glib/gvarianttypeinfo.h

   1 /*
   2  * Copyright © 2008 Ryan Lortie
   3  * Copyright © 2010 Codethink Limited
   4  *
   5  * This library is free software; you can redistribute it and/or
   6  * modify it under the terms of the GNU Lesser General Public
   7  * License as published by the Free Software Foundation; either
   8  * version 2 of the License, or (at your option) any later version.
   9  *
  10  * This library is distributed in the hope that it will be useful,
  11  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  12  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  13  * Lesser General Public License for more details.
  14  *
  15  * You should have received a copy of the GNU Lesser General Public
  16  * License along with this library; if not, see <http://www.gnu.org/licenses/>.
  17  *
  18  * Author: Ryan Lortie <desrt@desrt.ca>
  19  */
  20
  21 #ifndef __G_VARIANT_TYPE_INFO_H__
  22 #define __G_VARIANT_TYPE_INFO_H__
  23
  24 #include <glib/gvarianttype.h>
  25
  26 #define G_VARIANT_TYPE_INFO_CHAR_MAYBE      'm'
  27 #define G_VARIANT_TYPE_INFO_CHAR_ARRAY      'a'
  28 #define G_VARIANT_TYPE_INFO_CHAR_TUPLE      '('
  29 #define G_VARIANT_TYPE_INFO_CHAR_DICT_ENTRY '{'
  30 #define G_VARIANT_TYPE_INFO_CHAR_VARIANT    'v'
  31 #define g_variant_type_info_get_type_char(info) \
  32   (g_variant_type_info_get_type_string(info)[0])
  33
  34 typedef struct _GVariantTypeInfo GVariantTypeInfo;
  35
  36 /* < private >
  37  * GVariantMemberInfo:
  38  *
  39  * This structure describes how to construct a GVariant instance
  40  * corresponding to a given child of a tuple or dictionary entry in a
  41  * very short constant time.  It contains the typeinfo of the child,
  42  * along with 4 constants that allow the bounds of the child's
  43  * serialised data within the container's serialised data to be found
  44  * very efficiently.
  45  *
  46  * Since dictionary entries are serialised as if they were tuples of 2
  47  * items, the term "tuple" will be used here in the general sense to
  48  * refer to tuples and dictionary entries.
  49  *
  50  * BACKGROUND:
  51  *   The serialised data for a tuple contains an array of "offsets" at
  52  *   the end.  There is one "offset" in this array for each
  53  *   variable-sized item in the tuple (except for the last one).  The
  54  *   offset points to the end point of that item's serialised data.  The
  55  *   procedure for finding the start point is described below.  An
  56  *   offset is not needed for the last item because the end point of the
  57  *   last item is merely the end point of the container itself (after
  58  *   the offsets array has been accounted for).  An offset is not needed
  59  *   for fixed-sized items (like integers) because, due to their fixed
  60  *   size, the end point is a constant addition to the start point.
  61  *
  62  *   It is clear that the starting point of a given item in the tuple is
  63  *   determined by the items that precede it in the tuple.  Logically,
  64  *   the start point of a particular item in a given type of tuple can
  65  *   be determined entirely by the end point of the nearest
  66  *   variable-sized item that came before it (or from the start of the
  67  *   container itself in case there is no preceding variable-sized
  68  *   item).  In the case of "(isis)" for example, in order to find out
  69  *   the start point of the last string, one must start at the end point
  70  *   of the first string, align to 4 (for the integer's alignment) and
  71  *   then add 4 (for storing the integer).  That's the point where the
  72  *   string starts (since no special alignment is required for strings).
  73  *
  74  *   Of course, this process requires iterating over the types in the
  75  *   tuple up to the item of interest.  As it turns out, it is possible
  76  *   to determine 3 constants 'a', 'b', and 'c' for each item in the
  77  *   tuple, such that, given the ending offset of the nearest previous
  78  *   variable-sized item (prev_end), a very simple calculation can be
  79  *   performed to determine the start of the item of interest.
  80  *
  81  * The constants in this structure are used as follows:
  82  *
  83  * First, among the array of offets contained in the tuple, 'i' is the
  84  * index of the offset that refers to the end of the variable-sized item
  85  * preceding the item of interest.  If no variable-sized items precede
  86  * this item, then 'i' will be -1.
  87  *
  88  * Let 'prev_end' be the end offset of the previous item (or 0 in the
  89  * case that there was no such item).  The start address of this item
  90  * can then be calculate using 'a', 'b', and 'c':
  91  *
  92  *    item_start = ((prev_end + a) & b) | c;
  93  *
  94  * For details about how 'a', 'b' and 'c' are calculated, see the
  95  * comments at the point of the implementation in gvariantypeinfo.c.
  96  *
  97  * The end address of the item is then determined in one of three ways,
  98  * according to the 'end_type' field.
  99  *
 100  *   - FIXED: For fixed sized items, the end address is equal to the
 101  *     start address plus the fixed size.
 102  *
 103  *   - LAST: For the last variable sized item in the tuple, the end
 104  *     address is equal to the end address of the tuple, minus the size
 105  *     of the offset array.
 106  *
 107  *   - OFFSET: For other variable sized items, the next index past 'i'
 108  *     (ie: 'i + 1') must be consulted to find the end of this item.
 109  */
 110
 111 typedef struct
 112 {
 113   GVariantTypeInfo *type_info;
 114
 115   gsize i, a;
 116   gint8 b, c;
 117
 118   guint8 ending_type;
 119 } GVariantMemberInfo;
 120
 121 #define G_VARIANT_MEMBER_ENDING_FIXED   0
 122 #define G_VARIANT_MEMBER_ENDING_LAST    1
 123 #define G_VARIANT_MEMBER_ENDING_OFFSET  2
 124
 125 /* query */
 126 GLIB_AVAILABLE_IN_ALL
 127 const gchar *                   g_variant_type_info_get_type_string     (GVariantTypeInfo   *typeinfo);
 128
 129 GLIB_AVAILABLE_IN_ALL
 130 void                            g_variant_type_info_query               (GVariantTypeInfo   *typeinfo,
 131                                                                          guint              *alignment,
 132                                                                          gsize              *size);
 133
 134 /* array */
 135 GLIB_AVAILABLE_IN_ALL
 136 GVariantTypeInfo *              g_variant_type_info_element             (GVariantTypeInfo   *typeinfo);
 137 GLIB_AVAILABLE_IN_ALL
 138 void                            g_variant_type_info_query_element       (GVariantTypeInfo   *typeinfo,
 139                                                                          guint              *alignment,
 140                                                                          gsize              *size);
 141
 142 /* structure */
 143 GLIB_AVAILABLE_IN_ALL
 144 gsize                           g_variant_type_info_n_members           (GVariantTypeInfo   *typeinfo);
 145 GLIB_AVAILABLE_IN_ALL
 146 const GVariantMemberInfo *      g_variant_type_info_member_info         (GVariantTypeInfo   *typeinfo,
 147                                                                          gsize               index);
 148
 149 /* new/ref/unref */
 150 GLIB_AVAILABLE_IN_ALL
 151 GVariantTypeInfo *              g_variant_type_info_get                 (const GVariantType *type);
 152 GLIB_AVAILABLE_IN_ALL
 153 GVariantTypeInfo *              g_variant_type_info_ref                 (GVariantTypeInfo   *typeinfo);
 154 GLIB_AVAILABLE_IN_ALL
 155 void                            g_variant_type_info_unref               (GVariantTypeInfo   *typeinfo);
 156 GLIB_AVAILABLE_IN_ALL
 157 void                            g_variant_type_info_assert_no_infos     (void);
 158
 159 #endif /* __G_VARIANT_TYPE_INFO_H__ */