On Sat 31/Jan/2015 13:55:24 +0100 Sam Varshavchik wrote:
> Alessandro Vesely writes:
>>
>> 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
>> line
>> it would have interpreted it as a severity code. But, even if stderr is
>> line-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.
That's right what I was hoping. I found the following in pipe(7):
POSIX.1-2001 says that write(2)s of less than PIPE_BUF bytes must be
atomic: the output data is written to the pipe as a contiguous
sequence. Writes of more than PIPE_BUF bytes may be nonatomic: the
kernel may interleave the data with data written by other processes.
POSIX.1-2001 requires PIPE_BUF to be at least 512 bytes. (On Linux,
PIPE_BUF is 4096 bytes.)
> If stderr is line-buffered, these short log lines should not get intermingled.
I thought it was line buffered, but I was wrong. setbuf(3) says "stderr is
always unbuffered by default". Calling setlinebuf(stderr) should fix that.
Thanks for the hint
Ale
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