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SubjectRe: send(), sendmsg(), sendto() not thread-safe
Date
Followup to:  <OFE8460E54.0C8D85D8-ON8525716F.0074F22F-8825716F.0076D537@us.ibm.com>
By author: Mark A Smith <mark1smi@us.ibm.com>
In newsgroup: linux.dev.kernel
>
> I discovered that in some cases, send(), sendmsg(), and sendto() are not
> thread-safe. Although the man page for these functions does not specify
> whether these functions are supposed to be thread-safe, my reading of the
> POSIX/SUSv3 specification tells me that they should be. I traced the
> problem to tcp_sendmsg(). I was very curious about this issue, so I wrote
> up a small page to describe in more detail my findings. You can find it at:
> http://www.almaden.ibm.com/cs/people/marksmith/sendmsg.html .
>
> Thanks,
> Mark A. Smith
>
> PS. I am using the term "thread" in the general sense, this is a problem
> independent of pthreads, etc. The problem occurs when two processes
> (whether or not they share an address space) send on the same socket (and
> some other low-resource conditions exist).
>

User error. Writes onto a streaming socket (or a pipe) are
thread-safe, *but not necessarily atomic*, if the size exceeds PIPE_BUF.

If you want atomicity you either have to do your own locking, or use a
DGRAM or SEQPACKET socket.

-hpa

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