Re: recv and errno during a connection reset/closed by peer

[Igor Pechtchanski <pechtcha at cs dot nyu dot edu>]

On Thu, 24 Mar 2005, Brian Dessent wrote:

> Peter Stephens wrote:
>
> > When in non-blocking mode I thought I would be able to get a return
> > from recv of '-1' and then check errno, but it never seems to be
> > anything but '11', or EAGAIN.  This seems to be true whether I
> > MSG_PEEK or not.
> >
> > I have included my code below.  The intention is that for recv returns
> > greater than zero, there is a message and I should process it and get
> > ready for the next one.  For recv returns of '0' I should do nothing
> > and for recv returns of '-1' I should handle per errno.
> >
> > Seems easy enough, but no matter what I have tried I can only get a recv
> > return of EAGAIN.
> > ...
> >          rcv_length = recv(threadarg->new_fd,NULL,NULL,MSG_PEEK);
>
> Try passing a buffer and length to recv().  The Cygwin code does not
> attempt to do anything with the socket if buf = NULL and len = 0.  (You
> can look at it yourself, file winsup/cygwin/net.cc, functions
> cygwin_recv() and cygwin_recvfrom().)  How would you ever expect recv()
> to return >0 when you don't give it a buffer to put the data into?

AIUI, recv with MSG_PEEK is supposed to return the length of the waiting
data without putting anything in the buffer.

> The POSIX standard doesn't say anything about the behavior of recv()
> when buf=NULL so what you're trying to do must be some nonstandard quirk
> of other systems' libc.

The SUSv6 page on recv also doesn't mention the possibility of the buffer
being NULL.  I'd suggest to the OP to pass in a dummy 1-character buffer
to recv() with MSG_PEEK, e.g.,

	char c;
	...
        rcv_length = recv(threadarg->new_fd,&c,1,MSG_PEEK);

HTH,
	Igor
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