Subject: Re: libssh2_channel_write_ex() returning 0

Re: libssh2_channel_write_ex() returning 0

From: Nathan Myers <>
Date: Wed, 05 Dec 2012 13:34:48 -0800

I wrote:
> 3. In release 1.4.3, src/channel.c, _libssh2_channel_write(), line 2017:
> return (rc==LIBSSH2_ERROR_EAGAIN?rc:0);
> Here rc is the result from _libssh2_transport_read(session).
> When the output buffer is full, but the input buffer wasn't, this
> returns 0. It seems to me (and please correct me if I am mistaken)
> that libssh2_channel_write_ex() should only return 0 if the buflen
> argument is itself zero. I think that the correct line here would
> be simply
> This fixes an infinite loop in typical fwrite()-like code that
> assumes that (the equivalent of) write() will prefer reporting
> EAGAIN over returning zero.

On a related note... if the output socket gets shut down or otherwise
blocked when the send window is exactly full, it's not clear to me how
libssh2_channel_write_ex() will ever return anything other than 0 or
LIBSSH2_ERROR_EAGAIN. Presumably calling send() would return an error,
allowing a flag (local.eof?) to be set, but how can it ever get called?

Shouldn't _ssh_transport_read() call send_existing()? Would that break
the logjam?

Nathan Myers

Received on 2012-12-05