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Re: kern/45352: pty(4)/tty(4) have a 1024 bytes transfer limit

The following reply was made to PR kern/45352; it has been noted by GNATS.

From: Matthew Mondor <>
Subject: Re: kern/45352: pty(4)/tty(4) have a 1024 bytes transfer limit
Date: Sun, 25 Sep 2011 05:39:57 -0400

 On Fri, 23 Sep 2011 15:43:04 -0400 (Christos Zoulas) wrote:
 > I have updated the patch on
 I had some trouble with the patch then noticed that there were actually
 -current commits; so reverted to the -current files, then rebuilt a
 kernel and booted it.
 The sysctl setting works great, and with an actual MTU of 3000 this
 time, performance was over 800KB/s for the same file and my pty-udp
 software, vs the max ~170KB/s I was getting with the 1024 bytes limit!
 However, there still seems to be a problem with the ioctl(2), as that
 one doesn't seem to have an effect, somehow:
         opt = (int)buffer_size;
         if (ioctl(fd, TIOCSQSIZE, &opt) == -1)
                 err(EXIT_FAILURE, "Couldn't set tty(4) buffer size");
 There is no error but the reads are still reaching the sysctl-set limit
 despite trying to set a larger buffer (that's of course still between
 1024 and 65536).
 Also, would there be a problem with setting the tty buffer size to 4096
 by default, rather than 1024?  This would then match the default Linux
 limit, and allow tools like rp-ppoe with common MTU sizes to work
 properly as-is as well.
 BTW, for those who wondered how rp-pppoe could have worked, note that
 it was previously possible with our 1024 limit, if not exceeding it too
 much (write size), to read in non-blocking mode until EAGAIN to obtain
 all bytes, yet at the expense of more syscalls and I/O, and the
 occasional loss of a few ending bytes depending on packet size.  I've
 not checked the rp-pppoe code in a while, but it might have done just
 Interestingly, when doing earlier searches about the problem I was
 having before posting the initial PR, OS/X would also have the 1024
 limit, and Solaris would have an even worse 255 limit; I'm not sure if
 those are defaults or hard-coded, though.
 Thanks again,

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