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GVariant: say that serialized form needs an out-of-band length

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This confused me for a while, because it isn't the same as D-Bus.
Like GVariant, the D-Bus serialization needs an out-of-band
endianness and type indicator, but unlike GVariant, serialized
D-Bus objects encapsulate their own length (often by starting with
a byte-count). This does come at some redundancy cost, so I can see
why the more efficient GVariant format does this the way it does;
but it's a difference between D-Bus and GVariant that seems worth
calling out.

It's also relevant for the designers of file or message-framing
formats: with D-Bus serialization it would be feasible to say "the file
starts with a little-endian D-Bus variant, followed by...",
but in GVariant you wouldn't be able to deserialize the variant
unless you either assume that it extends to end-of-file, or have
an explicit length.

Bug: https://bugzilla.gnome.org/show_bug.cgi?id=736975
Reviewed-by: Ryan Lortie
This commit is contained in:
Simon McVittie 2014-09-19 15:05:04 +01:00
parent 174ebaefcc
commit 7db1baf590
1 changed files with 7 additions and 5 deletions
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12
glib/gvariant.c
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@ -71,10 +71,11 @@
* in the gio library, for those.) * in the gio library, for those.)
* *
* For space-efficiency, the #GVariant serialisation format does not * For space-efficiency, the #GVariant serialisation format does not
* automatically include the variant's type or endianness, which must * automatically include the variant's length, type or endianness,
* either be implied from context (such as knowledge that a particular * which must either be implied from context (such as knowledge that a
* file format always contains a little-endian %G_VARIANT_TYPE_VARIANT) * particular file format always contains a little-endian
* or supplied out-of-band (for instance, a type and/or endianness * %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 * indicator could be placed at the beginning of a file, network message
* or network stream). * or network stream).
* *
@ -107,7 +108,8 @@
* *
* This is the memory that is used for storing GVariant data in * This is the memory that is used for storing GVariant data in
* serialised form. This is what would be sent over the network or * serialised form. This is what would be sent over the network or
* what would end up on disk. * 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, * The amount of memory required to store a boolean is 1 byte. 16,
* 32 and 64 bit integers and double precision floating point numbers * 32 and 64 bit integers and double precision floating point numbers