84 lines
2.5 KiB
Text
84 lines
2.5 KiB
Text
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Thoughts about different UNIX-IPC,
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Nico Schottelius, 2005-04-28 (Last Modified: 2005-06-11)
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Howto cinit communicates with
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a) cinit-forks
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b) any other program
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a) cinit-forks
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a.1) shared memory
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Access could generally be done with shared memory. The problem with
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that is, that -lrt is required, which implicits -lpthread, which
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makes linking staticly problematic (at least for glibc).
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a.2) pipes
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A maximum of ~510 pipes could be used ( (1024-3)/2 ),
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1024 is the maximum opened file deskriptors,
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minus 3 (stdin/stdout/stderr) already opened, divided
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by two, because two fds needed per client.
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Pipes cannot be used easily through forks of forks.
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a.3) fifos
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FIFOs are easy to use, but you would need to create two FIFOs
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for _every_ service, as with only two FIFOs we cannot reliable
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detect, _who_ is writting to us currently and who wants to read.
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The same limit as above is true for fifo, too: A maximum of ~510
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services.
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a.4) system-v-ipc
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No documentation found nor tested.
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a.5) sockets
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Are indeed a very clean way. There's only one problem:
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bind() fails on read-only mounted devices:
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- The socket either does not exists and cannot be created
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- or the socket exists, but bind() refuses to reuse it
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(error: Address already in use)
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Imho bind() should even honour the socket-option SO_REUSEADDR,
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which allows to re-use a socket, if there's no other program bound to.
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As far as I can see, SO_REUSEADDR is only honoured, if socket is of
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type PF_INET (we use PF_UNIX) and POSIX does only specify how to
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check for support, but not that sockets have to be able to use
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SO_REUSEADDR (see posix.aardvark.bug.report).
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That way, we are forced to mount a temporarily filesystem on
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/etc/cinit/tmp and create the socket below this directory. This is not
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the clean and easy solution one would wish. Still, sockets
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seem to be the cleanest and most reliable way to have IPC for
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this situation.
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See socket(2), bind(2), listen(2), accept(2), socket(7) and unix(7)
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for help.
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b) any other program
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What you can do is to tell cinit to
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- reboot,
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- halt,
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- poweroff,
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- update itself (hot-reboot)
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- and to start a rescue mode (see using.rescue).
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Simple send cinit a signal, what to do:
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HUP: reboot
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USR1: halt
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USR2: rescue
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TERM: poweroff
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CONT: update
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See signal(2) and signal(7) for help and serv/sig_reboot.c for implementation.
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