-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:
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.