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