Date: Mon, 2 Jun 2003 10:49:17 -0500 From: Dan Nelson <dnelson@allantgroup.com> To: Matthew Hagerty <matthew@mundomateo.com> Cc: freebsd-hackers@freebsd.org Subject: Re: Are write() calls guaranteed atomic? Message-ID: <20030602154917.GA97655@dan.emsphone.com> In-Reply-To: <1553.216.120.158.65.1054566440.squirrel@www.mundomateo.com> References: <1553.216.120.158.65.1054566440.squirrel@www.mundomateo.com>
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In the last episode (Jun 02), Matthew Hagerty said:
> I'm writing a server that receives data via a named pipe (FIFO) and
> will always have more than one other process sending data to it, and
> I'm concerned with preventing data multiplexing. The data coming in
> will be lines of text delimited with a newline, but the processes
> writing the data have no knowledge of the FIFO, they simply think
> they are writing to a file. This being the case, the processes
> writing the data give no regard to PIPE_BUF and may send data in
> longer lengths (but probably never longer than 2K to 4K.)
>
> Will the kernel ensure that the data write() will be delivered to the
> FIFO atomically even if the data is larger than PIPE_BUF, such that
> two or more successive read() calls will retrieve the data in order?
Pipes are always FIFO; it's part of their definition. From SUSv3:
A read on the file descriptor fildes[0] shall access data written to
the file descriptor fildes[1] on a first-in-first-out basis.
To ensure that your writes don't get interleaved with writes from other
processes, you do need to limit your write sizes to PIPE_BUF or less
bytes:
Write requests of {PIPE_BUF} bytes or less shall not be interleaved
with data from other processes doing writes on the same pipe. Writes
of greater than {PIPE_BUF} bytes may have data interleaved, on
arbitrary boundaries, with writes by other processes, whether or not
the O_NONBLOCK flag of the file status flags is set.
If you cannot modify the clients, try changing your server to create a
Unix domain socket instead of a named pipe (the clients shouldn't see
any difference).
--
Dan Nelson
dnelson@allantgroup.com
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