sockdown performs the shutdown(2) system call on one of its file descriptors specified by fd. The possible values for how are
The default fd is 1 (stdout) and the default how is 1.0 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
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 \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.
sh -c " cat blah.txt & cat > blah.ps "
So, we use sockdown to close half of the pipe
client$ hose server 3000 -in -out \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.
sh -c " ( cat blah.txt ; sockdown ) & cat > blah.ps "
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.
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.
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 sockdown 1 2 would do the job.
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