Use
g_main_context_invoke_full()
to invoke functions in other threads, assuming every thread has a thread
default main context which runs throughout the lifetime of that thread
()
Use
GTask
to run a function in the background without caring about the specific
thread used ()
Liberally use assertions to check which context executes each function, and add these assertions when first writing the code ()
Explicitly document contexts a function is expected to be called in, a callback will be invoked in, or a signal will be emitted in ()
Beware of g_idle_add()
and similar functions which
implicitly use the global-default main context
()
GMainContext
?
GMainContext
is a generalized implementation of an
event loop,
useful for implementing polled file I/O or event-based widget systems
(such as GTK+). It is at the core of almost every GLib application. To
understand GMainContext
requires understanding
poll() and polled I/O.
A GMainContext
has a set of
GSource
s
which are ‘attached’ to it, each of which can be thought of as an expected
event with an associated callback function which will be invoked when that
event is received; or equivalently as a set of file descriptors (FDs) to
check. An event could be a timeout or data being received on a socket, for
example. One iteration of the event loop will:
Prepare sources, determining if any of them are ready to dispatch immediately.
Poll the sources, blocking the current thread until an event is received for one of the sources.
Check which of the sources received an event (several could have).
Dispatch callbacks from those sources.
This is explained very well in the GLib documentation.
At its core, GMainContext
is just a poll()
loop,
with the preparation, check and dispatch stages of the loop corresponding
to the normal preamble and postamble in a typical poll()
loop
implementation, such as listing 1 from
this article.
Typically, some complexity is needed in non-trivial
poll()
-using applications to track the lists of FDs which are
being polled. Additionally, GMainContext
adds a lot of useful
functionality which vanilla poll()
doesn’t support. Most
importantly, it adds thread safety.
GMainContext
is completely thread safe, meaning that a
GSource
can be created in one thread and attached to a
GMainContext
running in another thread. (See
also: .) A typical use for this might be to allow
worker threads to control which sockets are being listened to by a
GMainContext
in a central I/O thread. Each
GMainContext
is ‘acquired’ by a thread for each iteration
it’s put through. Other threads cannot iterate a GMainContext
without acquiring it, which guarantees that a GSource
and its
FDs will only be polled by one thread at once (since each
GSource
is attached to at most one
GMainContext
). A GMainContext
can be swapped
between threads across iterations, but this is expensive.
GMainContext
is used instead of poll()
mostly
for convenience, as it transparently handles dynamically managing
the array of FDs to pass to poll()
, especially when operating
over multiple threads. This is done by encapsulating FDs in
GSource
s, which decide whether those FDs should be passed to
the poll()
call on each ‘prepare’ stage of the main context
iteration.
GMainLoop
?
GMainLoop
is essentially the following few lines of code, once reference counting
and locking have been removed (from
g_main_loop_run()
):
loop->is_running = TRUE;
while (loop->is_running)
{
g_main_context_iteration (context, TRUE);
}
Plus a fourth line in
g_main_loop_quit()
which sets loop->is_running = FALSE
and which will cause
the loop to terminate once the current main context iteration ends.
Hence, GMainLoop
is a convenient, thread-safe way of running
a GMainContext
to process events until a desired exit
condition is met, at which point g_main_loop_quit()
should be
called. Typically, in a UI program, this will be the user clicking ‘exit’.
In a socket handling program, this might be the final socket closing.
It is important not to confuse main contexts with main loops. Main contexts do the bulk of the work: preparing source lists, waiting for events, and dispatching callbacks. A main loop simply iterates a context.
One of the important features of GMainContext
is its support
for ‘default’ contexts. There are two levels of default context: the
thread-default, and the global-default. The global-default (accessed using
g_main_context_default()
) is run by GTK+ when
gtk_main()
is called. It’s also used for timeouts
(g_timeout_add()
) and idle callbacks
(g_idle_add()
) — these won’t be dispatched unless the default
context is running! (See:
.)
Thread-default contexts are a later addition to GLib (since version 2.22),
and are generally used for I/O operations which need to run and dispatch
callbacks in a thread. By calling
g_main_context_push_thread_default()
before starting an I/O
operation, the thread-default context is set and the I/O operation can add
its sources to that context. The context can then be run in a new main
loop in an I/O thread, causing the callbacks to be dispatched on that
thread’s stack rather than on the stack of the thread running the
global-default main context. This allows I/O operations to be run entirely
in a separate thread without explicitly passing a specific
GMainContext
pointer around everywhere.
Conversely, by starting a long-running operation with a specific
thread-default context set, the calling code can guarantee that the
operation’s callbacks will be emitted in that context, even if the
operation itself runs in a worker thread. This is the principle behind
GTask
:
when a new GTask
is created, it stores a reference to the
current thread-default context, and dispatches its completion callback in
that context, even if the task itself is run using
g_task_run_in_thread()
.
For example, the code below will run a GTask
which performs
two writes in parallel from a thread. The callbacks for the writes will
be dispatched in the worker thread, whereas the callback from the task
as a whole will be dispatched in the interesting_context
.
typedef struct {
GMainLoop *main_loop;
guint n_remaining;
} WriteData;
/* This is always called in the same thread as thread_cb() because
* it’s always dispatched in the @worker_context. */
static void
write_cb (GObject *source_object,
GAsyncResult *result,
gpointer user_data)
{
WriteData *data = user_data;
GOutputStream *stream = G_OUTPUT_STREAM (source_object);
GError *error = NULL;
gssize len;
/* Finish the write. */
len = g_output_stream_write_finish (stream, result, &error);
if (error != NULL)
{
g_error ("Error: %s", error->message);
g_error_free (error);
}
/* Check whether all parallel operations have finished. */
write_data->n_remaining--;
if (write_data->n_remaining == 0)
{
g_main_loop_quit (write_data->main_loop);
}
}
/* This is called in a new thread. */
static void
thread_cb (GTask *task,
gpointer source_object,
gpointer task_data,
GCancellable *cancellable)
{
/* These streams come from somewhere else in the program: */
GOutputStream *output_stream1, *output_stream;
GMainContext *worker_context;
GBytes *data;
const guint8 *buf;
gsize len;
/* Set up a worker context for the writes’ callbacks. */
worker_context = g_main_context_new ();
g_main_context_push_thread_default (worker_context);
/* Set up the writes. */
write_data.n_remaining = 2;
write_data.main_loop = g_main_loop_new (worker_context, FALSE);
data = g_task_get_task_data (task);
buf = g_bytes_get_data (data, &len);
g_output_stream_write_async (output_stream1, buf, len,
G_PRIORITY_DEFAULT, NULL, write_cb,
&write_data);
g_output_stream_write_async (output_stream2, buf, len,
G_PRIORITY_DEFAULT, NULL, write_cb,
&write_data);
/* Run the main loop until both writes have finished. */
g_main_loop_run (write_data.main_loop);
g_task_return_boolean (task, TRUE); /* ignore errors */
g_main_loop_unref (write_data.main_loop);
g_main_context_pop_thread_default (worker_context);
g_main_context_unref (worker_context);
}
/* This can be called from any thread. Its @callback will always be
* dispatched in the thread which currently owns
* @interesting_context. */
void
parallel_writes_async (GBytes *data,
GMainContext *interesting_context,
GCancellable *cancellable,
GAsyncReadyCallback callback,
gpointer user_data)
{
GTask *task;
g_main_context_push_thread_default (interesting_context);
task = g_task_new (NULL, cancellable, callback, user_data);
g_task_set_task_data (task, data,
(GDestroyNotify) g_bytes_unref);
g_task_run_in_thread (task, thread_cb);
g_object_unref (task);
g_main_context_pop_thread_default (interesting_context);
}
Several functions implicitly add sources to the global-default main
context. They should not be used in threaded code. Instead, use
g_source_attach()
with the GSource
created by
the replacement function from the table below.
Implicit use of the global-default main context means the callback functions are invoked in the main thread, typically resulting in work being brought back from a worker thread into the main thread.
No use |
Use en su lugar |
|
|
|
|
|
|
So to delay some computation in a worker thread, use the following code:
static guint
schedule_computation (guint delay_seconds)
{
GSource *source = NULL;
GMainContext *context;
guint id;
/* Get the calling context. */
context = g_main_context_get_thread_default ();
source = g_timeout_source_new_seconds (delay_seconds);
g_source_set_callback (source, do_computation, NULL, NULL);
id = g_source_attach (source, context);
g_source_unref (source);
/* The ID can be used with the same @context to
* cancel the scheduled computation if needed. */
return id;
}
static void
do_computation (gpointer user_data)
{
/* … */
}
GMainContext
en una biblioteca
At a high level, library code must not make changes to main contexts which
could affect the execution of an application using the library, for
example by changing when the application’s GSource
s are
dispatched. There are various best practices which can be followed to aid
this.
Never iterate a context created outside the library, including the
global-default or thread-default contexts. Otherwise,
GSource
s created in the application may be dispatched
when the application is not expecting it, causing
re-entrancy
problems for the application code.
Always remove GSource
s from a main context before dropping
the library’s last reference to the context, especially if it may have
been exposed to the application (for example, as a thread-default).
Otherwise the application may keep a reference to the main context and
continue iterating it after the library has returned, potentially causing
unexpected source dispatches in the library. This is equivalent to not
assuming that dropping the library’s last reference to a main context will
finalize that context.
If the library is designed to be used from multiple threads, or in a context-aware fashion, always document which context each callback will be dispatched in. For example, “callbacks will always be dispatched in the context which is the thread-default at the time of the object’s construction”. Developers using the library’s API need to know this information.
Use g_main_context_invoke()
to ensure callbacks are
dispatched in the right context. It’s much easier than manually using
g_idle_source_new()
to transfer work between contexts.
(See: .)
Libraries should never use g_main_context_default()
(or,
equivalently, pass NULL
to a GMainContext
-typed
parameter). Always store and explicitly use a specific
GMainContext
, even if it often points to some default
context. This makes the code easier to split out into threads in future,
if needed, without causing hard-to-debug problems caused by callbacks
being invoked in the wrong context.
Always write things asynchronously internally (using
GTask
where appropriate), and keep
synchronous wrappers at the very top level of an API, where they can be
implemented by calling g_main_context_iteration()
on a
specific GMainContext
. Again, this makes future refactoring
easier. This is demonstrated in the above example: the thread uses
g_output_stream_write_async()
rather than
g_output_stream_write()
.
Always match pushes and pops of the thread-default main context:
g_main_context_push_thread_default()
and
g_main_context_pop_thread_default()
.
The ‘right context’ is the thread-default main context of the thread the function should be executing in. This assumes the typical case that every thread has a single main context running in a main loop. A main context effectively provides a work or message queue for the thread — something which the thread can periodically check to determine if there is work pending from another thread. Putting a message on this queue – invoking a function in another main context – will result in it eventually being dispatched in that thread.
For example, if an application does a long and CPU-intensive computation it should schedule this in a background thread so that UI updates in the main thread are not blocked. The results of the computation, however, might need to be displayed in the UI, so some UI update function must be called in the main thread once the computation’s complete.
Furthermore, if the computation function can be limited to a single thread, it becomes easy to eliminate the need for locking a lot of the data it accesses. This assumes that other threads are implemented similarly and hence most data is only accessed by a single thread, with threads communicating by message passing. This allows each thread to update its data at its leisure, which significantly simplifies locking.
For some functions, there might be no reason to care which context they’re executed in, perhaps because they’re asynchronous and hence do not block the context. However, it is still advisable to be explicit about which context is used, since those functions may emit signals or invoke callbacks, and for reasons of thread safety it’s necessary to know which threads those signal handlers or callbacks are going to be invoked in.
For example, the progress callback in
g_file_copy_async()
is documented as being called in the thread-default main context
at the time of the initial call.
The core principle of invoking a function in a specific context is
simple, and is walked through below to explain the concepts. In practice
the convenience method,
g_main_context_invoke_full()
should be used instead.
A
GSource
has to be added to the target GMainContext
, which will invoke
the function when it’s dispatched. This GSource
should almost
always be an idle source created with
g_idle_source_new()
,
but this doesn’t have to be the case. It could be a timeout source
so that the function is executed after a delay, for example.
The GSource
will be
dispatched as soon as it’s ready,
calling the function on the thread’s stack. In the case of an idle source,
this will be as soon as all sources at a higher priority have been
dispatched — this can be tweaked using the idle source’s priority
parameter with
g_source_set_priority()
.
The source will typically then be destroyed so the function is only
executed once (though again, this doesn’t have to be the case).
Data can be passed between threads as the user_data
passed to
the GSource
’s callback. This is set on the source using
g_source_set_callback()
,
along with the callback function to invoke. Only a single pointer
is provided, so if multiple data fields need passing, they must be wrapped
in an allocated structure.
The example below demonstrates the underlying principles, but there are convenience methods explained below which simplify things.
/* Main function for the background thread, thread1. */
static gpointer
thread1_main (gpointer user_data)
{
GMainContext *thread1_main_context = user_data;
GMainLoop *main_loop;
/* Set up the thread’s context and run it forever. */
g_main_context_push_thread_default (thread1_main_context);
main_loop = g_main_loop_new (thread1_main_context, FALSE);
g_main_loop_run (main_loop);
g_main_loop_unref (main_loop);
g_main_context_pop_thread_default (thread1_main_context);
g_main_context_unref (thread1_main_context);
return NULL;
}
/* A data closure structure to carry multiple variables between
* threads. */
typedef struct {
gchar *some_string; /* owned */
guint some_int;
GObject *some_object; /* owned */
} MyFuncData;
static void
my_func_data_free (MyFuncData *data)
{
g_free (data->some_string);
g_clear_object (&data->some_object);
g_free (data);
}
static void
my_func (const gchar *some_string,
guint some_int,
GObject *some_object)
{
/* Do something long and CPU intensive! */
}
/* Convert an idle callback into a call to my_func(). */
static gboolean
my_func_idle (gpointer user_data)
{
MyFuncData *data = user_data;
my_func (data->some_string, data->some_int, data->some_object);
return G_SOURCE_REMOVE;
}
/* Function to be called in the main thread to schedule a call to
* my_func() in thread1, passing the given parameters along. */
static void
invoke_my_func (GMainContext *thread1_main_context,
const gchar *some_string,
guint some_int,
GObject *some_object)
{
GSource *idle_source;
MyFuncData *data;
/* Create a data closure to pass all the desired variables
* between threads. */
data = g_new0 (MyFuncData, 1);
data->some_string = g_strdup (some_string);
data->some_int = some_int;
data->some_object = g_object_ref (some_object);
/* Create a new idle source, set my_func() as the callback with
* some data to be passed between threads, bump up the priority
* and schedule it by attaching it to thread1’s context. */
idle_source = g_idle_source_new ();
g_source_set_callback (idle_source, my_func_idle, data,
(GDestroyNotify) my_func_data_free);
g_source_set_priority (idle_source, G_PRIORITY_DEFAULT);
g_source_attach (idle_source, thread1_main_context);
g_source_unref (idle_source);
}
/* Main function for the main thread. */
static void
main (void)
{
GThread *thread1;
GMainContext *thread1_main_context;
/* Spawn a background thread and pass it a reference to its
* GMainContext. Retain a reference for use in this thread
* too. */
thread1_main_context = g_main_context_new ();
g_thread_new ("thread1", thread1_main,
g_main_context_ref (thread1_main_context));
/* Maybe set up your UI here, for example. */
/* Invoke my_func() in the other thread. */
invoke_my_func (thread1_main_context,
"some data which needs passing between threads",
123456, some_object);
/* Continue doing other work. */
}
This invocation is uni-directional: it calls
my_func()
in thread1
, but there’s no way to
return a value to the main thread. To do that, the same principle needs
to be used again, invoking a callback function in the main thread. It’s
a straightforward extension which isn’t covered here.
To maintain thread safety, data which is potentially accessed by
multiple threads must make those accesses mutually exclusive using a
mutex.
Data potentially accessed by multiple threads:
thread1_main_context
, passed in the fork call to
thread1_main
; and some_object
, a reference to
which is passed in the data closure. Critically, GLib guarantees that
GMainContext
is thread safe, so sharing
thread1_main_context
between threads is safe. The example
assumes that other code accessing some_object
is thread
safe.
Note that some_string
and some_int
cannot be
accessed from both threads, because copies of them are passed
to thread1
, rather than the originals. This is a standard
technique for making cross-thread calls thread safe without requiring
locking. It also avoids the problem of synchronizing freeing
some_string
.
Similarly, a reference to some_object
is transferred to
thread1
, which works around the issue of synchronizing
destruction of the object (see ).
g_idle_source_new()
is used rather than the simpler
g_idle_add()
so the GMainContext
to attach to
can be specified.
g_main_context_invoke_full()
This is simplified greatly by the convenience method,
g_main_context_invoke_full()
.
It invokes a callback so that the specified GMainContext
is
owned during the invocation. Owning a main context is almost always
equivalent to running it, and hence the function is invoked in the
thread for which the specified context is the thread-default.
g_main_context_invoke()
can be used instead if the user
data does not need to be freed by a GDestroyNotify
callback
after the invocation returns.
Modifying the earlier example, the invoke_my_func()
function can be replaced by the following:
static void
invoke_my_func (GMainContext *thread1_main_context,
const gchar *some_string,
guint some_int,
GObject *some_object)
{
MyFuncData *data;
/* Create a data closure to pass all the desired variables
* between threads. */
data = g_new0 (MyFuncData, 1);
data->some_string = g_strdup (some_string);
data->some_int = some_int;
data->some_object = g_object_ref (some_object);
/* Invoke the function. */
g_main_context_invoke_full (thread1_main_context,
G_PRIORITY_DEFAULT, my_func_idle,
data,
(GDestroyNotify) my_func_data_free);
}
Consider what happens if invoke_my_func()
were called
from thread1
, rather than from the main thread. With the
original implementation, the idle source would be added to
thread1
’s context and dispatched on the context’s next
iteration (assuming no pending dispatches with higher priorities).
With the improved implementation,
g_main_context_invoke_full()
will notice that the
specified context is already owned by the thread (or ownership can be
acquired by it), and will call my_func_idle()
directly,
rather than attaching a source to the context and delaying the
invocation to the next context iteration.
This subtle behavior difference doesn’t matter in most cases, but is
worth bearing in mind since it can affect blocking behavior
(invoke_my_func()
would go from taking negligible time,
to taking the same amount of time as my_func()
before
returning).
It is useful to document which thread each function should be called in, in the form of an assertion:
g_assert (g_main_context_is_owner (expected_main_context));
If that’s put at the top of each function, any assertion failure will highlight a case where a function has been called from the wrong thread. It is much easier to write these assertions when initially developing code, rather than debugging race conditions which can easily result from a function being called in the wrong thread.
This technique can also be applied to signal emissions and callbacks,
improving type safety as well as asserting the right context is used. Note
that signal emission via
g_signal_emit()
is synchronous, and doesn’t involve a main context at all.
For example, instead of using the following when emitting a signal:
guint param1; /* arbitrary example parameters */
gchar *param2;
guint retval = 0;
g_signal_emit_by_name (my_object, "some-signal",
param1, param2, &retval);
Se puede usar lo siguiente:
static guint
emit_some_signal (GObject *my_object,
guint param1,
const gchar *param2)
{
guint retval = 0;
g_assert (g_main_context_is_owner (expected_main_context));
g_signal_emit_by_name (my_object, "some-signal",
param1, param2, &retval);
return retval;
}
GTask
GTask
provides a slightly different approach to invoking functions in other
threads, which is more suited to the case where a function should be
executed in some background thread, but not a specific one.
GTask
takes a data closure and a function to execute, and
provides ways to return the result from this function. It handles
everything necessary to run that function in an arbitrary thread belonging
to some thread pool internal to GLib.
By combining
g_main_context_invoke_full()
and GTask
, it is possible to run a task in a specific context
and effortlessly return its result to the current context:
/* This will be invoked in thread1. */
static gboolean
my_func_idle (gpointer user_data)
{
GTask *task = G_TASK (user_data);
MyFuncData *data;
gboolean retval;
/* Call my_func() and propagate its returned boolean to
* the main thread. */
data = g_task_get_task_data (task);
retval = my_func (data->some_string, data->some_int,
data->some_object);
g_task_return_boolean (task, retval);
return G_SOURCE_REMOVE;
}
/* Whichever thread this is invoked in, the @callback will be
* invoked in, once my_func() has finished and returned a result. */
static void
invoke_my_func_with_result (GMainContext *thread1_main_context,
const gchar *some_string,
guint some_int,
GObject *some_object,
GAsyncReadyCallback callback,
gpointer user_data)
{
MyFuncData *data;
/* Create a data closure to pass all the desired variables
* between threads. */
data = g_new0 (MyFuncData, 1);
data->some_string = g_strdup (some_string);
data->some_int = some_int;
data->some_object = g_object_ref (some_object);
/* Create a GTask to handle returning the result to the current
* thread-default main context. */
task = g_task_new (NULL, NULL, callback, user_data);
g_task_set_task_data (task, data,
(GDestroyNotify) my_func_data_free);
/* Invoke the function. */
g_main_context_invoke_full (thread1_main_context,
G_PRIORITY_DEFAULT, my_func_idle,
task,
(GDestroyNotify) g_object_unref);
}