Are write() calls guaranteed atomic?

Matthew Hagerty matthew at
Mon Jun 2 09:57:08 PDT 2003

> 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 at


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...)


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