is sendto and recvfrom thread safe?

is sendto and recvfrom thread safe?

[Lee Chin]

Hi,
If I have a global socket file descriptor, can I call sendto and recvfrom on that file descriptor concurrently with out using semaphores around the call to sendto and recvfrom?

What about for TCP sockets?

Thanks
Lee
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Re: is sendto and recvfrom thread safe?

[Stephen Hemminger]

On Thu, 21 Aug 2003 01:06:45 -0500
"Lee Chin" <leechin@mail.com> wrote:

> Hi,
> If I have a global socket file descriptor, can I call sendto and recvfrom on that file descriptor concurrently with out using semaphores around the call to sendto and recvfrom?
> 
> What about for TCP sockets?
> 
> Thanks
> Lee

It depends on your definition of thread safe.
Sendto and recvfrom are for UDP which is a datagram protocol; therefore
each call sends and receives a separate data unit.  The application protocol
has to be atomic to work with UDP anyway. Your application has to deal
with out of order, and dropped packets.

If you use TCP sockets with threaded applications you better do application
level locking per socket and have enough information in your protocol to describe
the length of each operation.  TCP is a byte stream, and with multiple threads sending
it is possible to "mix the streams".  Multiple threads reading will get out of
order data.

This is true of all OS's that I know about that support threads 
(Windows, Solaris, Linux, ...).

Re: is sendto and recvfrom thread safe?

[Lee Chin]

This sort  of helps... my question is with respect to UDP sockets, for sendto and recvfrom, can I concurrently have two threads at the same time write (sendto) on a same file descriptor?  How about readfrom and sendt on the same file descriptor at the same time?

Thanks
Lee
----- Original Message -----
From: Stephen Hemminger <shemminger@osdl.org>
Date: Thu, 21 Aug 2003 09:16:49 -0700
To: "Lee Chin" <leechin@mail.com>
Subject: Re: is sendto and recvfrom thread safe?

> On Thu, 21 Aug 2003 01:06:45 -0500
> "Lee Chin" <leechin@mail.com> wrote:
> 
> > Hi,
> > If I have a global socket file descriptor, can I call sendto and recvfrom on that file descriptor concurrently with out using semaphores around the call to sendto and recvfrom?
> > 
> > What about for TCP sockets?
> > 
> > Thanks
> > Lee
> 
> It depends on your definition of thread safe.
> Sendto and recvfrom are for UDP which is a datagram protocol; therefore
> each call sends and receives a separate data unit.  The application protocol
> has to be atomic to work with UDP anyway. Your application has to deal
> with out of order, and dropped packets.
> 
> If you use TCP sockets with threaded applications you better do application
> level locking per socket and have enough information in your protocol to describe
> the length of each operation.  TCP is a byte stream, and with multiple threads sending
> it is possible to "mix the streams".  Multiple threads reading will get out of
> order data.
> 
> This is true of all OS's that I know about that support threads 
> (Windows, Solaris, Linux, ...).

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Re: is sendto and recvfrom thread safe?

[Stephen Hemminger]

On Thu, 21 Aug 2003 15:02:26 -0500
"Lee Chin" <leechin@mail.com> wrote:

> This sort  of helps... my question is with respect to UDP sockets, for sendto and recvfrom, can I concurrently have two threads at the same time write (sendto) on a same file descriptor?  How about readfrom and sendt on the same file descriptor at the same time?

Yes, but assume two threads A (writes a one message containing "aaaa") 
and B which (writes two messages "b" and "c").

You can be assured that B's data won't show up inside A's message:
	ie. never aabaa
but you can not assume anything about the order they will be received.

If you want more detail read the book.
 W. Richard Stevens, UNIX network programming, Volume 1

http://www.kohala.com/start/
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