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<refentry id="binding.md">
<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 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) is very 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 create operator, set parameters, execute, extract results. 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 mst 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>
<para>
libvips has a couple of extra things to let you examine the arguments and types of an operator at runtime. Use vips_lib.vips_argument_map() to loop over all the arguments of an operator, and vips_object_get_argument() to fetch the type and flags of a specific argument.
</para>
<para>
Use vips_operation_get_flags() to get general information about an operator.
</para>
</refsect3>
<refsect3 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 not-varargs 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 type-safety) with <literal>VImage::call()</literal>, or use the member functions on <literal>VImage</literal> to get type-safe 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 id="dynamic-language-with-ffi">
<title>Dynamic language with FFI</title>
<para>
Languages like Ruby, Python, JavaScript and Lua 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. This makes these bindings self-writing: they only contain a small amount of code and just expose everything they find in the libvips class hierarchy.
</para>
</refsect3>
<refsect3 id="dynamic-langauge-without-ffi">
<title>Dynamic langauge without FFI</title>
<para>
PHP does not have FFI, unfortunately, so for this language a small native 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 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 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>