fix up C docs intro
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John Cupitt
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@ -15,7 +15,7 @@
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<refpurpose>How to use the VIPS library from C</refpurpose>
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</refnamediv>
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<refsect1 id="using-C">
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<refsect1 id="using-C-intro">
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<title>Introduction</title>
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<para>
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VIPS comes with a convenient, high-level C API. You should read the API
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@ -23,9 +23,12 @@
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overview. The <command>vips</command> program is handy for getting a
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summary of an operation's parameters.
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</para>
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</refsect1>
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<refsect1 id="using-C-startup">
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<title>Library startup</title>
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<para>
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When your program starts, use <code>VIPS_INIT()</code>
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When your program starts, use VIPS_INIT()
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to start up the VIPS library. You should pass it the name
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of your program, usually <code>argv[0]</code>. Use
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vips_shutdown() when you exit.
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@ -33,17 +36,20 @@
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<para>
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You can add the VIPS flags to your %GObject command-line processing
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with <code>vips_get_option_group()</code>, see below.
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with vips_add_option_entries(), see below.
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</para>
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</refsect1>
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<refsect1 id="using-C-vipsimage">
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<title>The #VipsImage class</title>
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<para>
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The basic data object is the #VipsImage. You can create an
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image from a file on disc or from an area of memory, either
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as a C-style array, or as a formatted object, like JPEG. See
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vips_image_new_from_file() and friends. Loading an
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image is fast. VIPS read just enough of the image to be able to get
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the various properties, such as width in pixels. It delays reading
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any pixels until they are really needed.
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image is fast: VIPS read just enough of the image to be able to get
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the various properties, such as width, but no decoding occurs until
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pixel values are really needed.
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</para>
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<para>
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@ -54,9 +60,12 @@
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See <link linkend="libvips-header">VIPS Header</link> to read about
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image properties.
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</para>
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</refsect1>
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<refsect1 id="using-C-ref">
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<title>Reference counting</title>
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<para>
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VIPS is based on the %GObject library and is therefore refcounted.
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VIPS is based on the %GObject library and is therefore reference counted.
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vips_image_new_from_file() returns an object with a count of 1.
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When you are done with an image, use g_object_unref() to dispose of it.
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If you pass an image to an operation and that operation needs to keep a
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@ -66,25 +75,45 @@
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</para>
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<para>
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Use things like vips_invert() to manipulate your images. See <link
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linkend="VipsOperation">VIPS Operations</link> for information on
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See #VipsOperation for more detail on VIPS
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reference counting conventions.
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</para>
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</refsect1>
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<refsect1 id="using-C-operations">
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<title>VIPS operations</title>
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<para>
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Use things like vips_invert() to manipulate your images. See
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#VipsOperation
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for information on
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running operations on images. When you are done, you can write
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the final image to a disc file, to a formatted memory buffer, or to
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C-style memory array. See vips_image_write_to_file() and friends.
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</para>
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</refsect1>
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<refsect1 id="using-C-pixels">
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<title>Getting pixels</title>
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<para>
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Use #VipsRegion to read pixels out of images. You can use
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VIPS_IMAGE_ADDR() as well, but this can need a large amount of
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memory to work. See <link linkend="extending">extending</link>
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for an introduction to writing your own operations.
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</para>
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</refsect1>
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<refsect1 id="using-C-errors">
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<title>Error handling</title>
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<para>
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VIPS keeps a log of error message, see <link
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linkend="libvips-error">VIPS Error</link> to find out how to get and
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set the error log.
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clear the error log.
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</para>
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</refsect1>
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<refsect1 id="using-C-example">
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<title>Example</title>
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<example>
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<title>VIPS from C example</title>
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@ -100,55 +129,55 @@
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int
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main( int argc, char **argv )
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{
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GOptionContext *context;
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GOptionGroup *main_group;
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GError *error = NULL;
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VipsImage *in;
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double mean;
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VipsImage *out;
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GOptionContext *context;
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GOptionGroup *main_group;
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GError *error = NULL;
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VipsImage *in;
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double mean;
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VipsImage *out;
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if( VIPS_INIT( argv[0] ) )
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vips_error_exit( NULL );
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if( VIPS_INIT( argv[0] ) )
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vips_error_exit( NULL );
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context = g_option_context_new( "hello infile outfile - VIPS demo" );
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context = g_option_context_new( "hello infile outfile - VIPS demo" );
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main_group = g_option_group_new( NULL, NULL, NULL, NULL, NULL );
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g_option_context_set_main_group( context, main_group );
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g_option_context_add_group( context, vips_get_option_group() );
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main_group = g_option_group_new( NULL, NULL, NULL, NULL, NULL );
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g_option_context_set_main_group( context, main_group );
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vips_add_option_entries( main_group );
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if( !g_option_context_parse( context, &argc, &argv, &error ) ) {
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if( error ) {
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fprintf( stderr, "%s\n", error->message );
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g_error_free( error );
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}
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if( !g_option_context_parse( context, &argc, &argv, &error ) ) {
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if( error ) {
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fprintf( stderr, "%s\n", error->message );
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g_error_free( error );
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}
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vips_error_exit( NULL );
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}
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vips_error_exit( NULL );
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}
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if( argc != 3 )
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vips_error_exit( "usage: %s infile outfile", 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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if( argc != 3 )
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vips_error_exit( "usage: %s infile outfile", 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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printf( "image width = %d\n", vips_image_get_width( in ) );
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printf( "image width = %d\n", vips_image_get_width( in ) );
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if( vips_avg( in, &mean, NULL ) )
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vips_error_exit( NULL );
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if( vips_avg( in, &mean, NULL ) )
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vips_error_exit( NULL );
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printf( "mean pixel value = %g\n", mean );
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printf( "mean pixel value = %g\n", mean );
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if( vips_invert( in, &out, NULL ) )
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vips_error_exit( NULL );
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if( vips_invert( in, &out, NULL ) )
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vips_error_exit( NULL );
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g_object_unref( in );
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g_object_unref( in );
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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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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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g_object_unref( out );
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return( 0 );
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return( 0 );
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}
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</programlisting>
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</example>
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@ -160,7 +160,7 @@ vips_get_argv0( void )
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* }
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* ]|
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*
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* See also: vips_shutdown(), vips_get_option_group(), vips_version(),
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* See also: vips_shutdown(), vips_add_option_entries(), vips_version(),
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* vips_guess_prefix(), vips_guess_libdir().
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*
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* Returns: 0 on success, -1 otherwise
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@ -1008,7 +1008,7 @@ vips_version( int flag )
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* @leak: turn leak checking on or off
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*
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* Turn on or off vips leak checking. See also --vips-leak and
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* vips_get_option_group().
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* vips_add_option_entries().
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*
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* You should call this very early in your program.
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*/
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