1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
|
.\" $Id: sockdown.html,v 1.4 1998/10/28 16:07:57 thoth Exp $ Copyright 1995 by Robert Forsman
.TH SOCKDOWN 1 "July 7, 1998"
.SH NAME
sockdown \- shutdown(2) a socket
netpipes 4.2
.SH SYNOPSIS
\fBsockdown\fP
[ \fIfd\fP
[\fIhow\fP] ]
.SH DESCRIPTION
\fBsockdown\fP
performs the shutdown(2) system call on one of its file descriptors
specified by \fIfd\fP. The possible values for \fIhow\fP are
.TS H
box;
|lw(0.4i)|lw(2i)|.
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
.TE
The default \fIfd\fP is 1 (stdout) and the default \fIhow\fP is 1.
.SH EXAMPLES
Imagine you have a machine that can perform a service (in this case
conversion from ASCII to fancy postscript) :
.nf
server$ faucet 3000 \-\-in \-\-out enscript \-2rGhp \-
.fi
You may then connect to it with a hose. However, the first example enters deadlock :
.nf
client$ hose server 3000 \-in \-out \\
sh \-c " cat blah.txt & cat > blah.ps "
.fi
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
.nf
client$ hose server 3000 \-in \-out \\
sh \-c " ( cat blah.txt ; sockdown ) & cat > blah.ps "
.fi
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:
.nf
client$ hose server 3000 \-slave < blah.txt > blah.ps
.fi
Ian Stirling <root@mauve.demon.co.uk>
informs me that \fBsockdown\fP can be used in Linux's
/proc/\fIpid\fP/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
\fBstrace\fP 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:
.nf
bash# sockdown 1 2 > /proc/991/fd/5
.fi
.SH ERRORS
\fBSocket operation on non\-socket\fP
\fBInvalid argument (seen on Solaris)\fP
The \fIfd\fP 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.
\fBBad file number\fP
You gave it a bad file number for \fIfd\fP. 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.
.SH SEE ALSO
netpipes (1)
faucet (1),
hose (1),
getpeername (1),
socket (2),
shutdown (2),
.SH 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 \fBsockdown\fP 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 \fBshutdown \-a\fP semantics on hose since
a \fBsockdown 1 2\fP would do the job.
.SH CREDITS
Ian Stirling <root@mauve.demon.co.uk>,
for the idea of using this program in /proc on a Linux machine.
.SH 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
MERCHANTABILITY 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.
.SH AUTHOR
Robert Forsman
thoth@purplefrog.com
Purple Frog Software
http://web.purplefrog.com/~thoth/
|
