Re: Introspection binary format comments
- From: Matthias Clasen <mclasen redhat com>
- To: Owen Taylor <otaylor redhat com>
- Cc: gtk-devel-list gnome org
- Subject: Re: Introspection binary format comments
- Date: Sat, 26 Feb 2005 12:21:33 -0500
I have tried to incorporate most of the feedback in the next draft of
the binary metadata format. One question I have not decided yet is
hashes vs directories:
- do we want hashes ?
- if yes, where do we want them ? Likely candidates are a hash to
lookup blobs listed in the directory, and a hash per interface
to look up the functions in that interface. With the extensibility
precautions added in the new draft, we should be able to add hashes
as they turn out to be necessary.
- are they optional or mandatory ?
- if they are mandatory, do we still need the arrays which they
complement ?
Here is the list of changes wrt to the first draft:
- drop comments about _GOBJ_METADATA
- drop string pool, strings can appear anywhere
- use 'blob' as collective name for the various blob types
- rename 'type' field in blobs to 'blob_type'
- rename 'type_name' and 'type_init' fields to 'gtype_name',
'gtype_init'
- shrink directory entries to 12 bytes
- merge struct and boxed blobs
- split interface blobs into enum, object and interface blobs
- add an 'unregistered' flag to struct and enum blobs
- add a 'wraps_vfunc' flag to function blobs and link them to
the vfuncs they wrap
- restrict value blobs to only occur inside enums and flags again
- add constant blobs, allow them toplevel, in interfaces and in objects
- rename 'receiver_owns_value' and 'receiver_owns_container' to
'transfer_ownership' and 'transfer_container_ownership'
- add a 'struct_offset' field to virtual function and field blobs
- add 'dipper' and 'optional' flags to arg blobs
- add a 'true_stops_emit' flag to signal blobs
- add variable blob sizes to header
- store offsets to signature blobs instead of including them directly
- change the type offset to be measured in words rather than bytes
Matthias
Version 0.2
Changes since 0.1:
- drop comments about _GOBJ_METADATA
- drop string pool, strings can appear anywhere
- use 'blob' as collective name for the various blob types
- rename 'type' field in blobs to 'blob_type'
- rename 'type_name' and 'type_init' fields to 'gtype_name', 'gtype_init'
- shrink directory entries to 12 bytes
- merge struct and boxed blobs
- split interface blobs into enum, object and interface blobs
- add an 'unregistered' flag to struct and enum blobs
- add a 'wraps_vfunc' flag to function blobs and link them to
the vfuncs they wrap
- restrict value blobs to only occur inside enums and flags again
- add constant blobs, allow them toplevel, in interfaces and in objects
- rename 'receiver_owns_value' and 'receiver_owns_container' to
'transfer_ownership' and 'transfer_container_ownership'
- add a 'struct_offset' field to virtual function and field blobs
- add 'dipper' and 'optional' flags to arg blobs
- add a 'true_stops_emit' flag to signal blobs
- add variable blob sizes to header
- store offsets to signature blobs instead of including them directly
- change the type offset to be measured in words rather than bytes
GObject binary metadata
-----------------------
The format of GObject metadata 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 outside of interface blobs, string references are
stored as offsets relative to the start of the metadata. One possibility
is to store all strings in a string pool at the end of the metadata.
Overview
--------
The metadata has the following general format.
metadata ::= header, directory, blobs, types, annotations
directory ::= list of entries
entry ::= blob type, name, namespace, offset
blob ::= function|callback|struct|boxed|enum|flags|object|interface|constant
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 (74 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 size;
guint32 namespace;
guint16 entry_blob_size; /* 12 */
guint16 function_blob_size; /* 16 */
guint16 callback_blob_size; /* 12 */
guint16 signal_blob_size; /* 12 */
guint16 vfunc_blob_size; /* 16 */
guint16 arg_blob_size; /* 8 */
guint16 property_blob_size; /* 8 */
guint16 field_blob_size; /* 8 */
guint16 value_blob_size; /* 16 */
guint16 annotation_blob_size; /* 12 */
guint16 constant_blob_size; /* 16 */
guint16 signature_blob_size; /* 4 */
guint16 enum_blob_size; /* 22 */
guint16 struct_blob_size; /* 36 */
guint16 interface_blob_size; /* 28 */
}
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 blobs in this metadata. 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.
size: The size of the metadata.
namespace:
Offset of the namespace string in the metadata.
entry_blob_size:
function_blob_size:
callback_blob_size:
signal_blob_size:
vfunc_blob_size:
arg_blob_size:
property_blob_size:
field_blob_size:
value_blob_size:
annotation_blob_size:
constant_blob_size:
The sizes of fixed-size blobs. Recording this information here
allows to write parser which continue to work if the format is
extended by adding new fields to the end of the fixed-size blobs.
signature_blob_size:
enum_blob_size:
struct_blob_size:
interface_blob_size:
For variable-size blobs, the size of the struct up to the first
flexible array member. Recording this information here allows to
write parser which continue to work if the format is extended by
adding new fields before the first flexible array member in
variable-size blobs.
Directory entry (12 bytes)
struct DirectoryEntry
{
guint16 blob_type;
guint is_local : 1;
guint reserved :15;
guint32 name;
guint32 offset;
}
blob_type:
The type of blob this entry points to:
1 function
2 callback
3 struct
4 boxed
5 enum
6 flags
7 object
8 interface
9 constant
is_local: Wether this entry refers to a blob in this metadata.
name: The name of the entry.
offset: If is_local is set, this is the offset of the blob in the metadata.
Otherwise, it is the offset of the namespace in which the blob has
to be looked up by name.
All blobs pointed to by a directory entry start with the same layout for
the first 8 bytes (the reserved flags may be used by some interface types)
struct InterfacePrefix
{
guint16 blob_type;
guint deprecated : 1;
guint reserved :15;
guint32 name;
}
blob_type:
An integer specifying the type of the blob, see DirectoryEntry
for details.
deprecated:
Wether the blob is deprecated.
name: The name of the blob.
The SignatureBlob is shared between Functions,
Callbacks, Signals and VirtualFunctions.
SignatureBlob (4 + 8 * n_arguments bytes)
struct SignatureBlob
{
SimpleTypeBlob 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.
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 (16 bytes)
struct FunctionBlob
{
guint16 blob_type; /* 1 */
guint deprecated : 1;
guint is_setter : 1;
guint is_getter : 1;
guint is_constructor : 1;
guint wraps_vfunc : 1;
guint reserved : 1;
guint index :10;
guint32 name;
guint32 c_name;
guint32 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.
wraps_vfunc:
The function is a simple wrapper for a virtual function.
index: Index of the property that this function is a setter or getter of
in the array of properties of the containing interface, or index
of the virtual function that this function wraps.
signature:
Offset of the SignatureBlob describing the parameter types and the
return value type.
CallbackBlob (12 bytes)
struct CallbackBlob
{
guint16 blob_type; /* 2 */
guint deprecated : 1;
guint reserved :15;
guint32 name;
guint32 signature;
}
signature:
Offset of the SignatureBlob describing the parameter types and the
return value type.
ArgBlob (8 bytes)
struct ArgBlob
{
guint32 name;
guint in : 1;
guint out : 1;
guint dipper : 1;
guint null_ok : 1;
guint optional : 1;
guint transfer_ownership : 1;
guint transfer_container_ownership : 1;
guint is_return_value : 1;
guint reserved : 8:
SimpleTypeBlob 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*.
dipper: The parameter is a pointer to a struct or object that will
receive an output of the function.
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.
optional:
For an out parameter, indicates that NULL may be passed in
if the value is not needed.
transfer_ownership:
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.
transfer_container_ownership:
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.
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.
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.
Unlike other offsets, this is in words (ie 32bit units) rather
than bytes.
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 (12 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 true_stops_emit : 1;
guint reserved : 5;
guint16 class_closure;
guint32 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.
true_stops_emit:
Wether the signal has true-stops-emit semantics
class_closure:
The index of the class closure in the list of virtual functions
of the interface on which the signal is defined.
signature:
Offset of the SignatureBlob describing the parameter types and the
return value type.
VirtualFunctionBlob (16 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;
guint16 struct_offset;
guint16 reserved;
guint32 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.
struct_offset:
The offset of the function pointer in the class struct.
signature:
Offset of the SignatureBlob describing the parameter types and the
return value type.
FieldBlob (8 bytes)
struct FieldBlob
{
guint32 name;
guint readable : 1;
guint writable : 1;
guint reserved : 6;
guint8 bits;
guint16 struct_offset;
guint16 reserved;
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.
struct_offset:
The offset of the field in the struct.
type: The type of the field.
ValueBlob (16 bytes)
Values commonly occur in enums and flags, but we also allow them to occur
in interfaces or freestanding, to describe constants.
struct ValueBlob
{
guint deprecated : 1;
guint reserved :31;
guint32 name;
guint32 short_name;
guint32 value;
}
short_name:
A short name for the value;
value: The numerical value;
GTypeBlob (8 bytes)
struct GTypeBlob
{
guint32 gtype_name;
guint32 gtype_init;
}
gtype_name:
The name under which the interface is registered with GType.
gtype_init:
The symbol name of the get_type() function which registers the type.
StructBlob (12 + 8 * n_fields + x * n_functions)
struct StructBlob
{
guint16 blob_type; /* 3: struct, 4: boxed */
guint deprecated : 1;
guint unregistered : 1;
guint reserved :14;
guint32 name;
GTypeBlob gtype;
guint16 n_fields;
guint16 n_functions;
FieldBlob fields[];
FunctionBlob functions[];
}
unregistered:
If this is set, the type is not registered with GType.
gtype: For types which are registered with GType, contains the
information about the GType. Otherwise unused.
n_fields:
n_functions:
The lengths of the arrays.
fields: An array of n_fields FieldBlobs.
functions:
An array of n_functions FunctionBlobs. The described functions
should be considered as methods of the struct.
EnumBlob (22 + 16 * n_values)
struct EnumBlob
{
guint16 blob_type; /* 5: enum, 6: flags */
guint deprecated : 1;
guint unregistered : 1;
guint reserved :14;
guint32 name;
GTypeBlob gtype;
guint16 n_values;
guint16 reserved;
ValueBlob values[];
}
unregistered:
If this is set, the type is not registered with GType.
gtype: For types which are registered with GType, contains the
information about the GType. Otherwise unused.
n_values:
The lengths of the values arrays.
values: Describes the enum values.
ObjectBlob (36 + x bytes)
struct ObjectBlob
{
guint16 blob_type; /* 7 */
guint deprecated : 1;
guint reserved :15;
guint32 name;
GTypeBlob gtype;
guint16 parent;
guint16 n_interfaces;
guint16 n_fields;
guint16 n_properties;
guint16 n_methods;
guint16 n_signals;
guint16 n_virtual_functions;
guint16 n_constants;
guint16 reserved;
guint16 interfaces[];
FieldBlob fields[];
PropertyBlob properties[];
FunctionBlob methods[];
SignalBlob signals[];
VirtualFunctionBlob virtual_functions[];
ConstantBlob constants;
}
gtype: Contains the information about the GType.
parent: The directory index of the parent interface. This is only set for
objects.
n_interfaces:
n_fields:
n_properties:
n_methods:
n_signals:
n_virtual_functions:
n_constants:
The lengths of the arrays.
Up to 16bits of padding may be inserted between the arrays to ensure that they
start on a 32bit boundary.
interfaces:
An array of indices of directory entries for the implemented
interfaces.
fields: Describes the fields.
functions:
Describes the methods, constructors, setters and getters.
properties:
Describes the properties.
signals: Describes the signals.
virtual_functions:
Describes the virtual functions.
constants:
Describes the constants.
InterfaceBlob (28 + x bytes)
struct InterfaceBlob
{
guint16 blob_type; /* 8 */
guint deprecated : 1;
guint reserved :15;
guint32 name;
GTypeBlob gtype;
guint16 n_prerequisites;
guint16 n_properties;
guint16 n_methods;
guint16 n_signals;
guint16 n_virtual_functions;
guint16 n_constants;
guint16 prerequisites[];
PropertyBlob properties[];
FunctionBlob methods[];
SignalBlob signals[];
VirtualFunctionBlob virtual_functions[];
ConstantBlob constants[];
}
n_prerequisites:
n_properties:
n_methods:
n_signals:
n_virtual_functions:
n_constants:
The lengths of the arrays.
Up to 16bits of padding may be inserted between the arrays to ensure that they
start on a 32bit boundary.
prerequisites:
An array of indices of directory entries for required interfaces.
functions:
Describes the methods, constructors, setters and getters.
properties:
Describes the properties.
signals: Describes the signals.
virtual_functions:
Describes the virtual functions.
constants:
Describes the constants.
ConstantBlob (16 bytes)
struct ConstantBlob
{
guint16 blob_type; /* 9 */
guint deprecated : 1;
guint reserved :15;
guint32 name;
SimpleTypeBlob type;
guint16 size;
guint32 offset;
}
type: The type of the value. In most cases this should be a numeric
type or string.
size: The size of the value in bytes.
offset: The offset of the value in the metadata.
AnnotationBlob (12 bytes)
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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