1nbdkit-ocaml-plugin(3) NBDKIT nbdkit-ocaml-plugin(3)
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6 nbdkit-ocaml-plugin - writing nbdkit plugins in OCaml
7
9 nbdkit /path/to/plugin.so [arguments...]
10 nbdkit plugin [arguments...]
12
14 This manual page describes how to write nbdkit plugins in natively
15 compiled OCaml code. This requires OCaml ≥ 4.03.
16
18 For an example plugin written in OCaml, see:
19 https://gitlab.com/nbdkit/nbdkit/blob/master/plugins/ocaml/example.ml
20 Broadly speaking, OCaml nbdkit plugins work like C ones, so you should
22 read nbdkit-plugin(3) first.
23 You should also look at NBDKit(3) which describes the plugin interface
25 for OCaml plugins.
26 Your OCaml code should call "NBDKit.register_plugin" like this when the
28 plugin is loaded:
29 NBDKit.register_plugin
31 ~name: "myplugin"
32 ~version: "1.0"
33 ~open_connection: myplugin_open
34 ~get_size: myplugin_get_size
35 ~pread: myplugin_pread
36 ~thread_model:
37 (fun () -> NBDKit.THREAD_MODEL_SERIALIZE_ALL_REQUESTS)
38 (* etc *)
39 ()
40 "~name", "~open_connection", "~get_size" and "~pread" are required.
42 All other parameters are optional.
43 Compiling an OCaml nbdkit plugin
45 OCaml nbdkit plugins are natively compiled into shared object ("*.so")
46 files which nbdkit loads like regular C plugins.
47 After writing your OCaml plugin ("myplugin.ml"), compile and link it
49 using this command:
50 ocamlopt.opt -output-obj -runtime-variant _pic \
52 -o nbdkit-myplugin-plugin.so \
53 unix.cmxa NBDKit.cmx myplugin.ml \
54 -cclib -lnbdkitocaml
55 You can then use "nbdkit-myplugin-plugin.so" as an nbdkit plugin (see
57 nbdkit(1), nbdkit-plugin(3)):
58 nbdkit ./nbdkit-myplugin-plugin.so [args ...]
60 or if the ".so" file is installed in the $plugindir directory:
62 nbdkit myplugin [args ...]
64 Handle
66 Your "open_connection" callback can return an OCaml value of any type.
67 The same value is passed back to the per-connection callbacks like
68 "get_size" and "pread".
69 Typically (although this is not a requirement) you define your own
71 handle struct in your plugin:
72 type handle = {
74 (* any per-connection data you want to store goes here *)
75 h_id : int; (* this is just an example field *)
76 h_readonly : bool;
77 }
78 let id = ref 0
80 let myplugin_open readonly =
81 (* return a newly allocated handle *)
82 incr id;
83 { h_id = !id; h_readonly = readonly }
84 let myplugin_get_size handle =
86 printf "handle ID = %d\n" handle.h_id;
87 (* ... *)
88 If you don't need to store per-connection data, "open_connection" can
90 return "()".
91 Errors
93 Plugins can return errors from methods by raising an exception.
94 If you need to control which errno is sent back to the client you have
96 to call "NBDKit.set_error" before raising the exception.
97 Note if you call some function in the OCaml "Unix" module or another
99 library which fails, then the errno of the failing system call will not
100 be returned to the client. You have to catch the exception and call
101 "NBDKit.set_error" before re-raising the exception if you need to
102 control this.
103 Threads
105 One of the optional parameters of "NBDKit.register_plugin" is
106 "~thread_model", which must return one of the values in the table
107 below:
108 "NBDKit.THREAD_MODEL_SERIALIZE_CONNECTIONS"
110 "NBDKit.THREAD_MODEL_SERIALIZE_ALL_REQUESTS"
111 "NBDKit.THREAD_MODEL_SERIALIZE_REQUESTS"
112 "NBDKit.THREAD_MODEL_PARALLEL"
113 If this optional parameter is not provided, the thread model defaults
115 to "NBDKit.THREAD_MODEL_PARALLEL". The garbage collector lock is
116 acquired when calling into OCaml code and because of this callbacks are
117 never truly concurrent.
118 For more information on thread models, see "THREADS" in
120 nbdkit-plugin(3).
121 Debugging
123 You can add debugging messages which are printed only when nbdkit is in
124 verbose mode by calling:
125 NBDKit.debug fs [...]
127 This function works like "Printf.printf".
129 OCaml scripts
131 Using nbdkit-cc-plugin(1) it is possible to write OCaml plugins which
132 are compiled just before use, and so appear to work more like scripts.
133
135 OCaml plugins first appeared in nbdkit 1.2.
136
138 NBDKit(3), nbdkit(1), nbdkit-plugin(3), ocamlopt(1).
139
141 Richard W.M. Jones
142
144 Copyright (C) 2014-2021 Red Hat Inc.
145
147 Redistribution and use in source and binary forms, with or without
148 modification, are permitted provided that the following conditions are
149 met:
150 • Redistributions of source code must retain the above copyright
152 notice, this list of conditions and the following disclaimer.
153 • Redistributions in binary form must reproduce the above copyright
155 notice, this list of conditions and the following disclaimer in the
156 documentation and/or other materials provided with the
157 distribution.
158 • Neither the name of Red Hat nor the names of its contributors may
160 be used to endorse or promote products derived from this software
161 without specific prior written permission.
162 THIS SOFTWARE IS PROVIDED BY RED HAT AND CONTRIBUTORS ''AS IS'' AND ANY
164 EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
165 IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
166 PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL RED HAT OR CONTRIBUTORS BE
167 LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
168 CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
169 SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR
170 BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY,
171 WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR
172 OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF
173 ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
174nbdkit-1.32.5 2023-01-03 nbdkit-ocaml-plugin(3)