Re: Introspection API
- From: Matthias Clasen <mclasen redhat com>
- To: gtk-devel-list gnome org, language-bindings gnome org
- Cc:
- Subject: Re: Introspection API
- Date: Thu, 24 Feb 2005 15:20:51 -0500
I have written up a draft spec for the format of the binary metadata
to back up the repository api which I posted earlier. I hope to have
some initial code implementing both of these soon.
Comments highly appreciated,
Matthias
GObject binary metadata
-----------------------
GObject metadata is stored in a variable with the name _G_METADATA in the shared
object it describes. The format of the data is strongly influenced by the Mozilla
XPCOM format.
Some of the differences to XPCOM include:
- type information is stored not quite as compactly (XPCOM stores it inline in
function descriptions in variable-sized blobs of 1 to n bytes. We store 16 bits
of type information for each parameter, which is enough to encode simple types
inline. Complex (e.g. recursive) types are stored out of line in a separate
list of types.
- String data is stored in a string pool, string references are stored as
offsets relative to the start of the metadata.
Overview
--------
The metadata has the following general format.
metadata ::= header, directory, interfaces, types, annotations, string pool
directory ::= list of entries
entry ::= interface type, name, namespace, offset
interface ::= function|callback|struct|value|object|interface|enum|flags|boxed
annotations ::= list of annotations, sorted by offset
annotation ::= offset, key, value
Details
-------
We describe the fragments that make up the metadata in the form of C structs (although
some fall short of being valid C structs since they contain multiple flexible arrays).
Header (48 bytes)
struct Header
{
gchar[16] magic;
guint8 major_version;
guint8 minor_version;
guint16 reserved;
guint16 n_entries;
guint16 n_local_entries;
guint32 directory;
guint32 types;
guint32 annotations;
guint32 string_pool;
guint32 size;
guint32 namespace;
}
magic: The string "GOBJ\nMETADATA\r\n\032". This was inspired by XPCOM,
which in turn borrowed from PNG.
major_version,
minor_version:
The version of the metadata format. Minor version changes indicate
compatible changes and should still allow the metadata to be parsed
by a parser designed for the same major_version.
n_entries:
The number of entries in the directory.
n_local_entries:
The number of entries referring to interfaces in this blob. The
local entries must occur before the unresolved entries.
directory:
Offset of the directory in the metadata.
FIXME: need to specify if and how the directory is sorted
types: Offset of the list of types in the metadata.
annotations:
Offset of the list of annotations in the metadata.
string_pool:
Offset of the string pool in the metadata. The strings are stored
as zero-terminated strings
size: The size of the metadata.
namespace:
Offset of the namespace in the string pool.
Directory entry (16 bytes)
struct DirectoryEntry
{
guint32 type;
guint32 name;
guint32 namespace;
guint32 interface;
}
type: The type of interface this entry points to:
1 function
2 callback
3 struct
4 object
5 interface
6 enum
7 flags
8 boxed
9 value
name: The name of the entry.
namespace:
For references to interfaces which are in a different metadata blob,
this contains the namespace to look in. For references to interfaces
in the same blob, it is 0.
interface:
For references to interfaces in the same blob, this is the offset of
the interface in the metadata. For references to interfaces which are
in a different blob, this is 0.
All interfaces pointed to by directory entries start with the same layout for
the first 8 bytes (the reserved flags may be used by some interface types)
struct InterfacePrefix {
guint16 type;
guint deprecated : 1;
guint reserved :15;
guint32 name;
}
type: An integer specifying the type of interface, see DirectoryEntry
for details
deprecated:
Wether the interface is deprecated
name: The name of the interface.
The SignatureBlob is shared between Functions,
Callbacks, Signals and VirtualFunctions.
SignatureBlob (4 + 8 * n_arguments bytes)
struct SignatureBlob
{
guint16 return_type;
guint may_return_null : 1;
guint caller_owns_return_value : 1;
guint caller_owns_return_container : 1;
guint reserved : 5;
guint8 n_arguments;
ArgBlob[] arguments;
}
return_type:
Describes the type of the return value. See details below for TypeBlob.
may_return_null:
Only relevant for pointer types. Indicates whether the caller
must expect NULL as a return value.
caller_owns_return_value:
If set, the caller is responsible for freeing the return value
if it is no longer needed.
caller_owns_return_container:
This flag is only relevant if the return type is a container type.
If the flag is set, the caller is resonsible for freeing the container,
but not its contents
n_arguments:
The number of arguments that this function expects, also the length
of the array of ArgBlobs
arguments:
An array of ArgBlob for the arguments of the function.
FunctionBlob (12 + x bytes)
struct FunctionBlob {
guint16 type; /* 1 */
guint deprecated : 1;
guint is_setter : 1;
guint is_getter : 1;
guint is_constructor : 1;
guint reserved : 2;
guint property :10;
guint32 name;
guint32 c_name;
SignatureBlob signature;
}
c_name: The symbol which can be used to obtain the function pointer with dlsym().
deprecated
The function is deprecated.
is_setter
The function is a setter for a property. Language bindings may prefer to
not bind individual setters and rely on the generic g_object_set().
is_getter
The function is a getter for a property. Language bindings may prefer to
not bind individual getters and rely on the generic g_object_get().
is_constructor
The function acts as a constructor for the object it is contained in.
property
Index of the property that this function is a setter or getter of in the
array of properties of the containing interface
signature:
Describes the parameter types and the return value type.
CallbackBlob (8 + x bytes)
struct CallbackBlob
{
guint16 type; /* 2 */
guint deprecated : 1;
guint reserved :15;
guint32 name;
SignatureBlob signature;
}
signature:
Describes the parameter types and the return value type.
ArgumentBlob (8 bytes)
struct ArgBlob
{
guint32 name;
guint in : 1;
guint out : 1;
guint is_return_value : 1;
guint null_ok : 1;
guint receiver_owns_value : 1;
guint receiver_owns_container : 1;
guint reserved :10:
guint16 arg_type;
}
name: A suggested name for the parameter.
in: The parameter is an input to the function
out: The parameter is used to return an output of the function.
Parameters can be both in and out. Out parameters implicitly
add another level of indirection to the parameter type. Ie if
the type is uint32 in an out parameter, the function actually
takes an uint32*.
is_return_value:
The parameter should be considered the return value of the function.
Only out parameters can be marked as return value, and there can be
at most one per function call. If an out parameter is marked as
return value, the actual return value of the function should be
either void or a boolean indicating the success of the call.
null_ok: Only meaningful for types which are passed as pointers.
For an in parameter, indicates if it is ok to pass NULL in, for
an out parameter, wether it may return NULL. Note that NULL is a
valid GList and GSList value, thus null_ok will normally be set for
parameters of these types.
receiver_owns_value:
For an in parameter, indicates that the function takes over ownership
of the parameter value. For an out parameter, it indicates that the
caller is responsible for freeing the return value.
receiver_owns_container:
For container types, indicates that the ownership of the container,
but not of its contents is transferred. This is typically the case
for out parameters returning lists of statically allocated things.
arg_type:
Describes the type of the parameter. See details below.
Types are specified by two bytes. If the high byte is zero, the low byte
describes a basic type, otherwise the 16bit number is an offset relative to
header->types and points to a TypeBlob.
SimpleTypeBlob (2 bytes)
union SimpleTypeBlob
{
struct
{
guint8 reserved; /* 0 */
guint is_pointer :1;
guint reserved :2;
guint tag :5;
};
guint16 offset;
}
is_pointer:
indicates wether the type is passed by reference.
tag: specifies what kind of type is described, as follows:
0 void
1 boolean (booleans are passed as ints)
2 int8
3 uint8
4 int16
5 uint16
6 int32
7 uint32
8 int32
9 int64
10 uint64
11 float
12 double
13 string (these are zero-terminated char* and assumed to be
in UTF-8, for other data, use uint8[])
14 GString
For string and GString, is_pointer will always be set.
offset: Offset relative to header->types that points to a TypeBlob.
TypeBlob (4 or more bytes)
union TypeBlob
{
ArrayTypeBlob array_type;
InterfaceTypeBlob interface_type;
OneParameterTypeBlob one_parameter_type;
TwoParameterTypeBlob two_parameter_type;
ErrorTypeBlob error_type;
}
ArrayTypeBlob (4 bytes)
Arrays have a tag value of 20. They are passed by reference, thus is_pointer is
always 1.
struct ArrayTypeBlob
{
guint is_pointer :1; /* 1 */
guint reserved :2;
guint tag :5; /* 20 */
guint zero_terminated :1;
guint has_length :1;
guint length :6;
SimpleTypeBlob type;
}
zero_terminated:
Indicates that the array must be terminated by a suitable NULL value.
has_length:
Indicates that length points to a parameter specifying the length of the
array. If both has_length and zero_terminated are set, the convention
is to pass -1 for the length if the array is zero-terminated.
FIXME: what does this mean for types of field and properties ?
length: The index of the parameter which is used to pass the length of the
array. The parameter must be an integer type and have the same direction
as this one.
type: The type of the array elements.
InterfaceTypeBlob (4 bytes)
struct InterfaceTypeBlob
{
guint is_pointer :1;
guint reserved :2;
guint tag :5; /* 21 */
guint8 reserved;
guint16 interface;
}
Types which are described by an entry in the metadata have a tag value of 21.
If the interface is an enum of flags type, is_pointer is 0, otherwise it is 1.
interface:
Index of the directory entry for the interface.
OneParameterTypeBlob (4 bytes)
GLists have a tag value of 22, GSLists have a tag value of 23. They are passed
by reference, thus is_pointer is always 1.
struct OneParameterTypeBlob
{
guint is_pointer :1; /* 1 */
guint reserved :2;
guint tag :5; /* 22 or 23 */
guint8 reserved;
SimpleTypeBlob type;
}
type: Describes the type of the list elements.
TwoParameterTypeBlob (8 bytes)
GHashTables have a tag value of 24. They are passed by reference, thus is_pointer
is always 1.
struct TwoParameterTypeBlob
{
guint is_pointer :1; /* 1 */
guint reserved :2;
guint tag :5; /* 24 */
guint8 reserved;
guint16 reserved;
SimpleTypeBlob type1;
SimpleTypeBlob type2;
}
type1: Describes the type of the keys in the table. This will most commonly be
int or string.
type2: Describes the type of the value in the table. This may be void*.
ErrorTypeBlob (4 + 4 * n_domains bytes)
struct ErrorTypeBlob
{
guint is_pointer :1; /* 1 */
guint reserved :2;
guint tag :5; /* 25 */
guint8 reserved;
guint16 n_domains;
guint32 domains[];
}
n_domains:
The number of domains to follow
domains: Offsets of ErrorDomainBlobs describing the possible error domains.
ErrorDomainBlob (10 bytes)
struct ErrorDomainBlob
{
guint32 name;
guint32 get_quark;
guint16 error_codes;
}
name: The name of the error domain
get_quark:
The symbol name of the function which must be called to obtain the
GQuark for the error domain.
error_codes:
Index of the InterfaceBlob describing the enumeration which lists
the possible error codes.
PropertyBlob (8 bytes)
struct PropertyBlob
{
guint32 name;
guint deprecated :1;
guint readable :1;
guint writable :1;
guint construct :1;
guint construct_only :1;
guint reserved :11;
SimpleTypeBlob type;
}
name: The name of the property.
readable:
writable:
construct:
construct_only:
The ParamFlags used when registering the property.
type: Describes the type of the property.
SignalBlob (8 + x bytes)
struct SignalBlob
{
guint32 name;
guint deprecated : 1;
guint run_first : 1;
guint run_last : 1;
guint run_cleanup : 1;
guint no_recurse : 1;
guint detailed : 1;
guint action : 1;
guint no_hooks : 1;
guint has_class_closure : 1;
guint reserved : 6;
guint16 class_closure;
SignatureBlob signature;
}
name: The name of the signal.
run_first:
run_last:
run_cleanup:
no_recurse:
detailed:
action:
no_hooks: The flags used when registering the signal.
has_class_closure:
Set if the signal has a class closure.
class_closure:
The index of the class closure in the list of virtual functions
of the interface on which the signal is defined.
signature:
Describes the parameter types and the return value type.
VirtualFunctionBlob (8 + x bytes)
struct VirtualFunctionBlob
{
guint32 name;
guint must_chain_up : 1;
guint must_be_implemented : 1;
guint must_not_be_implemented : 1;
guint is_class_closure : 1;
guint reserved :12;
guint16 signal;
SignatureBlob signature;
}
name: The name of the virtual function.
must_chain_up:
If set, every implementation of this virtual function must
chain up to the implementation of the parent class.
must_be_implemented:
If set, every derived class must override this virtual function.
must_not_be_implemented:
If set, derived class must not override this virtual function.
is_class_closure:
Set if this virtual function is the class closure of a signal.
signal:
The index of the signal in the list of signals of the interface
to which this virtual function belongs.
signature:
The signature of the virtual function.
FieldBlob (8 bytes)
struct FieldBlob
{
guint32 name;
guint readable : 1;
guint writable : 1;
guint reserved : 6;
guint8 bits;
SimpleTypeBlob type;
}
name: The name of the field.
readable:
writable: How the field may be accessed.
bits: If this field is part of a bitfield, the number of bits which it
uses, otherwise 0.
type: The type of the field.
ValueBlob (32 bytes)
Values commonly occur in enums and flags, but we also allow them to occur
in interfaces or freestanding, to describe constants.
struct ValueBlob
{
guint16 type; /* 9 */
guint deprecated : 1;
guint reserved :15;
guint32 name;
guint32 short_name;
guint32 value;
}
short_name:
A short name for the value;
value: The numerical value;
StructBlob (12 + 8 * n_fields + x * n_functions)
struct StructBlob
{
guint16 type; /* 3 */
guint deprecated : 1;
guint reserved :15;
guint32 name;
guint16 n_fields;
guint16 n_functions;
FieldBlob fields[];
FunctionBlob functions[];
}
n_fields:
n_functions:
The lengths of the arrays.
fields: An array of n_fields FieldBlobs. The fields must be described in
the order in which they occur in the struct.
functions:
An array of n_functions FunctionDescriptors. The described functions
should be considered as methods of the struct.
InterfaceBlob
All GType registered types are described by an InterfaceBlob. Some parts of
a descriptor will be zero, depending on what kind of type is being described.
Interfaces have no parent and no fields, enums and flags have only values, boxed
types may have fields and functions.
struct InterfaceBlob
{
guint16 type; /* 4 for object, 5 for interface, 6 for enum, 7 for flags, 8 for boxed */
guint deprecated : 1;
guint reserved :15;
guint32 name;
guint32 type_name;
guint32 type_init;
guint16 n_interfaces;
guint16 n_values;
guint16 n_fields;
guint16 n_properties;
guint16 n_methods;
guint16 n_signals;
guint16 n_virtual_functions;
guint16 parent;
guint16 interfaces[];
ValueBlob values[];
FieldBlob fields[];
PropertyBlob properties[];
FunctionBlob methods[];
SignalBlob signals[];
VirtualFunctionBlob virtual_functions[];
}
type_name:
The name under which the interface is registered with GType.
type_init:
The symbol name of the get_type() function which registers the type.
n_interfaces:
n_values:
n_fields:
n_properties:
n_methods:
n_signals:
n_virtual_functions:
The lengths of the arrays.
parent: The directory index of the parent interface. This is only set for
objects.
Up to 16bits of padding may be inserted between the arrays to ensure that they
start on a 32bit boundary.
interfaces:
An array of direcory indices which for objects point to the implemented
interfaces, while they point to prerequisited for interfaces. For enums,
flags or boxed types this array is empty.
values: Describes the constant values defined in this interface.
fields: Describes the fields. For enums, flags and interfaces this array is
empty.
functions:
Describes the methods, constructors, setters and getters. For enums
and flags, this array is empty.
properties:
Describes the properties. For enums flags and boxed types, this array
is empty.
signals: Describes the signals. For enums flags and boxed types, this array
is empty.
virtual_functions:
Describes the virtual functions. For enums flags and boxed types,
this array is empty.
struct AnnotationBlob
{
guint32 offset;
guint32 name;
guint32 value;
}
offset: The offset of the interface to which this annotation refers. Annotations are
kept sorted by offset, so that the annotations of an interface can be found
by a binary search.
name: The name of the annotation, a string.
value: The value of the annotation (also a string)
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