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GVariant Format Strings

GVariant Format Strings — varargs conversion of GVariants

Variable Argument Conversions

This page attempts to document how to perform variable argument conversions with GVariant.

Conversions occur according to format strings. A format string is a two-way mapping between a single GVariant value and one or more C values.

A conversion from C values into a GVariant value is made using the g_variant_new() function. A conversion from a GVariant into C values is made using the g_variant_get() function.

Syntax

This section exhaustively describes all possibilities for GVariant format strings. There are no valid forms of format strings other than those described here. Please note that the format string syntax is likely to expand in the future.

Valid format strings have one of the following forms:

  • any type string

  • a type string prefixed with a '@'

  • '&s' '&o', '&g', '^as', '^a&s', '^ao', '^a&o','^ay', '^&ay', '^aay' or '^a&ay'.

  • any format string, prefixed with an 'm'

  • a sequence of zero or more format strings, concatenated and enclosed in parentheses

  • an opening brace, followed by two format strings, followed by a closing brace (subject to the constraint that the first format string correspond to a type valid for use as the key type of a dictionary)

Symbols

The following table describes the rough meaning of symbols that may appear inside a GVariant format string. Each symbol is described in detail in its own section, including usage examples.

Symbol

Meaning

b, y, n, q, i, u, x, t, h, d

Used for building or deconstructing boolean, byte and numeric types. See Numeric Types below.

s, o, g

Used for building or deconstructing string types. See Strings below.

v

Used for building or deconstructing variant types. See Variants below.

a

Used for building or deconstructing arrays. See Arrays below.

m

Used for building or deconstructing maybe types. See Maybe Types below.

()

Used for building or deconstructing tuples. See Tuples below.

{}

Used for building or deconstructing dictionary entries. See Dictionaries below.

@

Used as a prefix for a GVariant type string (not a prefix for a format string, so @as is a valid format string but @^as is not). Denotes that a pointer to a GVariant should be used in place of the normal C type or types. For g_variant_new() this means that you must pass a non-NULL (GVariant *); if it is a floating reference, ownership will be taken, as if by using g_variant_ref_sink(). For g_variant_get() this means that you must pass a pointer to a (GVariant *) for the value to be returned by reference or NULL to ignore the value. See GVariant * below.

*, ?, r

Exactly equivalent to @*, @? and @r. Provided only for completeness so that all GVariant type strings can be used also as format strings. See GVariant * below.

&

Used as a prefix for a GVariant type string (not a prefix for a format string, so &s is a valid format string but &@s is not). Denotes that a C pointer to serialised data should be used in place of the normal C type. See Pointers below.

^

Used as a prefix on some specific types of format strings. See Convenience Conversions below.

Numeric Types

Characters: b, y, n, q, i, u, x, t, h, d

Variable argument conversions from numeric types work in the most obvious way possible. Upon encountering one of these characters, g_variant_new() takes the equivalent C type as an argument. g_variant_get() takes a pointer to the equivalent C type (or NULL to ignore the value).

The equivalent C types are as follows:

Character

Equivalent C type

b

gboolean

y

guchar

n

gint16

q

guint16

i

gint32

u

guint32

x

gint64

t

guint64

h

gint32

d

gdouble

Note that in C, small integer types in variable argument lists are promoted up to int or unsigned int as appropriate, and read back accordingly. int is 32 bits on every platform on which GLib is currently supported. This means that you can use C expressions of type int with g_variant_new() and format characters 'b', 'y', 'n', 'q', 'i', 'u' and 'h'. Specifically, you can use integer literals with these characters.

When using the 'x' and 't' characters, you must ensure that the value that you provide is 64 bit. This means that you should use a cast or make use of the G_GINT64_CONSTANT or G_GUINT64_CONSTANT macros.

No type promotion occurs when using g_variant_get() since it operates with pointers. The pointers must always point to a memory region of exactly the correct size.

Examples

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GVariant *value1, *value2, *value3, *value4;

value1 = g_variant_new ("y", 200);
value2 = g_variant_new ("b", TRUE);
value3 = g_variant_new ("d", 37.5):
value4 = g_variant_new ("x", G_GINT64_CONSTANT (998877665544332211));

{
  gdouble floating;
  gboolean truth;
  gint64 bignum;


  g_variant_get (value1, "y", NULL);      /* ignore the value. */
  g_variant_get (value2, "b", &truth);
  g_variant_get (value3, "d", &floating);
  g_variant_get (value4, "x", &bignum);
}

Strings

Characters: s, o, g

String conversions occur to and from standard nul-terminated C strings. Upon encountering an 's', 'o' or 'g' in a format string, g_variant_new() takes a (const gchar *) and makes a copy of it. NULL is not a valid string; use maybe types to encode that. If the 'o' or 'g' characters are used, care must be taken to ensure that the passed string is a valid DBus object path or DBus type signature, respectively.

Upon encounting 's', 'o' or 'g', g_variant_get() takes a pointer to a (gchar *) (ie: (gchar **)) and sets it to a newly-allocated copy of the string. It is appropriate to free this copy using g_free(). NULL may also be passed to indicate that the value of the string should be ignored (in which case no copy is made).

Examples

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GVariant *value1, *value2, *value3;

value1 = g_variant_new ("s", "hello world!");
value2 = g_variant_new ("o", "/must/be/a/valid/path");
value3 = g_variant_new ("g", "iias");

#if 0
  g_variant_new ("s", NULL);      /* not valid: NULL is not a string. */
#endif

{
  gchar *result;

  g_variant_get (value1, "s", &result);
  g_print ("It was '%s'\n", result);
  g_free (result);
}

Variants

Characters: v

Upon encountering a 'v', g_variant_new() takes a (GVariant *). The value of the GVariant is used as the contents of the variant value.

Upon encountering a 'v', g_variant_get() takes a pointer to a (GVariant *) (ie: (GVariant **) ). It is set to a new reference to a GVariant instance containing the contents of the variant value. It is appropriate to free this reference using g_variant_unref(). NULL may also be passed to indicate that the value should be ignored (in which case no new reference is created).

Examples

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GVariant *x, *y;

/* the following two lines are equivalent: */
x = g_variant_new ("v", y);
x = g_variant_new_variant (y);

/* as are these: */
g_variant_get (x, "v", &y);
y = g_variant_get_variant (x);

Arrays

Characters: a

Upon encountering an 'a' character followed by a type string, g_variant_new() will take a (GVariantBuilder *) that has been created as an array builder for an array of the type given in the type string. The builder will have g_variant_builder_end() called on it and the result will be used as the value. As a special exception, if the given type string is a definite type, then NULL may be given to mean an empty array of that type.

Upon encountering an 'a' character followed by a type string, g_variant_get() will take a pointer to a (GVariantIter *) (ie: (GVariantIter **)). A new heap-allocated iterator is created and returned, initialised for iterating over the elements of the array. This iterator should be freed when you are done with it, using g_variant_iter_free(). NULL may also be given to indicate that the value of the array should be ignored.

Examples

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GVariantBuilder *builder;
GVariant *value;

builder = g_variant_builder_new (G_VARIANT_TYPE ("as"));
g_variant_builder_add (builder, "s", "when");
g_variant_builder_add (builder, "s", "in");
g_variant_builder_add (builder, "s", "the");
g_variant_builder_add (builder, "s", "course");
value = g_variant_new ("as", builder);
g_variant_builder_unref (builder);

{
  GVariantIter *iter;
  gchar *str;

  g_variant_get (value, "as", &iter);
  while (g_variant_iter_loop (iter, "s", &str))
    g_print ("%s\n", str);
  g_variant_iter_free (iter);
}

g_variant_unref (value);

Maybe Types

Characters: m

Maybe types are handled in two separate ways depending on the format string that follows the 'm'. The method that is used currently depends entirely on the character immediately following the 'm'.

The first way is used with format strings starting with 'a', 's', 'o', 'g', 'v', '@', '*', '?', 'r', '&', or '^'. In all of these cases, for non-maybe types, g_variant_new() takes a pointer to a non-NULL value and g_variant_get() returns (by reference) a non-NULL pointer. When any of these format strings are prefixed with an 'm', the type of arguments that are collected does not change in any way, but NULL becomes a permissable value, to indicate the Nothing case.

Note that the "special exception" introduced in the array section for constructing empty arrays is ignored here. Using a NULL pointer with the format string 'mas' constructs the Nothing value -- not an empty array.

The second way is used with all other format strings. For g_variant_new() an additional gboolean argument is collected and for g_variant_get() an additional (gboolean *). Following this argument, the arguments that are normally collected for the equivalent non-maybe type will be collected.

If FALSE is given to g_variant_new() then the Nothing value is constructed and the collected arguments are ignored. Otherwise (if TRUE was given), the arguments are used in the normal way to create the Just value.

If NULL is given to g_variant_get() then the value is ignored. If a non-NULL pointer is given then it is used to return by reference whether the value was Just. In the case that the value was Just, the gboolean will be set to TRUE and the value will be stored in the arguments in the usual way. In the case that the value was Nothing, the gboolean will be set to FALSE and the arguments will be collected in the normal way but have their values set to binary zero.

Examples

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GVariant *value1, *value2, *value3, *value4, *value5, *value6;
value1 = g_variant_new ("ms", "Hello world");
value2 = g_variant_new ("ms", NULL);
value3 = g_variant_new ("(m(ii)s)", TRUE, 123, 456, "Done");
value4 = g_variant_new ("(m(ii)s)", FALSE, -1, -1, "Done");          /* both '-1' are ignored. */
value5 = g_variant_new ("(m@(ii)s)", NULL, "Done");

{
  GVariant *contents;
  const gchar *cstr;
  gboolean just;
  gint32 x, y;
  gchar *str;

  g_variant_get (value1, "ms", &str);
  if (str != NULL)
    g_print ("str: %s\n", str);
  else
    g_print ("it was null\n");
  g_free (str);


  g_variant_get (value2, "m&s", &cstr);
  if (cstr != NULL)
    g_print ("str: %s\n", cstr);
  else
    g_print ("it was null\n");
  /* don't free 'cstr' */


  /* NULL passed for the gboolean *, but two 'gint32 *' still collected */
  g_variant_get (value3, "(m(ii)s)", NULL, NULL, NULL, &str);
  g_print ("string is %s\n", str);
  g_free (str);

  /* note: &s used, so g_free() not needed */
  g_variant_get (value4, "(m(ii)&s)", &just, &x, &y, &cstr);
  if (just)
    g_print ("it was (%d, %d)\n", x, y);
  else
    g_print ("it was null\n");
  g_print ("string is %s\n", cstr);
  /* don't free 'cstr' */


  g_variant_get (value5, "(m*s)", &contents, NULL); /* ignore the string. */
  if (contents != NULL)
    {
      g_variant_get (contents, "(ii)", &x, &y);
      g_print ("it was (%d, %d)\n", x, y);
      g_variant_unref (contents);
    }
  else
    g_print ("it was null\n");
}

Tuples

Characters: ()

Tuples are handled by handling each item in the tuple, in sequence. Each item is handled in the usual way.

Examples

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GVariant *value1, *value2;

value1 = g_variant_new ("(s(ii))", "Hello", 55, 77);
value2 = g_variant_new ("()");

{
  gchar *string;
  gint x, y;

  g_variant_get (value1, "(s(ii))", &string, &x, &y);
  g_print ("%s, %d, %d\n", string, x, y);
  g_free (string);

  g_variant_get (value2, "()");   /* do nothing... */
}

Dictionaries

Characters: {}

Dictionary entries are handled by handling first the key, then the value. Each is handled in the usual way.

Examples

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GVariantBuilder *b;
GVariant *dict;

b = g_variant_builder_new (G_VARIANT_TYPE ("a{sv}"));
g_variant_builder_add (b, "{sv}", "name", g_variant_new_string ("foo"));
g_variant_builder_add (b, "{sv}", "timeout", g_variant_new_int32 (10));
dict = g_variant_builder_end (b);

GVariant *

Characters: @, *, ?, r

Upon encountering a '@' in front of a type string, g_variant_new() takes a non-NULL pointer to a GVariant and uses its value directly instead of collecting arguments to create the value. The provided GVariant must have a type that matches the type string following the '@'. '*' is the same as '@*' (ie: take a GVariant of any type). '?' is the same as '@?' (ie: take a GVariant of any basic type). 'r' is the same as '@r' (ie: take a GVariant of any tuple type).

Upon encountering a '@' in front of a type string, g_variant_get() takes a pointer to a (GVariant *) (ie: a (GVariant **)) and sets it to a new reference to a GVariant containing the value (instead of deconstructing the value into C types in the usual way). NULL can be given to ignore the value. '*', '?' and 'r' are handled in a way analogous to what is stated above.

You can always use '*' as an alternative to '?', 'r' or any use of '@'. Using the other characters where possible is recommended, however, due to the improvements in type safety and code self-documentation.

Examples

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GVariant *value1, *value2;

value1 = g_variant_new ("(i@ii)", 44, g_variant_new_int32 (55), 66);

/* note: consumes floating reference count on 'value1' */
value2 = g_variant_new ("(@(iii)*)", value1, g_variant_new_string ("foo"));

{
  const gchar *string;
  GVariant *tmp;
  gsize length;
  gint x, y, z;

  g_variant_get (value2, "((iii)*)", &x, &y, &z, &tmp);
  string = g_variant_get_string (tmp, &length);
  g_print ("it is %d %d %d %s (length=%d)\n", x, y, z, string, (int) length);
  g_variant_unref (tmp);

  /* quick way to skip all the values in a tuple */
  g_variant_get (value2, "(rs)", NULL, &string); /* or "(@(iii)s)" */
  g_print ("i only got the string: %s\n", string);
  g_free (string);
}

Pointers

Characters: &

The '&' character is used to indicate that serialised data should be directly exchanged via a pointer.

Currently, the only use for this character is when it is applied to a string (ie: '&s', '&o' or '&g'). For g_variant_new() this has absolutely no effect. The string is collected and duplicated normally. For g_variant_get() it means that instead of creating a newly allocated copy of the string, a pointer to the serialised data is returned. This pointer should not be freed. Validity checks are performed to ensure that the string data will always be properly nul-terminated.

Examples

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{
  const gchar *str;
  GVariant *value;

  value = g_variant_new ("&s", "hello world");
  g_variant_get (value, "&s", &str);
  g_print ("string is: %s\n", str);
  /* no need to free str */
}

Convenience Conversions

Characters: ^

The '^' character currently supports conversion to and from bytestrings or to and from arrays of strings or bytestrings. It has a number of forms.

In all forms, when used with g_variant_new() one pointer value is collected from the variable arguments and passed to a function (as given in the table below). The result of that function is used as the value for this position. When used with g_variant_get() one pointer value is produced by using the function (given in the table) and returned by reference.

Conversion

Used with g_variant_new()

Used with g_variant_get()

^as

equivalent to g_variant_new_strv()

equivalent to g_variant_dup_strv()

^a&s

equivalent to g_variant_get_strv()

^ao

equivalent to g_variant_new_objv()

equivalent to g_variant_dup_objv()

^a&o

equivalent to g_variant_get_objv()

^ay

equivalent to g_variant_new_bytestring()

equivalent to g_variant_dup_bytestring()

^&ay

equivalent to g_variant_get_bytestring()

^aay

equivalent to g_variant_new_bytestring_array()

equivalent to g_variant_dup_bytestring_array()

^a&ay

equivalent to g_variant_get_bytestring_array()

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