000054CA2C5F.00007A7D: Found-Virusdrop >> msg,id=00000000005DC04E.0000000054CA2C5F.00007A7C: Found-Virus> Is there a \n at the end of the line written to stderr? Yes, I have something like so: vfprintf(stderr, fmt, ap); va_end(ap); fputc('\n', stderr);That "INFO:" arrived from a sibling process. They both inherit fd 2 as a pipe to courierfilter. If courierfilter had seen "INFO:" at the beginning of a lineit would have interpreted it as a severity code. But, even if stderr isline-buffered, two processes can happen to flush their buffers simultaneously.
Each flush is atomic. Somewhere buried in the man pages is a guarantee that if a write() to a pipe is below a certain size, it is guaranteed to be atomic, and won't get comingled with anything else that writes to the same pipe.
If stderr is line-buffered, these short log lines should not get intermingled. But is' possible that vfprintf explicitly flushes the output stream. I recall seeing that behavior somewhere, so even something like that may not work right.
The best approach is to vsnprintf the whole thing, including \n into a buffer, and manually write() it to stderr.
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