+++ /dev/null
-<!-- ##### SECTION Title ##### -->
-DataDescriptor API
-
-<!-- ##### SECTION Short_Description ##### -->
-Describing the data
-
-<!-- ##### SECTION Long_Description ##### -->
-<para>In order to allow GRAS to send data over the network (or simply to
-dupplicate it in SG), you have to describe the structure of data attached
-with each message. This mecanism is stolen from NWS message passing
-interface.</para>
-
-<para>For each message, you have to declare a structure representing the
-data to send as payload with the message.</para>
-
-<refsect2>
- <title>Sending (or receiving) simple structures</title>
- <para>Let's imagin you want to declare a <command>STORE_STATE</command>
- message, which will send some data to the memory server for inclusion in
- the database. Here is the structure we want to send:</para>
-
-<literallayout>
- struct state {
- char id[STATE_NAME_SIZE];
- int rec_size;
- int rec_count;
- double seq_no;
- double time_out;
- };
-</literallayout>
-
- <para>And here is the structure description GRAS needs to be able to send
- this over the network:</para>
-
-<literallayout>
- const static DataDescriptor stateDescriptor[] =
- {SIMPLE_MEMBER(CHAR_TYPE, STATE_NAME_SIZE, offsetof(struct state, id)),
- SIMPLE_MEMBER(INT_TYPE, 1, offsetof(struct state, rec_size)),
- SIMPLE_MEMBER(INT_TYPE, 1, offsetof(struct state, rec_count)),
- SIMPLE_MEMBER(DOUBLE_TYPE, 1, offsetof(struct state, seq_no)),
- SIMPLE_MEMBER(DOUBLE_TYPE, 1, offsetof(struct state, time_out))};
-</literallayout>
-
- <para>Contrary to what one could think when you first see it, it's pretty
- easy. A structure descriptor is a list of descriptions, describing each
- field of the structure. For example, for the first field, you say that
- the base type is <command>CHAR_TYPE</command>, that there is
- <command>STATE_NAME_SIZE</command> element of this type and that it's
- position in the structure is computed by <command>offsetof(struct state,
- id)</command>. This leads to two remarks:</para>
-
- <itemizedlist>
- <listitem>
- <para>it's impossible to send dynamic sized strings that way. It's a
- known limitation, but I think we can live with it.</para>
- </listitem>
- <listitem>
- <para>Yes, the <command>offsetof(struct state, id)</command>
- construction is C ANSI and is portable.</para>
- </listitem>
- </itemizedlist>
-</refsect2>
-
-<refsect2>
- <title>Sending (or receiving) complex structure</title>
- <para>How to send non-flat structures, do you ask? It's not harder. Let's
- imagin you want to send the following structure:</para>
-
-<literallayout>
- typedef struct {
- unsigned long address;
- unsigned long port;
- } CliqueMember;
-
- typedef struct {
- char name[MAX_CLIQUE_NAME_SIZE];
- double whenGenerated;
- double instance;
- char skill[MAX_SKILL_SIZE];
- char options[MAX_OPTIONS_SIZE];
- double period;
- double timeOut;
- CliqueMember members[MAX_MEMBERS];
- unsigned int count;
- unsigned int leader;
- } Clique;
-</literallayout>
-
- <para>As you can see, this structure contains an array of another user
- defined structure. To be able to send <command>struct Clique</command>,
- you have to describe each structures that way:</para>
-
-<literallayout>
- static const DataDescriptor cliqueMemberDescriptor[] =
- {SIMPLE_MEMBER(UNSIGNED_LONG_TYPE, 1, offsetof(CliqueMember, address)),
- SIMPLE_MEMBER(UNSIGNED_LONG_TYPE, 1, offsetof(CliqueMember, port))};
-
- static const DataDescriptor cliqueDescriptor[] =
- {SIMPLE_MEMBER(CHAR_TYPE, MAX_CLIQUE_NAME_SIZE, offsetof(Clique, name)),
- SIMPLE_MEMBER(DOUBLE_TYPE, 1, offsetof(Clique, whenGenerated)),
- SIMPLE_MEMBER(DOUBLE_TYPE, 1, offsetof(Clique, instance)),
- SIMPLE_MEMBER(CHAR_TYPE, MAX_SKILL_SIZE, offsetof(Clique, skill)),
- SIMPLE_MEMBER(CHAR_TYPE, MAX_OPTIONS_SIZE, offsetof(Clique, options)),
- SIMPLE_MEMBER(DOUBLE_TYPE, 1, offsetof(Clique, period)),
- SIMPLE_MEMBER(DOUBLE_TYPE, 1, offsetof(Clique, timeOut)),
- {STRUCT_TYPE, MAX_MEMBERS, offsetof(Clique, members),
- (DataDescriptor *)&cliqueMemberDescriptor, cliqueMemberDescriptorLength,
- PAD_BYTES(CliqueMember, port, unsigned long, 1)},
- SIMPLE_MEMBER(UNSIGNED_INT_TYPE, 1, offsetof(Clique, count)),
- SIMPLE_MEMBER(UNSIGNED_INT_TYPE, 1, offsetof(Clique, leader))};
-</literallayout>
-
- <para>So, even if less natural, it is possible to send structures
- containing structures with these tools.</para>
-
- <para>You can see that it's not only impossible to send dynamic-sized
- strings, it impossible to send dynamic-sized arrays. Here,
- <command>MAX_MEMBERS</command> is the maximum of members a clique can
- contain. In NWS, this value is defined to 100. <warning><para>I'm not
- sure, but I think that all the 100 values are sent each time, even if
- there is only 3 non-null members. Yes, that's
- bad.</para></warning></para>
-
- <warning><para>The DataDescriptor_t MUST be const. Malloc'ing them and
- then casting them on argument passing IS NOT OK. This is because we get
- the number of elements in the array with the sizeof(dd)/sizeof(dd[0]).
- </para></warning>
-</refsect2>
-
-<!-- ##### SECTION See_Also ##### -->
-<para>
-
-</para>
-
-<!-- ##### ENUM DataTypes ##### -->
-<para>
-
-</para>
-
-@CHAR_TYPE:
-@DOUBLE_TYPE:
-@FLOAT_TYPE:
-@INT_TYPE:
-@LONG_TYPE:
-@SHORT_TYPE:
-@UNSIGNED_INT_TYPE:
-@UNSIGNED_LONG_TYPE:
-@UNSIGNED_SHORT_TYPE:
-@STRUCT_TYPE:
-
-<!-- ##### MACRO SIMPLE_DATA ##### -->
-<para>
-
-</para>
-
-@type:
-@repetitions:
-
-
-<!-- ##### MACRO SIMPLE_MEMBER ##### -->
-<para>
-
-</para>
-
-@type:
-@repetitions:
-@offset:
-
-
-<!-- ##### MACRO PAD_BYTES ##### -->
-<para>
-
-</para>
-
-@structType:
-@lastMember:
-@memberType:
-@repetitions:
-
-