228 lines
8.9 KiB
XML
228 lines
8.9 KiB
XML
<?xml version="1.0" encoding="utf-8" ?>
|
||
<!DOCTYPE article PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
|
||
"http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd">
|
||
<refentry id="binding">
|
||
|
||
|
||
<para>
|
||
<refmeta> <refentrytitle>How to write bindings</refentrytitle> <manvolnum>3</manvolnum> <refmiscinfo>libvips</refmiscinfo> </refmeta>
|
||
</para>
|
||
<para>
|
||
<refnamediv> <refname>Binding</refname> <refpurpose>Writing bindings for libvips</refpurpose> </refnamediv>
|
||
</para>
|
||
<para>
|
||
There are full libvips bindings for quite a few environments now: C, C++, command-line, Ruby, PHP, Lua, Python and JavaScript (node).
|
||
</para>
|
||
<para>
|
||
This chapter runs through the four main styles that have been found to work well. If you want to write a new binding, one of these should be close to what you need.
|
||
</para>
|
||
<refsect3 xml:id="dont-bind-the-top-level-c-api">
|
||
<title>Don’t bind the top-level C API</title>
|
||
<para>
|
||
The libvips C API (vips_add() and so on) was designed to be easy for humans to write. It is inconvenient and dangerous to use from other languages due to its heavy use of varargs.
|
||
</para>
|
||
<para>
|
||
It’s much better to use the layer below. This lower layer is structured as:
|
||
</para>
|
||
<itemizedlist>
|
||
<listitem>
|
||
<para>
|
||
Create operator. You can use vips_operation_new() to make a new <literal>VipsOperation</literal> object from an operator nickname, like <literal>"add"</literal>.
|
||
</para>
|
||
</listitem>
|
||
<listitem>
|
||
<para>
|
||
Set parameters. You can loop over the operation with vips_argument_map() to get the name and type of each input argument. For each argument, you need to get the value from your language, convert to a <literal>GValue</literal>, then use g_object_set_property() to set that value on the operator.
|
||
</para>
|
||
</listitem>
|
||
<listitem>
|
||
<para>
|
||
Execute with vips_cache_operation_build().
|
||
</para>
|
||
</listitem>
|
||
<listitem>
|
||
<para>
|
||
Extract results. Again, you loop over the operator arguments with vips_argument_map(), but instead of inputs, this time you look for output arguments. You extract their value with g_object_get_property(), and pass the value back to your language.
|
||
</para>
|
||
</listitem>
|
||
</itemizedlist>
|
||
<para>
|
||
For example, you can execute vips_invert() like this:
|
||
</para>
|
||
<programlisting language="c">
|
||
/* compile with
|
||
*
|
||
* gcc -g -Wall callvips.c `pkg-config vips --cflags --libs`
|
||
*
|
||
*/
|
||
|
||
#include <vips/vips.h>
|
||
|
||
int
|
||
main( int argc, char **argv )
|
||
{
|
||
VipsImage *in;
|
||
VipsImage *out;
|
||
VipsOperation *op;
|
||
VipsOperation *new_op;
|
||
GValue gvalue = { 0 };
|
||
|
||
if( VIPS_INIT( argv[0] ) )
|
||
/* This shows the vips error buffer and quits with a fail exit
|
||
* code.
|
||
*/
|
||
vips_error_exit( NULL );
|
||
|
||
/* This will print a table of any ref leaks on exit, very handy for
|
||
* development.
|
||
*/
|
||
vips_leak_set( TRUE );
|
||
|
||
if( argc != 3 )
|
||
vips_error_exit( "usage: %s input-filename output-filename",
|
||
argv[0] );
|
||
|
||
if( !(in = vips_image_new_from_file( argv[1], NULL )) )
|
||
vips_error_exit( NULL );
|
||
|
||
/* Create a new operator from a nickname. NULL for unknown operator.
|
||
*/
|
||
op = vips_operation_new( "invert" );
|
||
|
||
/* Init a gvalue as an image, set it to in, use the gvalue to set the
|
||
* operator property.
|
||
*/
|
||
g_value_init( &gvalue, VIPS_TYPE_IMAGE );
|
||
g_value_set_object( &gvalue, in );
|
||
g_object_set_property( G_OBJECT( op ), "in", &gvalue );
|
||
g_value_unset( &gvalue );
|
||
|
||
/* We no longer need in: op will hold a ref to it as long as it needs
|
||
* it.
|
||
*/
|
||
g_object_unref( in );
|
||
|
||
/* Call the operation. This will look up the operation+args in the vips
|
||
* operation cache and either return a previous operation, or build
|
||
* this one. In either case, we have a new ref we must release.
|
||
*/
|
||
if( !(new_op = vips_cache_operation_build( op )) ) {
|
||
g_object_unref( op );
|
||
vips_error_exit( NULL );
|
||
}
|
||
g_object_unref( op );
|
||
op = new_op;
|
||
|
||
/* Now get the result from op. g_value_get_object() does not ref the
|
||
* object, so we need to make a ref for out to hold.
|
||
*/
|
||
g_value_init( &gvalue, VIPS_TYPE_IMAGE );
|
||
g_object_get_property( G_OBJECT( op ), "out", &gvalue );
|
||
out = VIPS_IMAGE( g_value_get_object( &gvalue ) );
|
||
g_object_ref( out );
|
||
g_value_unset( &gvalue );
|
||
|
||
/* All done: we can unref op. The output objects from op actually hold
|
||
* refs back to it, so before we can unref op, we must unref them.
|
||
*/
|
||
vips_object_unref_outputs( VIPS_OBJECT( op ) );
|
||
g_object_unref( op );
|
||
|
||
if( vips_image_write_to_file( out, argv[2], NULL ) )
|
||
vips_error_exit( NULL );
|
||
|
||
g_object_unref( out );
|
||
|
||
return( 0 );
|
||
}
|
||
</programlisting>
|
||
</refsect3>
|
||
<refsect3 xml:id="compiled-language-which-can-call-c">
|
||
<title>Compiled language which can call C</title>
|
||
<para>
|
||
The C++ binding uses this lower layer to define a function called <literal>VImage::call()</literal> which can call any libvips operator with a set of variable arguments.
|
||
</para>
|
||
<para>
|
||
A small Python program walks the set of all libvips operators and generates a set of static bindings. For example:
|
||
</para>
|
||
<programlisting language="c++">
|
||
VImage VImage::invert( VOption *options )
|
||
{
|
||
VImage out;
|
||
|
||
call( "invert", (options ? options : VImage::option()) ->
|
||
set( "in", *this ) ->
|
||
set( "out", &out ) );
|
||
|
||
return( out );
|
||
}
|
||
</programlisting>
|
||
<para>
|
||
So from C++ you can call any libvips operator (though without static typechecking) with <literal>VImage::call()</literal>, or use the member functions on <literal>VImage</literal> to get type-checked calls for at least the required operator arguments.
|
||
</para>
|
||
<para>
|
||
The <literal>VImage</literal> class also adds automatic reference counting, constant expansion, operator overloads, and various other useful features.
|
||
</para>
|
||
</refsect3>
|
||
<refsect3 xml:id="dynamic-language-with-ffi">
|
||
<title>Dynamic language with FFI</title>
|
||
<para>
|
||
Languages like Ruby, Python, JavaScript and LuaJIT can’t call C directly, but they do support FFI. The bindings for these languages work rather like C++, but use FFI to call into libvips and run operations.
|
||
</para>
|
||
<para>
|
||
Since these languages are dynamic, they can add another trick: they intercept the method-missing hook and attempt to run any method calls not implemented by the <literal>Image</literal> class as libvips operators. In effect, the binding is generated at runtime.
|
||
</para>
|
||
</refsect3>
|
||
<refsect3 xml:id="dynamic-language-without-ffi">
|
||
<title>Dynamic language without FFI</title>
|
||
<para>
|
||
PHP does not have a useful FFI, unfortunately, so for this language a small C module implements the general <literal>vips_call()</literal> function for PHP language types, and a larger pure PHP layer makes it convenient to use.
|
||
</para>
|
||
</refsect3>
|
||
<refsect3 xml:id="gobject-introspection">
|
||
<title>gobject-introspection</title>
|
||
<para>
|
||
The C source code to libvips has been marked up with special comments describing the interface in a standard way. These comments are read by the <literal>gobject-introspection</literal> package when libvips is compiled and used to generate a typelib, a description of how to call the library. Many languages have gobject-introspection packages: all you need to do to call libvips from your favorite language is to start g-o-i, load the libvips typelib, and you should have the whole library available. For example, from Python it’s as simple as:
|
||
</para>
|
||
<programlisting language="python">
|
||
from gi.repository import Vips
|
||
</programlisting>
|
||
<para>
|
||
You can now use all of the libvips introspection machinery, as noted above.
|
||
</para>
|
||
<para>
|
||
Unfortunately g-o-i has some strong disadvantages. It is not very portable, since you will need a g-o-i layer for whatever platform you are targetting; it does not cross-compile well, since typelibs include a lot of very-low level data (such as exact structure layouts); and installation for your users is likely to be tricky.
|
||
</para>
|
||
<para>
|
||
If you have a choice, I would recommend simply using FFI.
|
||
</para>
|
||
</refsect3>
|
||
<refsect3 xml:id="documentation">
|
||
<title>Documentation</title>
|
||
<para>
|
||
You can generate searchable docs from a <code>.gir</code> (the thing that is built from scanning libvips and which in turn turn the typelib is made from) with <command>g-ir-doc-tool</command>, for example:
|
||
</para>
|
||
<programlisting>
|
||
$ g-ir-doc-tool --language=Python -o ~/mydocs Vips-8.0.gir
|
||
</programlisting>
|
||
<para>
|
||
Then to view them, either:
|
||
</para>
|
||
<programlisting>
|
||
$ yelp ~/mydocs
|
||
</programlisting>
|
||
<para>
|
||
Or perhaps:
|
||
</para>
|
||
<programlisting>
|
||
$ cd ~/mydocs
|
||
$ yelp-build html .
|
||
</programlisting>
|
||
<para>
|
||
To make HTML docs. This is an easy way to see what you can call in the library.
|
||
</para>
|
||
</refsect3>
|
||
|
||
|
||
</refentry>
|