sockdown - Online in the Cloud

PROGRAM:

NAME

sockdown - shutdown(2) a socket

netpipes 4.2

SYNOPSIS

sockdown [ fd [how] ]

DESCRIPTION

sockdown 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 more reads or writes are allowed │
│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 finished, 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 special 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 descriptor 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.

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