gvariant: Document the need to cast varargs when constructing GVariants

Slightly expand on the documentation about casting varargs when
constructing GVariants, and link to it from all the functions where it’s
a necessary consideration.

Add an example of passing flags to a ‘t’ type variable (guint64).
Assuming the flags enum does not have many members, the flag variable
will be 32 bits wide, and needs an explicit cast to be passed into
g_variant_new() as a 64-bit value.

https://bugzilla.gnome.org/show_bug.cgi?id=712837

docs/reference/glib/gvariant-varargs.xml

@@ -466,6 +466,7 @@
     </tgroup>
    </informaltable>
 
+   <anchor id='gvariant-varargs'/>
    <para>
     Note that in C, small integer types in variable argument lists are promoted up to <link
     linkend='gint'><type>int</type></link> or <link linkend='guint'><type>unsigned int</type></link> as appropriate, and

glib/gvariant-parser.c

@@ -2428,6 +2428,10 @@ g_variant_parse (const GVariantType  *type,
  * #GVariant pointer will be returned unmodified, without adding any
  * additional references.
  *
+ * Note that the arguments in @app must be of the correct width for their types
+ * specified in @format when collected into the #va_list. See
+ * the <link linkend='gvariant-varargs'>GVariant varargs documentation</link>.
+ *
  * In order to behave correctly in all cases it is necessary for the
  * calling function to g_variant_ref_sink() the return result before
  * returning control to the user that originally provided the pointer.
@@ -2481,6 +2485,10 @@ g_variant_new_parsed_va (const gchar *format,
  * that case, the same arguments are collected from the argument list as
  * g_variant_new() would have collected.
  *
+ * Note that the arguments must be of the correct width for their types
+ * specified in @format. This can be achieved by casting them. See
+ * the <link linkend='gvariant-varargs'>GVariant varargs documentation</link>.
+ *
  * Consider this simple example:
  * |[<!-- language="C" --> 
  *  g_variant_new_parsed ("[('one', 1), ('two', %i), (%s, 3)]", 2, "three");
@@ -2530,6 +2538,10 @@ g_variant_new_parsed (const gchar *format,
  * calling g_variant_new_parsed() followed by
  * g_variant_builder_add_value().
  *
+ * Note that the arguments must be of the correct width for their types
+ * specified in @format_string. This can be achieved by casting them. See
+ * the <link linkend='gvariant-varargs'>GVariant varargs documentation</link>.
+ *
  * This function might be used as follows:
  *
  * |[<!-- language="C" --> 

glib/gvariant.c

@@ -5168,6 +5168,21 @@ g_variant_valist_get (const gchar **str,
  * 'r'; in essence, a new #GVariant must always be constructed by this
  * function (and not merely passed through it unmodified).
  *
+ * Note that the arguments must be of the correct width for their types
+ * specified in @format_string. This can be achieved by casting them. See
+ * the <link linkend='gvariant-varargs'>GVariant varargs documentation</link>.
+ *
+ * <programlisting>
+ * MyFlags some_flags = FLAG_ONE | FLAG_TWO;
+ * const gchar *some_strings[] = { "a", "b", "c", NULL };
+ * GVariant *new_variant;
+ *
+ * new_variant = g_variant_new ("(t^as)",
+ *                              /<!-- -->* This cast is required. *<!-- -->/
+ *                              (guint64) some_flags,
+ *                              some_strings);
+ * </programlisting>
+ *
  * Returns: a new floating #GVariant instance
  *
  * Since: 2.24
@@ -5214,6 +5229,10 @@ g_variant_new (const gchar *format_string,
  * @format_string, are collected from this #va_list and the list is left
  * pointing to the argument following the last.
  *
+ * Note that the arguments in @app must be of the correct width for their types
+ * specified in @format_string when collected into the #va_list. See
+ * the <link linkend='gvariant-varargs'>GVariant varargs documentation</link>.
+ *
  * These two generalisations allow mixing of multiple calls to
  * g_variant_new_va() and g_variant_get_va() within a single actual
  * varargs call by the user.
@@ -5365,6 +5384,10 @@ g_variant_get_va (GVariant     *value,
  * This call is a convenience wrapper that is exactly equivalent to
  * calling g_variant_new() followed by g_variant_builder_add_value().
  *
+ * Note that the arguments must be of the correct width for their types
+ * specified in @format_string. This can be achieved by casting them. See
+ * the <link linkend='gvariant-varargs'>GVariant varargs documentation</link>.
+ *
  * This function might be used as follows:
  *
  * |[<!-- language="C" -->