Date: Mon, 2 Jun 2003 12:57:05 -0400 (EDT) From: "Matthew Hagerty" <matthew@mundomateo.com> To: freebsd-hackers@freebsd.org Subject: Re: Are write() calls guaranteed atomic? Message-ID: <1720.216.120.158.65.1054573025.squirrel@www.mundomateo.com> In-Reply-To: <20030602154917.GA97655@dan.emsphone.com> References: <1553.216.120.158.65.1054566440.squirrel@www.mundomateo.com> <20030602154917.GA97655@dan.emsphone.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
Dan,
Thanks for the info, very helpful! What reference did you get that from?
I searched high and low to find a definitive answer (like the one above)
before posting.
As for the clients, no, I don't have control over them. They are web
server child processes, Apache usually. I considered using a socket, but
I must have missed something since I didn't realize you could have a local
socket that looks and smells like a file to external processes? Based on
your post, can I assume that I can create a socket that can be accessed
using open() and write() by external processes?
On my way to RTFM... man socket (again...)
Thanks,
Matthew
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