SOCKDOWN(1)SOCKDOWN(1)NAMEsockdown - shutdown(2) a socket
netpipes 4.2
SYNOPSISsockdown [ fd [how] ]
DESCRIPTIONsockdown performs the shutdown(2) system call on one of its file
descriptors specified by fd. The possible values for how are
│ │ │
│writeonly │ convert to write-only file descriptor │
│0 │ convert to write-only file descriptor │
│writeonly │ symbolic for same as above │
│1 │ convert to read-only file descriptor │
│readonly │ symbolic for same as above │
│2 │ complete shutdown. no reads or writes allowed in the future │
│totally │ symbolic for same as above │
The default fd is 1 (stdout) and the default how is 1.
EXAMPLES
Imagine you have a machine that can perform a service (in this case
conversion from ASCII to fancy postscript) :
server$ faucet 3000 --in --out enscript -2rGhp -
You may then connect to it with a hose. However, the first example
enters deadlock :
client$ hose server 3000 -in -out \
sh -c " cat blah.txt & cat > blah.ps "
The enscript blocks waiting for input from the socket because not all
of the client processes have exited. While the cat blah.txt is fin‐
ished, the cat > blah.ps is not, and will not be finished until the
remote enscript process finishes writing. The enscript process will
not finish writing until it is finished reading, but that
client->server half of the socket is still open and will not be closed
until all the client processes are done. The result is deadlock.
So, we use sockdown to close half of the pipe
client$ hose server 3000 -in -out \
sh -c " ( cat blah.txt ; sockdown ) & cat > blah.ps "
This way when the cat blah.txt is done, half of the socket is shut down
and the remote enscript process runs out of input, causing it to flush
its output and exit, so eventually the whole mess finishes cleanly.
Note: the & on the hose is necessary to prevent another deadlock. If
we simply used the ; to serialize the two cat processes it is possible
that the enscript would fill up its write buffer before the first cat
was done causing both processes to block and preventing the second cat
from draining the pipe.
Of course, that idiomatic usage of hose is so useful that it is a spe‐
cial form:
client$ hose server 3000 -slave < blah.txt > blah.ps
Ian Stirling <root@mauve.demon.co.uk> informs me that sockdown can be
used in Linux's /proc/pid/fd/ directories to tear down hung network
connections. I have since used this myself on a wedged MOMspider. To
try this, you have to know the PID of the program and the file descrip‐
tor of the wedged socket (can sometimes be found by running strace and
see if the program is stuck in a read(2) system call). If the PID is
991 and the socket's descriptor is 5, you do this as root:
bash# sockdown 1 2 > /proc/991/fd/5
ERRORS
Socket operation on non-socket
Invalid argument (seen on Solaris) The fd you specified does not refer
to a socket. This happens when you run sockdown by itself (it is
unlikely that any of the file descriptors attached to an interactive
shell are actually sockets) or if you goof up your faucet/hose command
and forgot to dup(2) one of your descriptors.
Bad file number You gave it a bad file number for fd. If you have
enough skill to actually generate this error, you probably know what is
wrong.
If you encounter any other errors, clue me in.
SEE ALSO
netpipes (1) faucet (1), hose (1), getpeername (1), socket (2), shut‐
down (2),
NOTES
Any normal human would assume a program this simple has to be bug free,
but I am an experienced programmer.
Just avoid doing anything funky like passing sockdown strings and it
should serve you well. You should not have to pass it any arguments
unless you are doing something fairly funky.
Perhaps I should ditch the shutdown -a semantics on hose since a sock‐
down 1 2 would do the job.
CREDITS
Ian Stirling <root@mauve.demon.co.uk>, for the idea of using this pro‐
gram in /proc on a Linux machine.
COPYRIGHT
Copyright (C) 1995-1998 Robert Forsman
This program is free software; you can redistribute it and/or modify it
under the terms of the GNU General Public License as published by the
Free Software Foundation; either version 2 of the License, or (at your
option) any later version.
This program is distributed in the hope that it will be useful, but
WITHOUT ANY WARRANTY; without even the implied warranty of MER‐
CHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General
Public License for more details.
You should have received a copy of the GNU General Public License along
with this program; if not, write to the Free Software Foundation, Inc.,
675 Mass Ave, Cambridge, MA 02139, USA.
AUTHOR
Robert Forsman
thoth@purplefrog.com
Purple Frog Software
http://web.purplefrog.com/~thoth/
July 7, 1998 SOCKDOWN(1)
