glib.variant.VariantG is a variant datatype; it can contain one or more values
along with information about the type of the values.
A glib.variant.VariantG may contain simple types, like an integer, or a boolean value;
or complex types, like an array of two strings, or a dictionary of key
value pairs. A glib.variant.VariantG is also immutable: once it’s been created neither
its type nor its content can be modified further.
glib.variant.VariantG is useful whenever data needs to be serialized, for example when
sending method parameters in D-Bus, or when saving settings using
[gio.settings.Settings].
When creating a new glib.variant.VariantG, you pass the data you want to store in it
along with a string representing the type of data you wish to pass to it.
For instance, if you want to create a glib.variant.VariantG holding an integer value you
can use:
GVariant *v = g_variant_new ("u", 40);
The string u in the first argument tells glib.variant.VariantG that the data passed to
the constructor (40) is going to be an unsigned integer.
glib.variant.VariantG is heavily optimised for dealing with data in serialized
form. It works particularly well with data located in memory-mapped
files. It can perform nearly all deserialization operations in a
small constant time, usually touching only a single memory page.
Serialized glib.variant.VariantG data can also be sent over the network.
For space-efficiency, the glib.variant.VariantG serialization format does not
automatically include the variant’s length, type or endianness,
which must either be implied from context (such as knowledge that a
particular file format always contains a little-endian
G_VARIANT_TYPE_VARIANT which occupies the whole length of the file)
or supplied out-of-band (for instance, a length, type and/or endianness
indicator could be placed at the beginning of a file, network message
or network stream).
For convenience to C programmers, glib.variant.VariantG features powerful
varargs-based value construction and destruction. This feature is
designed to be embedded in other libraries.
There is a Python-inspired text language for describing glib.variant.VariantG
values. glib.variant.VariantG includes a printer for this language and a parser
with type inferencing.
glib.variant.VariantG tries to be quite efficient with respect to memory use.
This section gives a rough idea of how much memory is used by the
current implementation. The information here is subject to change
in the future.
The memory allocated by glib.variant.VariantG can be grouped into 4 broad
purposes: memory for serialized data, memory for the type
information cache, buffer management memory and memory for the
glib.variant.VariantG structure itself.
This is the memory that is used for storing glib.variant.VariantG data in
serialized form. This is what would be sent over the network or
what would end up on disk, not counting any indicator of the
endianness, or of the length or type of the top-level variant.
The amount of memory required to store a boolean is 1 byte. 16,
32 and 64 bit integers and double precision floating point numbers
use their ‘natural’ size. Strings (including object path and
signature strings) are stored with a nul terminator, and as such
use the length of the string plus 1 byte.
‘Maybe’ types use no space at all to represent the null value and
use the same amount of space (sometimes plus one byte) as the
equivalent non-maybe-typed value to represent the non-null case.
Arrays use the amount of space required to store each of their
members, concatenated. Additionally, if the items stored in an
array are not of a fixed-size (ie: strings, other arrays, etc)
then an additional framing offset is stored for each item. The
size of this offset is either 1, 2 or 4 bytes depending on the
overall size of the container. Additionally, extra padding bytes
are added as required for alignment of child values.
Tuples (including dictionary entries) use the amount of space
required to store each of their members, concatenated, plus one
framing offset (as per arrays) for each non-fixed-sized item in
the tuple, except for the last one. Additionally, extra padding
bytes are added as required for alignment of child values.
Variants use the same amount of space as the item inside of the
variant, plus 1 byte, plus the length of the type string for the
item inside the variant.
As an example, consider a dictionary mapping strings to variants.
In the case that the dictionary is empty, 0 bytes are required for
the serialization.
If we add an item ‘width’ that maps to the int32 value of 500 then
we will use 4 bytes to store the int32 (so 6 for the variant
containing it) and 6 bytes for the string. The variant must be
aligned to 8 after the 6 bytes of the string, so that’s 2 extra
bytes. 6 (string) + 2 (padding) + 6 (variant) is 14 bytes used
for the dictionary entry. An additional 1 byte is added to the
array as a framing offset making a total of 15 bytes.
If we add another entry, ‘title’ that maps to a nullable string
that happens to have a value of null, then we use 0 bytes for the
null value (and 3 bytes for the variant to contain it along with
its type string) plus 6 bytes for the string. Again, we need 2
padding bytes. That makes a total of 6 + 2 + 3 = 11 bytes.
We now require extra padding between the two items in the array.
After the 14 bytes of the first item, that’s 2 bytes required.
We now require 2 framing offsets for an extra two
bytes. 14 + 2 + 11 + 2 = 29 bytes to encode the entire two-item
dictionary.
For each glib.variant.VariantG type that currently exists in the program a type
information structure is kept in the type information cache. The
type information structure is required for rapid deserialization.
Continuing with the above example, if a glib.variant.VariantG exists with the
type a{sv} then a type information struct will exist for
a{sv}, {sv}, s, and v. Multiple uses of the same type
will share the same type information. Additionally, all
single-digit types are stored in read-only static memory and do
not contribute to the writable memory footprint of a program using
glib.variant.VariantG.
Aside from the type information structures stored in read-only
memory, there are two forms of type information. One is used for
container types where there is a single element type: arrays and
maybe types. The other is used for container types where there
are multiple element types: tuples and dictionary entries.
Array type info structures are 6 * sizeof (void *), plus the
memory required to store the type string itself. This means that
on 32-bit systems, the cache entry for a{sv} would require 30
bytes of memory (plus allocation overhead).
Tuple type info structures are 6 * sizeof (void *), plus `4 *
sizeof (void *)` for each item in the tuple, plus the memory
required to store the type string itself. A 2-item tuple, for
example, would have a type information structure that consumed
writable memory in the size of 14 * sizeof (void *) (plus type
string) This means that on 32-bit systems, the cache entry for
{sv} would require 61 bytes of memory (plus allocation overhead).
This means that in total, for our a{sv} example, 91 bytes of
type information would be allocated.
The type information cache, additionally, uses a glib.hash_table.HashTable to
store and look up the cached items and stores a pointer to this
hash table in static storage. The hash table is freed when there
are zero items in the type cache.
Although these sizes may seem large it is important to remember
that a program will probably only have a very small number of
different types of values in it and that only one type information
structure is required for many different values of the same type.
glib.variant.VariantG uses an internal buffer management structure to deal
with the various different possible sources of serialized data
that it uses. The buffer is responsible for ensuring that the
correct call is made when the data is no longer in use by
glib.variant.VariantG. This may involve a func@GLib.free or
even glib.mapped_file.MappedFile.unref.
One buffer management structure is used for each chunk of
serialized data. The size of the buffer management structure
is 4 * (void *). On 32-bit systems, that’s 16 bytes.
The size of a glib.variant.VariantG structure is 6 * (void *). On 32-bit
systems, that’s 24 bytes.
glib.variant.VariantG structures only exist if they are explicitly created
with API calls. For example, if a glib.variant.VariantG is constructed out of
serialized data for the example given above (with the dictionary)
then although there are 9 individual values that comprise the
entire dictionary (two keys, two values, two variants containing
the values, two dictionary entries, plus the dictionary itself),
only 1 glib.variant.VariantG instance exists — the one referring to the
dictionary.
If calls are made to start accessing the other values then
glib.variant.VariantG instances will exist for those values only for as long
as they are in use (ie: until you call glib.variant.VariantG.unref). The
type information is shared. The serialized data and the buffer
management structure for that serialized data is shared by the
child.
To put the entire example together, for our dictionary mapping
strings to variants (with two entries, as given above), we are
using 91 bytes of memory for type information, 29 bytes of memory
for the serialized data, 16 bytes for buffer management and 24
bytes for the glib.variant.VariantG instance, or a total of 160 bytes, plus
allocation overhead. If we were to use glib.variant.VariantG.getChildValue
to access the two dictionary entries, we would use an additional 48
bytes. If we were to have other dictionaries of the same type, we
would use more memory for the serialized data and buffer
management for those dictionaries, but the type information would
be shared.
glib.variant.VariantG is a variant datatype; it can contain one or more values along with information about the type of the values.
A glib.variant.VariantG may contain simple types, like an integer, or a boolean value; or complex types, like an array of two strings, or a dictionary of key value pairs. A glib.variant.VariantG is also immutable: once it’s been created neither its type nor its content can be modified further.
glib.variant.VariantG is useful whenever data needs to be serialized, for example when sending method parameters in D-Bus, or when saving settings using [gio.settings.Settings].
When creating a new glib.variant.VariantG, you pass the data you want to store in it along with a string representing the type of data you wish to pass to it.
For instance, if you want to create a glib.variant.VariantG holding an integer value you can use:
The string u in the first argument tells glib.variant.VariantG that the data passed to the constructor (40) is going to be an unsigned integer.
More advanced examples of glib.variant.VariantG in use can be found in documentation for [glib.variant.VariantG] format strings.
The range of possible values is determined by the type.
The type system used by glib.variant.VariantG is glib.variant_type.VariantType.
glib.variant.VariantG instances always have a type and a value (which are given at construction time). The type and value of a glib.variant.VariantG instance can never change other than by the glib.variant.VariantG itself being destroyed. A glib.variant.VariantG cannot contain a pointer.
glib.variant.VariantG is reference counted using glib.variant.VariantG.ref_ and glib.variant.VariantG.unref. glib.variant.VariantG also has floating reference counts — see glib.variant.VariantG.refSink.
glib.variant.VariantG is completely threadsafe. A glib.variant.VariantG instance can be concurrently accessed in any way from any number of threads without problems.
glib.variant.VariantG is heavily optimised for dealing with data in serialized form. It works particularly well with data located in memory-mapped files. It can perform nearly all deserialization operations in a small constant time, usually touching only a single memory page. Serialized glib.variant.VariantG data can also be sent over the network.
glib.variant.VariantG is largely compatible with D-Bus. Almost all types of glib.variant.VariantG instances can be sent over D-Bus. See glib.variant_type.VariantType for exceptions. (However, glib.variant.VariantG’s serialization format is not the same as the serialization format of a D-Bus message body: use GDBusMessage, in the GIO library, for those.)
For space-efficiency, the glib.variant.VariantG serialization format does not automatically include the variant’s length, type or endianness, which must either be implied from context (such as knowledge that a particular file format always contains a little-endian G_VARIANT_TYPE_VARIANT which occupies the whole length of the file) or supplied out-of-band (for instance, a length, type and/or endianness indicator could be placed at the beginning of a file, network message or network stream).
A glib.variant.VariantG’s size is limited mainly by any lower level operating system constraints, such as the number of bits in gobject.types.size_t. For example, it is reasonable to have a 2GB file mapped into memory with glib.mapped_file.MappedFile, and call glib.variant.VariantG.newFromData on it.
For convenience to C programmers, glib.variant.VariantG features powerful varargs-based value construction and destruction. This feature is designed to be embedded in other libraries.
There is a Python-inspired text language for describing glib.variant.VariantG values. glib.variant.VariantG includes a printer for this language and a parser with type inferencing.
Memory Use
glib.variant.VariantG tries to be quite efficient with respect to memory use. This section gives a rough idea of how much memory is used by the current implementation. The information here is subject to change in the future.
The memory allocated by glib.variant.VariantG can be grouped into 4 broad purposes: memory for serialized data, memory for the type information cache, buffer management memory and memory for the glib.variant.VariantG structure itself.
Serialized Data Memory
This is the memory that is used for storing glib.variant.VariantG data in serialized form. This is what would be sent over the network or what would end up on disk, not counting any indicator of the endianness, or of the length or type of the top-level variant.
The amount of memory required to store a boolean is 1 byte. 16, 32 and 64 bit integers and double precision floating point numbers use their ‘natural’ size. Strings (including object path and signature strings) are stored with a nul terminator, and as such use the length of the string plus 1 byte.
‘Maybe’ types use no space at all to represent the null value and use the same amount of space (sometimes plus one byte) as the equivalent non-maybe-typed value to represent the non-null case.
Arrays use the amount of space required to store each of their members, concatenated. Additionally, if the items stored in an array are not of a fixed-size (ie: strings, other arrays, etc) then an additional framing offset is stored for each item. The size of this offset is either 1, 2 or 4 bytes depending on the overall size of the container. Additionally, extra padding bytes are added as required for alignment of child values.
Tuples (including dictionary entries) use the amount of space required to store each of their members, concatenated, plus one framing offset (as per arrays) for each non-fixed-sized item in the tuple, except for the last one. Additionally, extra padding bytes are added as required for alignment of child values.
Variants use the same amount of space as the item inside of the variant, plus 1 byte, plus the length of the type string for the item inside the variant.
As an example, consider a dictionary mapping strings to variants. In the case that the dictionary is empty, 0 bytes are required for the serialization.
If we add an item ‘width’ that maps to the int32 value of 500 then we will use 4 bytes to store the int32 (so 6 for the variant containing it) and 6 bytes for the string. The variant must be aligned to 8 after the 6 bytes of the string, so that’s 2 extra bytes. 6 (string) + 2 (padding) + 6 (variant) is 14 bytes used for the dictionary entry. An additional 1 byte is added to the array as a framing offset making a total of 15 bytes.
If we add another entry, ‘title’ that maps to a nullable string that happens to have a value of null, then we use 0 bytes for the null value (and 3 bytes for the variant to contain it along with its type string) plus 6 bytes for the string. Again, we need 2 padding bytes. That makes a total of 6 + 2 + 3 = 11 bytes.
We now require extra padding between the two items in the array. After the 14 bytes of the first item, that’s 2 bytes required. We now require 2 framing offsets for an extra two bytes. 14 + 2 + 11 + 2 = 29 bytes to encode the entire two-item dictionary.
Type Information Cache
For each glib.variant.VariantG type that currently exists in the program a type information structure is kept in the type information cache. The type information structure is required for rapid deserialization.
Continuing with the above example, if a glib.variant.VariantG exists with the type a{sv} then a type information struct will exist for a{sv}, {sv}, s, and v. Multiple uses of the same type will share the same type information. Additionally, all single-digit types are stored in read-only static memory and do not contribute to the writable memory footprint of a program using glib.variant.VariantG.
Aside from the type information structures stored in read-only memory, there are two forms of type information. One is used for container types where there is a single element type: arrays and maybe types. The other is used for container types where there are multiple element types: tuples and dictionary entries.
Array type info structures are 6 * sizeof (void *), plus the memory required to store the type string itself. This means that on 32-bit systems, the cache entry for a{sv} would require 30 bytes of memory (plus allocation overhead).
Tuple type info structures are 6 * sizeof (void *), plus `4 * sizeof (void *)` for each item in the tuple, plus the memory required to store the type string itself. A 2-item tuple, for example, would have a type information structure that consumed writable memory in the size of 14 * sizeof (void *) (plus type string) This means that on 32-bit systems, the cache entry for {sv} would require 61 bytes of memory (plus allocation overhead).
This means that in total, for our a{sv} example, 91 bytes of type information would be allocated.
The type information cache, additionally, uses a glib.hash_table.HashTable to store and look up the cached items and stores a pointer to this hash table in static storage. The hash table is freed when there are zero items in the type cache.
Although these sizes may seem large it is important to remember that a program will probably only have a very small number of different types of values in it and that only one type information structure is required for many different values of the same type.
Buffer Management Memory
glib.variant.VariantG uses an internal buffer management structure to deal with the various different possible sources of serialized data that it uses. The buffer is responsible for ensuring that the correct call is made when the data is no longer in use by glib.variant.VariantG. This may involve a func@GLib.free or even glib.mapped_file.MappedFile.unref.
One buffer management structure is used for each chunk of serialized data. The size of the buffer management structure is 4 * (void *). On 32-bit systems, that’s 16 bytes.
GVariant structure
The size of a glib.variant.VariantG structure is 6 * (void *). On 32-bit systems, that’s 24 bytes.
glib.variant.VariantG structures only exist if they are explicitly created with API calls. For example, if a glib.variant.VariantG is constructed out of serialized data for the example given above (with the dictionary) then although there are 9 individual values that comprise the entire dictionary (two keys, two values, two variants containing the values, two dictionary entries, plus the dictionary itself), only 1 glib.variant.VariantG instance exists — the one referring to the dictionary.
If calls are made to start accessing the other values then glib.variant.VariantG instances will exist for those values only for as long as they are in use (ie: until you call glib.variant.VariantG.unref). The type information is shared. The serialized data and the buffer management structure for that serialized data is shared by the child.
Summary
To put the entire example together, for our dictionary mapping strings to variants (with two entries, as given above), we are using 91 bytes of memory for type information, 29 bytes of memory for the serialized data, 16 bytes for buffer management and 24 bytes for the glib.variant.VariantG instance, or a total of 160 bytes, plus allocation overhead. If we were to use glib.variant.VariantG.getChildValue to access the two dictionary entries, we would use an additional 48 bytes. If we were to have other dictionaries of the same type, we would use more memory for the serialized data and buffer management for those dictionaries, but the type information would be shared.