NAME

       nbdkit-captive - run nbdkit under another process and have it reliably cleaned up

SYNOPSIS

        nbdkit PLUGIN [...] --run "CMD ARGS ..."

        nbdkit --exit-with-parent PLUGIN [...]

DESCRIPTION

       You can run nbdkit under another process and have nbdkit reliably clean up.  There are two
       techniques depending on whether you want nbdkit to start the other process ("CAPTIVE
       NBDKIT"), or if you want the other process to start nbdkit ("EXIT WITH PARENT").

CAPTIVE NBDKIT

       You can run nbdkit as a "captive process", using the --run option.  This means that nbdkit
       runs as long as (for example) qemu(1) or guestfish(1) is running.  When those exit, nbdkit
       is killed.

       Some examples should make this clear.

       To run nbdkit captive under qemu:

        nbdkit file disk.img --run 'qemu -drive file=$nbd,if=virtio'

       On the qemu command line, $nbd is substituted automatically with the right NBD path so it
       can connect to nbdkit.  When qemu exits, nbdkit is killed and cleaned up automatically.

       Running nbdkit captive under guestfish:

        nbdkit file disk.img --run 'guestfish --format=raw -a $nbd -i'

       When guestfish exits, nbdkit is killed.

       The following shell variables are available in the --run argument:

       $uri
           A URI that refers to the nbdkit port or socket in the preferred form documented by the
           NBD project.  If nbdkit was started with the -e option to set the preferred export
           name, this is included in the URI.

       $nbd
           An older URL that refers to the nbdkit port or socket in a manner specific to certain
           tools.  This form does not include an export name, even if -e was used.

           Note there is some magic here, since qemu and guestfish URLs have a different format,
           so nbdkit tries to guess which you are running.  If the magic doesn't work, try using
           the variables below instead.

       $port
           If ≠ "", the port number that nbdkit is listening on.

       $unixsocket
           If ≠ "", the Unix domain socket that nbdkit is listening on.

       $exportname
           The default export name (which may be "") that nbdkit will advertise in response to
           NBD_OPT_LIST.  This comes from the -e (--exportname) command line option.

       --run implies --foreground.  It is not possible, and probably not desirable, to have
       nbdkit fork into the background when using --run.

       Even when running captive, nbdkit still listens on the regular TCP/IP port, unless you
       specify the -p/-U options.  If you want a truly private captive nbdkit, then you should
       create a private random Unix socket, like this:

        nbdkit -U - plugin [args] --run '...'

   Copying data in and out of plugins with captive nbdkit
       Captive nbdkit + qemu-img(1) can be used to copy data into and out of nbdkit plugins.  For
       example nbdkit-example1-plugin(1) contains an embedded disk image.  To copy it out:

        nbdkit -U - example1 --run 'qemu-img convert $nbd disk.img'

       If plugin requests have a high overhead (for example making HTTP requests to a remote
       server), adding nbdkit-readahead-filter(1) may help performance:

        nbdkit -U - --filter=readahead curl https://example.com/disk.img \
               --run 'qemu-img convert $nbd disk.img'

       If the source suffers from temporary network failures nbdkit-retry-filter(1) may help.

       To overwrite a file inside an uncompressed tar file (the file being overwritten must be
       the same size), use nbdkit-tar-plugin(1) like this:

        nbdkit -U - tar tar=data.tar file=disk.img \
          --run 'qemu-img convert -n disk.img $nbd'

EXIT WITH PARENT

       The --exit-with-parent option is almost the opposite of "CAPTIVE NBDKIT" described in the
       previous section.

       Running nbdkit with this option, for example from a script:

        nbdkit --exit-with-parent plugin ... &

       means that nbdkit will exit automatically if the parent program exits for any reason.
       This can be used to avoid complicated cleanups or orphaned nbdkit processes.

       --exit-with-parent is incompatible with forking into the background (because when we fork
       into the background we lose track of the parent process).  Therefore -f / --foreground is
       implied.

       This is currently implemented using a non-POSIX feature available in Linux ≥ 2.1.57 and
       FreeBSD ≥ 11.2, so it won't work on other operating systems (patches welcome to make it
       work).

       If the parent application is multithreaded, then (in the Linux implementation) if the
       parent thread exits, that will cause nbdkit to exit.  Thus in multithreaded applications
       you usually want to run "nbdkit --exit-with-parent" only from the main thread (unless you
       actually want nbdkit to exit with the thread, but that may not work reliably on all
       operating systems).

SEE ALSO

       nbdkit(1), prctl(2) (on Linux), procctl(2) (on FreeBSD).

AUTHORS

       Eric Blake

       Richard W.M. Jones

       Pino Toscano

COPYRIGHT

       Copyright (C) 2013-2018 Red Hat Inc.

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