custom libvips source and target in C

custom.c

/* Compile with:
 *
 * 	gcc -g -Wall custom.c `pkg-config vips --cflags --libs`
 *
 * Run with:
 *
 * 	./a.out ~/pics/k2.jpg x.jpg
 *
 */

#include <vips/vips.h>

static gint64 
my_read( VipsSourceCustom *source, void *buffer, gint64 length, void *user )
{
	FILE *in = (FILE *) user;

	size_t items;

	printf( "my_read: length = %zd\n", length );
	items = fread( buffer, sizeof( char ), length, in );
	printf( "    items = %zd\n", items );

	return( items );
}

static gint64 
my_seek( VipsSourceCustom *source, gint64 pos, int whence, void *user )
{
	FILE *in = (FILE *) user;

	long new_pos;

	printf( "my_seek: pos = %zd, whence = %d\n", pos, whence );
	if( fseek( in, pos, whence ) )
		return( -1 );
	new_pos = ftell( in );
	printf( "    new_pos = %zd\n", new_pos );

	return( new_pos );
}

static gint64 
my_write( VipsTargetCustom *target, const void *buffer, gint64 length, 
	void *user )
{
	FILE *out = (FILE *) user;

	size_t items;

	printf( "my_write: length = %zd\n", length );
	items = fwrite( buffer, sizeof( char ), length, out );
	printf( "    items = %zd\n", items );

	return( items );
}

static void
my_finish( VipsTargetCustom *target, void *user )
{
	FILE *out = (FILE *) user;

	printf( "my_finish:\n" );
	fclose( out );
}

int
main( int argc, char **argv )
{
	FILE *in;
	FILE *out;
	VipsSourceCustom *source;
	VipsTargetCustom *target;
	VipsImage *image;

	source = vips_source_custom_new();
	if( !(in = fopen( argv[1], "rb" )) ) 
		vips_error_exit( "unable to read %s", argv[1] );
	g_signal_connect( source, "read", G_CALLBACK( my_read ), in );
	g_signal_connect( source, "seek", G_CALLBACK( my_seek ), in );

	if( !(image = vips_image_new_from_source( VIPS_SOURCE( source ), "",
		"access", VIPS_ACCESS_SEQUENTIAL,
		NULL )) )
		vips_error_exit( "unable to read from %s as image", argv[1] );

	target = vips_target_custom_new();
	if( !(out = fopen( argv[2], "wb" )) ) 
		vips_error_exit( "unable to write %s", argv[2] );
	g_signal_connect( target, "write", G_CALLBACK( my_write ), out );
	g_signal_connect( target, "finish", G_CALLBACK( my_finish ), out );

	if( vips_image_write_to_target( image, ".jpg", VIPS_TARGET( target ), 
		"Q", 90,
		NULL ) )
		vips_error_exit( "unable to write to %s as image", argv[2] );

	g_object_unref( image );
	g_object_unref( source );
	g_object_unref( target );
	fclose( in );

	/* out is fclose()ed in finish.
	 */

	return( 0 );
}