multitee - Online in the Cloud

PROGRAM:

NAME

multitee - send multiple inputs to multiple outputs

SYNTAX

multitee [ -bsize ] [ -vQq ] [ fd-fd,fd,fd... ] ...

DESCRIPTION

multitee sends multiple inputs to multiple outputs. Given an argument of the form fdin-
fdout,fdout,fdout... it will send all input on file descriptor fdin to each descriptor
fdout. It will exit when all fdin are closed. Several arguments may specify outputs from
the same fdin.

-fdout and ,fdout are equivalent. If there is an error of any sort (including SIGPIPE) in
writing to fdout, multitee prints a warning on stderr and forgets fdout entirely. (This
doesn't affect reads on fdin.) If -fdout is replaced by :fdout then multitee will exit
upon any SIGPIPEs from that descriptor.

Furthermore, efd means that as soon as fdin reaches end of file, fd is considered to reach
EOF as well. multitee will warn about any input errors and then treat them like EOF.

Unlike tee, multitee tries its best to continue processing all descriptors even while some
of them are blocked. However, it will get stuck reading if someone else is reading the
descriptor and grabs the input first; it will get stuck writing if an input packet does
not fit in an output pipe. (If the output descriptor has NDELAY set, and multitee
receives EWOULDBLOCK, it writes one byte at a time to avoid pipe synchronization
problems.) While it is tempting to set the descriptors to non-blocking mode, this is
dangerous: other processes using the same open file may not be able to deal with NDELAY.
It is incredible that none of the major UNIX vendors or standards committees has come up
with true per-process non-blocking I/O. (Under BSD 4.3 and its variants, multitee could
send timer signals to itself rapidly to interrupt any blocking I/O. However, this cannot
work under BSD 4.2, and is generally more trouble than it's worth.) A program can set
NDELAY before invoking multitee if it knows that no other processes will use the same open
file.

multitee will also temporarily stop reading an input descriptor if more than 8192 bytes
are pending on one of its output descriptors. This does not affect independent fdin-fdout
pairs.

multitee has several flags:

-bsize Change input buffer size from 8192 to size. Unlike the previous version of
multitee, this version does not require output buffers, and does not copy
bytes anywhere between read() and write().

-v Verbose.

-q Quiet. multitee will not use stderr in any way (except, of course, if
descriptor 2 is specified in an argument).

-Q Normal level of verbosity.

EXIT VALUE

0 normally. 1 for usage messages. 3 if multitee runs out of memory. 4 in various
impossible situations.

DIAGNOSTICS

fatal: out of memory
multitee has run out of memory.

warning: cannot read descriptor
Self-explanatory.

warning: cannot write descriptor
Self-explanatory.

EXAMPLES

multitee 0-1,4,5 4>foo 5>bar

Same as tee foo bar except for better blocking behavior.

multitee 0:1 3:1 4:1,2 6:7

Merge several sources into the output, meanwhile copying 6 to 7 and recording 4's input in
2.

tcpclient servermachine smtp multitee 0:7 6:1e0

Same as mconnect on Suns. The e0 tells multitee to quit as soon as the network connection
closes.

RESTRICTIONS

multitee expects all descriptors involved to be open. Currently a closed descriptor acts
like an open descriptor which can never be written to.

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