Struct

GLibVariant

Description [src]

struct GVariant {
  /* No available fields */
}

GVariant is a variant datatype; it can contain one or more values along with information about the type of the values.

A GVariant 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 GVariant is also immutable: once it’s been created neither its type nor its content can be modified further.

GVariant is useful whenever data needs to be serialized, for example when sending method parameters in D-Bus, or when saving settings using GSettings.

When creating a new GVariant, 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 GVariant holding an integer value you can use:

  GVariant *v = g_variant_new ("u", 40);

The string “u” in the first argument tells GVariant that the data passed to the constructor (40) is going to be an unsigned integer.

More advanced examples of GVariant in use can be found in documentation for [GVariant format strings][gvariant-format-strings-pointers].

The range of possible values is determined by the type.

The type system used by GVariant is GVariantType.

GVariant instances always have a type and a value (which are given at construction time). The type and value of a GVariant instance can never change other than by the GVariant itself being destroyed. A GVariant cannot contain a pointer.

GVariant is reference counted using g_variant_ref() and g_variant_unref(). GVariant also has floating reference counts — see g_variant_ref_sink().

GVariant is completely threadsafe. A GVariant instance can be concurrently accessed in any way from any number of threads without problems.

GVariant 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 GVariant data can also be sent over the network.

GVariant is largely compatible with D-Bus. Almost all types of GVariant instances can be sent over D-Bus. See GVariantType for exceptions. (However, GVariants 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 GVariant 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 GVariants size is limited mainly by any lower level operating system constraints, such as the number of bits in #gsize. For example, it is reasonable to have a 2GB file mapped into memory with GMappedFile, and call g_variant_new_from_data() on it.

For convenience to C programmers, GVariant 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 GVariant values. GVariant includes a printer for this language and a parser with type inferencing.

Memory Use

GVariant 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 GVariant can be grouped into 4 broad purposes: memory for serialized data, memory for the type information cache, buffer management memory and memory for the GVariant structure itself.

Serialized Data Memory

This is the memory that is used for storing GVariant 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 byte 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 GVariant 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 GVariant 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 GVariant.

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 malloc 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 malloc 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 GHashTable 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

GVariant 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 GVariant. This may involve a g_free() or a g_slice_free() or even g_mapped_file_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 GVariant structure is 6 * (void *). On 32-bit systems, that’s 24 bytes.

GVariant structures only exist if they are explicitly created with API calls. For example, if a GVariant 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 GVariant instance exists — the one referring to the dictionary.

If calls are made to start accessing the other values then GVariant instances will exist for those values only for as long as they are in use (ie: until you call g_variant_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 GVariant instance, or a total of 160 bytes, plus malloc overhead. If we were to use g_variant_get_child_value() 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.

Available since:2.24

Constructors

g_variant_new

Creates a new GVariant instance.

Available since: 2.24

g_variant_new_array

Creates a new GVariant array from children.

Available since: 2.24

g_variant_new_boolean

Creates a new boolean GVariant instance — either TRUE or FALSE.

Available since: 2.24

g_variant_new_byte

Creates a new byte GVariant instance.

Available since: 2.24

g_variant_new_bytestring

Creates an array-of-bytes GVariant with the contents of string. This function is just like g_variant_new_string() except that the string need not be valid UTF-8.

Available since: 2.26

g_variant_new_bytestring_array

Constructs an array of bytestring GVariant from the given array of strings.

Available since: 2.26

g_variant_new_dict_entry

Creates a new dictionary entry GVariant. key and value must be non-NULL. key must be a value of a basic type (ie: not a container).

Available since: 2.24

g_variant_new_double

Creates a new double GVariant instance.

Available since: 2.24

g_variant_new_fixed_array

Constructs a new array GVariant instance, where the elements are of element_type type.

Available since: 2.32

g_variant_new_from_bytes

Constructs a new serialized-mode GVariant instance. This is the inner interface for creation of new serialized values that gets called from various functions in gvariant.c.

Available since: 2.36

g_variant_new_from_data

Creates a new GVariant instance from serialized data.

Available since: 2.24

g_variant_new_handle

Creates a new handle GVariant instance.

Available since: 2.24

g_variant_new_int16

Creates a new int16 GVariant instance.

Available since: 2.24

g_variant_new_int32

Creates a new int32 GVariant instance.

Available since: 2.24

g_variant_new_int64

Creates a new int64 GVariant instance.

Available since: 2.24

g_variant_new_maybe

Depending on if child is NULL, either wraps child inside of a maybe container or creates a Nothing instance for the given type.

Available since: 2.24

g_variant_new_object_path

Creates a D-Bus object path GVariant with the contents of string. string must be a valid D-Bus object path. Use g_variant_is_object_path() if you’re not sure.

Available since: 2.24

g_variant_new_objv

Constructs an array of object paths GVariant from the given array of strings.

Available since: 2.30

g_variant_new_parsed

Parses format and returns the result.

g_variant_new_parsed_va

Parses format and returns the result.

g_variant_new_printf

Creates a string-type GVariant using printf formatting.

Available since: 2.38

g_variant_new_signature

Creates a D-Bus type signature GVariant with the contents of string. string must be a valid D-Bus type signature. Use g_variant_is_signature() if you’re not sure.

Available since: 2.24

g_variant_new_string

Creates a string GVariant with the contents of string.

Available since: 2.24

g_variant_new_strv

Constructs an array of strings GVariant from the given array of strings.

Available since: 2.24

g_variant_new_take_string

Creates a string GVariant with the contents of string.

Available since: 2.38

g_variant_new_tuple

Creates a new tuple GVariant out of the items in children. The type is determined from the types of children. No entry in the children array may be NULL.

Available since: 2.24

g_variant_new_uint16

Creates a new uint16 GVariant instance.

Available since: 2.24

g_variant_new_uint32

Creates a new uint32 GVariant instance.

Available since: 2.24

g_variant_new_uint64

Creates a new uint64 GVariant instance.

Available since: 2.24

g_variant_new_va

This function is intended to be used by libraries based on GVariant that want to provide g_variant_new()-like functionality to their users.

Available since: 2.24

g_variant_new_variant

Boxes value. The result is a GVariant instance representing a variant containing the original value.

Available since: 2.24

Functions

g_variant_is_object_path

Determines if a given string is a valid D-Bus object path. You should ensure that a string is a valid D-Bus object path before passing it to g_variant_new_object_path().

Available since: 2.24

g_variant_is_signature

Determines if a given string is a valid D-Bus type signature. You should ensure that a string is a valid D-Bus type signature before passing it to g_variant_new_signature().

Available since: 2.24

g_variant_parse

Parses a GVariant from a text representation.

g_variant_parse_error_print_context

Pretty-prints a message showing the context of a GVariant parse error within the string for which parsing was attempted.

Available since: 2.40

g_variant_parse_error_quark
No description available.
g_variant_parser_get_error_quark

Same as g_variant_error_quark().

Instance methods

g_variant_byteswap

Performs a byteswapping operation on the contents of value. The result is that all multi-byte numeric data contained in value is byteswapped. That includes 16, 32, and 64bit signed and unsigned integers as well as file handles and double precision floating point values.

Available since: 2.24

g_variant_check_format_string

Checks if calling g_variant_get() with format_string on value would be valid from a type-compatibility standpoint. format_string is assumed to be a valid format string (from a syntactic standpoint).

Available since: 2.34

g_variant_classify

Classifies value according to its top-level type.

Available since: 2.24

g_variant_compare

Compares one and two.

Available since: 2.26

g_variant_dup_bytestring

Similar to g_variant_get_bytestring() except that instead of returning a constant string, the string is duplicated.

Available since: 2.26

g_variant_dup_bytestring_array

Gets the contents of an array of array of bytes GVariant. This call makes a deep copy; the return result should be released with g_strfreev().

Available since: 2.26

g_variant_dup_objv

Gets the contents of an array of object paths GVariant. This call makes a deep copy; the return result should be released with g_strfreev().

Available since: 2.30

g_variant_dup_string

Similar to g_variant_get_string() except that instead of returning a constant string, the string is duplicated.

Available since: 2.24

g_variant_dup_strv

Gets the contents of an array of strings GVariant. This call makes a deep copy; the return result should be released with g_strfreev().

Available since: 2.24

g_variant_equal

Checks if one and two have the same type and value.

Available since: 2.24

g_variant_get

Deconstructs a GVariant instance.

Available since: 2.24

g_variant_get_boolean

Returns the boolean value of value.

Available since: 2.24

g_variant_get_byte

Returns the byte value of value.

Available since: 2.24

g_variant_get_bytestring

Returns the string value of a GVariant instance with an array-of-bytes type. The string has no particular encoding.

Available since: 2.26

g_variant_get_bytestring_array

Gets the contents of an array of array of bytes GVariant. This call makes a shallow copy; the return result should be released with g_free(), but the individual strings must not be modified.

Available since: 2.26

g_variant_get_child

Reads a child item out of a container GVariant instance and deconstructs it according to format_string. This call is essentially a combination of g_variant_get_child_value() and g_variant_get().

Available since: 2.24

g_variant_get_child_value

Reads a child item out of a container GVariant instance. This includes variants, maybes, arrays, tuples and dictionary entries. It is an error to call this function on any other type of GVariant.

Available since: 2.24

g_variant_get_data

Returns a pointer to the serialized form of a GVariant instance. The returned data may not be in fully-normalised form if read from an untrusted source. The returned data must not be freed; it remains valid for as long as value exists.

Available since: 2.24

g_variant_get_data_as_bytes

Returns a pointer to the serialized form of a GVariant instance. The semantics of this function are exactly the same as g_variant_get_data(), except that the returned GBytes holds a reference to the variant data.

Available since: 2.36

g_variant_get_double

Returns the double precision floating point value of value.

Available since: 2.24

g_variant_get_fixed_array

Provides access to the serialized data for an array of fixed-sized items.

Available since: 2.24

g_variant_get_handle

Returns the 32-bit signed integer value of value.

Available since: 2.24

g_variant_get_int16

Returns the 16-bit signed integer value of value.

Available since: 2.24

g_variant_get_int32

Returns the 32-bit signed integer value of value.

Available since: 2.24

g_variant_get_int64

Returns the 64-bit signed integer value of value.

Available since: 2.24

g_variant_get_maybe

Given a maybe-typed GVariant instance, extract its value. If the value is Nothing, then this function returns NULL.

Available since: 2.24

g_variant_get_normal_form

Gets a GVariant instance that has the same value as value and is trusted to be in normal form.

Available since: 2.24

g_variant_get_objv

Gets the contents of an array of object paths GVariant. This call makes a shallow copy; the return result should be released with g_free(), but the individual strings must not be modified.

Available since: 2.30

g_variant_get_size

Determines the number of bytes that would be required to store value with g_variant_store().

Available since: 2.24

g_variant_get_string

Returns the string value of a GVariant instance with a string type. This includes the types G_VARIANT_TYPE_STRING, G_VARIANT_TYPE_OBJECT_PATH and G_VARIANT_TYPE_SIGNATURE.

Available since: 2.24

g_variant_get_strv

Gets the contents of an array of strings GVariant. This call makes a shallow copy; the return result should be released with g_free(), but the individual strings must not be modified.

Available since: 2.24

g_variant_get_type

Determines the type of value.

Available since: 2.24

g_variant_get_type_string

Returns the type string of value. Unlike the result of calling g_variant_type_peek_string(), this string is nul-terminated. This string belongs to GVariant and must not be freed.

Available since: 2.24

g_variant_get_uint16

Returns the 16-bit unsigned integer value of value.

Available since: 2.24

g_variant_get_uint32

Returns the 32-bit unsigned integer value of value.

Available since: 2.24

g_variant_get_uint64

Returns the 64-bit unsigned integer value of value.

Available since: 2.24

g_variant_get_va

This function is intended to be used by libraries based on GVariant that want to provide g_variant_get()-like functionality to their users.

Available since: 2.24

g_variant_get_variant

Unboxes value. The result is the GVariant instance that was contained in value.

Available since: 2.24

g_variant_hash

Generates a hash value for a GVariant instance.

Available since: 2.24

g_variant_is_container

Checks if value is a container.

Available since: 2.24

g_variant_is_floating

Checks whether value has a floating reference count.

Available since: 2.26

g_variant_is_normal_form

Checks if value is in normal form.

Available since: 2.24

g_variant_is_of_type

Checks if a value has a type matching the provided type.

Available since: 2.24

g_variant_iter_new

Creates a heap-allocated GVariantIter for iterating over the items in value.

Available since: 2.24

g_variant_lookup

Looks up a value in a dictionary GVariant.

Available since: 2.28

g_variant_lookup_value

Looks up a value in a dictionary GVariant.

Available since: 2.28

g_variant_n_children

Determines the number of children in a container GVariant instance. This includes variants, maybes, arrays, tuples and dictionary entries. It is an error to call this function on any other type of GVariant.

Available since: 2.24

g_variant_print

Pretty-prints value in the format understood by g_variant_parse().

Available since: 2.24

g_variant_print_string

Behaves as g_variant_print(), but operates on a GString.

Available since: 2.24

g_variant_ref

Increases the reference count of value.

Available since: 2.24

g_variant_ref_sink

GVariant uses a floating reference count system. All functions with names starting with g_variant_new_ return floating references.

Available since: 2.24

g_variant_store

Stores the serialized form of value at data. data should be large enough. See g_variant_get_size().

Available since: 2.24

g_variant_take_ref

If value is floating, sink it. Otherwise, do nothing.

g_variant_unref

Decreases the reference count of value. When its reference count drops to 0, the memory used by the variant is freed.

Available since: 2.24