Re: is sendto and recvfrom thread safe?

["Lee Chin" <leechin@mail.com>]

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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