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docs: Clarify costs of using the generic GObject C closure marshaller

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The libffi one is slower than type-specific generated ones, but is
generally better to use.

https://bugzilla.gnome.org/show_bug.cgi?id=744060
This commit is contained in:
Philip Withnall 2015-02-24 08:50:53 +00:00
parent e57741791e
commit 57d0ec57e4
2 changed files with 17 additions and 5 deletions
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3
docs/reference/gobject/tut_gsignal.xml
View File

@ -124,7 +124,8 @@ return_type function_callback (… , gpointer user_data);
A generic C closure marshaller is available as
<link linkend="g-cclosure-marshal-generic"><function>g_cclosure_marshal_generic</function></link>
which implements marshalling for all function types using libffi. Custom
marshallers for different types are not needed.
marshallers for different types are not needed apart from performance
critical code where the libffi-based marshaller may be too slow.
</para>
<para>

19
docs/reference/gobject/tut_howto.xml
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@ -1369,7 +1369,7 @@ file_signals[CHANGED] =
NULL /* closure */,
NULL /* accumulator */,
NULL /* accumulator data */,
g_cclosure_marshal_generic,
NULL /* C marshaller */,
G_TYPE_NONE /* return_type */,
0 /* n_params */,
NULL /* param_types */);
@ -1396,11 +1396,22 @@ maman_file_write (MamanFile *self,
</para>
<para>
The C signal marshaller should always be
The C signal marshaller should always be <literal>NULL</literal>, in which
case the best marshaller for the given closure type will be chosen by
GLib. This may be an internal marshaller specific to the closure type, or
<function>g_cclosure_marshal_generic</function>, which implements generic
conversion of arrays of parameters to C callback invocations. GLib used to
use type-specific generated marshallers, but that has been deprecated in
favour of the generic marshaller.
require the user to write or generate a type-specific marshaller and pass
that, but that has been deprecated in favour of automatic selection of
marshallers.
</para>
<para>
Note that <function>g_cclosure_marshal_generic</function> is slower than
non-generic marshallers, so should be avoided for performance critical
code. However, performance critical code should rarely be using signals
anyway, as emitting a signal blocks on emitting it to all listeners, which
has potentially unbounded cost.
</para>
</sect1>
</chapter>