nbdkit-cc-plugin - Man Page

write small nbdkit plugins in inline C (and other languages)

Synopsis

 nbdkit cc /path/to/plugin.c [CC=<CC>] [CFLAGS=<CFLAGS>]
                             [EXTRA_CFLAGS=<EXTRA_CFLAGS>]
 nbdkit cc - <<'EOF'
 ... C code ...
 EOF

Description

This plugin allows you to write small nbdkit(1) plugins in C (and some other languages).  When you use this plugin it compiles your source code to a temporary plugin and then jumps into your compiled plugin.  It is somewhat similar to nbdkit-sh-plugin(3), except for C source code.  This can also be used to write plugins which are “C scripts”.

Note this is not the way you normally write nbdkit plugins in C. To understand how to write plugins in C normally, read nbdkit-plugin(3).

Simple plugin example

Simple plugins from the nbdkit source tree can be compiled and run directly using commands such as:

 $ nbdkit cc plugins/example1/example1.c EXTRA_CFLAGS="-I. -Iinclude"

You can also read the source from stdin using -:

 $ nbdkit cc - EXTRA_CFLAGS="-I. -Iinclude" \
             < plugins/example1/example1.c

To replace the compiler flags:

 $ nbdkit cc plugins/example1/example1.c \
             CFLAGS="-O3 -mavx2 -fPIC -shared"

Compiler name and flags

The plugin parameters CC, CFLAGS and EXTRA_CFLAGS (written in uppercase) can be used to control which C compiler and C compiler flags are used.  If not set, the default compiler and flags from when nbdkit was itself compiled from source are used.  To see what those were you can do:

 $ nbdkit cc --dump-plugin
 ...
 CC=gcc
 CFLAGS=-g -O2 -fPIC -shared

The CFLAGS parameter overrides the built-in flags completely.  The EXTRA_CFLAGS parameter adds extra flags to the built-in flags.

Plugin API version

Plugins compiled this way must use the same API version as the cc plugin itself uses.  Currently this is NBDKIT_API_VERSION=2.

C plugin as a self-contained script

You can create a C plugin which is a self-contained script by adding the following lines at the top and ensuring the C source is executable (chmod +x plugin.c):

 #if 0
 exec nbdkit cc "$0" "$@"
 #endif

The script can be run as a command with additional nbdkit flags and plugin parameters, eg:

 ./plugin.c -f -v
 ./plugin.c -p 10000 --filter=cow
 ./plugin.c param=1

Using this plugin with other programming languages

This plugin can be used with most ahead-of-time compiled programming languages if they can create shared objects (.so files).  The only requirement is that the compiler (CC) supports an -o option to write a shared object.  Some examples follow.

Using this plugin with C++

 nbdkit cc CC=g++ source.cpp

Using this plugin with OCaml

 nbdkit cc CC=ocamlopt \
           CFLAGS="-output-obj -runtime-variant _pic NBDKit.cmx -cclib -lnbdkitocaml" \
           source.ml

See nbdkit-ocaml-plugin(3).

Parameters

The script name, or -, must appear as the first parameter.

CC=CC
CFLAGS=“CFLAGS
EXTRA_CFLAGS=“EXTRA_CFLAGS

Override the compiler and flags.  See “Compiler name and flags” above.

All other parameters on the command line are passed to the plugin.

Example

 $ nbdkit cc - <<'EOF'
 #include <stdint.h>
 #include <string.h>
 
 #define NBDKIT_API_VERSION 2
 #include <nbdkit-plugin.h>
 
 char data[10*1024*1024];
 
 #define THREAD_MODEL NBDKIT_THREAD_MODEL_PARALLEL
 
 static void *
 my_open (int readonly)
 {
   return NBDKIT_HANDLE_NOT_NEEDED;
 }
 
 static int64_t
 my_get_size (void *handle)
 {
   return (int64_t) sizeof (data);
 }
 
 static int
 my_pread (void *handle, void *buf,
           uint32_t count, uint64_t offset,
           uint32_t flags)
 {
   memcpy (buf, data+offset, count);
   return 0;
 }
 
 static int
 my_pwrite (void *handle, const void *buf,
            uint32_t count, uint64_t offset,
            uint32_t flags)
 {
   memcpy (data+offset, buf, count);
   return 0;
 }
 
 static struct nbdkit_plugin plugin = {
   .name              = "myplugin",
   .open              = my_open,
   .get_size          = my_get_size,
   .pread             = my_pread,
   .pwrite            = my_pwrite,
 };
 
 NBDKIT_REGISTER_PLUGIN(plugin)
 EOF

Files

$plugindir/nbdkit-cc-plugin.so

The plugin.

Use nbdkit --dump-config to find the location of $plugindir.

Version

nbdkit-cc-plugin first appeared in nbdkit 1.22.

See Also

nbdkit(1), nbdkit-plugin(3), nbdkit-sh-plugin(3).

Authors

Richard W.M. Jones

License

Redistribution and use in source and binary forms, with or without modification, are permitted provided that the following conditions are met:

THIS SOFTWARE IS PROVIDED BY RED HAT AND CONTRIBUTORS ''AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL RED HAT OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

Referenced By

nbdkit(1), nbdkit-plugin(3), nbdkit-release-notes-1.22(1).

2021-04-03 nbdkit-1.25.5