<page xmlns="http://projectmallard.org/1.0/" xmlns:its="http://www.w3.org/2005/11/its" xmlns:xi="http://www.w3.org/2003/XInclude" type="topic" id="threading" xml:lang="pt-BR">

  <info>

    <link type="guide" xref="index#specific-how-tos"/>

    <credit type="author copyright">

      <name>Philip Withnall</name>

      <email its:translate="no">philip.withnall@collabora.co.uk</email>

      <years>2015</years>

    </credit>

    <include xmlns="http://www.w3.org/2001/XInclude" href="cc-by-sa-3-0.xml"/>

    <desc>Movendo computação da thread principal para threads de trabalho</desc>

    <mal:credit xmlns:mal="http://projectmallard.org/1.0/" type="translator copyright">

      <mal:name>Rafael Fontenelle</mal:name>

      <mal:email>rafaelff@gnome.org</mal:email>

      <mal:years>2017</mal:years>

    </mal:credit>

  </info>

  <title>Threading</title>

  <synopsis>

    <title>Resumo</title>

    <list>

      <item>

        Do not use threads if at all possible.

        (<link xref="#when-to-use-threading"/>)

      
</item>

      <item>

        If threads have to be used, use GTask or

        GThreadPool and isolate the threaded code as much as

        possible.

        (<link xref="#using-threading"/>)

      
</item>

      <item>

        Use g_thread_join() to avoid leaking thread resources if

        using GThread manually.

        (<link xref="#using-threading"/>)

      
</item>

      <item>

        Be careful about the GMainContext which code is executed in

        if using threads. Executing code in the wrong context can cause race

        conditions, or block the main loop.

        (<link xref="#using-threading"/>)

      
</item>

    </list>

  </synopsis>

  <section id="when-to-use-threading">

    <title>When to Use Threading</title>

      When writing projects using GLib, the default approach should be to

      never use threads. Instead, make proper use of the

      <link xref="main-contexts">GLib main context</link> which, through the use

      of asynchronous operations,

      allows most blocking I/O operations to continue in the background while

      the main context continues to process other events. Analysis, review and

      debugging of threaded code becomes very hard, very quickly.

      Threading should only be necessary when using an external library which

      has blocking functions which need to be called from GLib code. If the

      library provides a non-blocking alternative, or one which integrates with

      a

      <link href="http://pubs.opengroup.org/onlinepubs/009695399/functions/poll.html">poll()</link>

      loop, that should be used in preference. If the blocking function really

      must be used, a thin wrapper should be written for it to convert it to the

      normal

      <link href="https://developer.gnome.org/gio/stable/GAsyncResult.html">GAsyncResult

      style</link> of GLib asynchronous function, running the blocking operation

      in a worker thread.

    <example>

      
Por exemplo:

int

some_blocking_function (void *param1,

                        void *param2);

        Should be wrapped as:

void

some_blocking_function_async (void                 *param1,

                              void                 *param2,

                              GCancellable         *cancellable,

                              GAsyncReadyCallback   callback,

                              gpointer              user_data);

int

some_blocking_function_finish (GAsyncResult        *result,

                               GError             **error);

        With an implementation something like:

      /* Closure for the call’s parameters. */

typedef struct {

  void *param1;

  void *param2;

} SomeBlockingFunctionData;

static void

some_blocking_function_data_free (SomeBlockingFunctionData *data)

{

  free_param (data->param1);

  free_param (data->param2);

  g_free (data);

}

static void

some_blocking_function_thread_cb (GTask         *task,

                                  gpointer       source_object,

                                  gpointer       task_data,

                                  GCancellable  *cancellable)

{

  SomeBlockingFunctionData *data = task_data;

  int retval;

  /* Handle cancellation. */

  if (g_task_return_error_if_cancelled (task))

    {

      return;

    }

  /* Run the blocking function. */

  retval = some_blocking_function (data->param1, data->param2);

  g_task_return_int (task, retval);

}

void

some_blocking_function_async (void                 *param1,

                              void                 *param2,

                              GCancellable         *cancellable,

                              GAsyncReadyCallback   callback,

                              gpointer              user_data)

{

  GTask *task = NULL;  /* owned */

  SomeBlockingFunctionData *data = NULL;  /* owned */

  g_return_if_fail (validate_param (param1));

  g_return_if_fail (validate_param (param2));

  g_return_if_fail (cancellable == NULL || G_IS_CANCELLABLE (cancellable));

  task = g_task_new (NULL, cancellable, callback, user_data);

  g_task_set_source_tag (task, some_blocking_function_async);

  /* Cancellation should be handled manually using mechanisms specific to

   * some_blocking_function(). */

  g_task_set_return_on_cancel (task, FALSE);

  /* Set up a closure containing the call’s parameters. Copy them to avoid

   * locking issues between the calling thread and the worker thread. */

  data = g_new0 (SomeBlockingFunctionData, 1);

  data->param1 = copy_param (param1);

  data->param2 = copy_param (param2);

  g_task_set_task_data (task, data, some_blocking_function_data_free);

  /* Run the task in a worker thread and return immediately while that continues

   * in the background. When it’s done it will call @callback in the current

   * thread default main context. */

  g_task_run_in_thread (task, some_blocking_function_thread_cb);

  g_object_unref (task);

}

int

some_blocking_function_finish (GAsyncResult  *result,

                               GError       **error)

{

  g_return_val_if_fail (g_task_is_valid (result,

                                         some_blocking_function_async), -1);

  g_return_val_if_fail (error == NULL || *error == NULL, -1);

  return g_task_propagate_int (G_TASK (result), error);

}

        See the

        <link href="https://developer.gnome.org/gio/stable/GAsyncResult.html">GAsyncResult

        documentation</link> for more details. A simple way to implement the

        worker thread is to use

        <link href="https://developer.gnome.org/gio/stable/GTask.html">GTask</link>

        and <link href="https://developer.gnome.org/gio/stable/GTask.html#g-task-run-in-thread">g_task_run_in_thread()</link>.

        (See also: <link xref="main-contexts#gtask"/>.)

    </example>

  </section>

  <section id="using-threading">

    <title>Using Threading</title>

      If GTask is not suitable for writing the worker thread, a

      more low-level approach must be used. This should be considered very

      carefully, as it is very easy to get threading code wrong in ways which

      will unpredictably cause bugs at runtime, cause deadlocks, or consume too

      many resources and terminate the program.

      A full manual on writing threaded code is beyond the scope of this

      document, but here are a number of guidelines to follow which should

      reduce the potential for bugs in threading code. The overriding principle

      is to reduce the amount of code and data which can be affected by

      threading — for example, reducing the number of threads, the complexity of

      worker thread implementation, and the amount of data shared between

      threads.

    <list>

      <item>

          Use <link href="https://developer.gnome.org/glib/stable/glib-Thread-Pools.html">GThreadPool</link>

          instead of manually creating

          <link href="https://developer.gnome.org/glib/stable/glib-Threads.html">GThreads</link>

          if possible. GThreadPool supports a work queue, limits on

          the number of spawned threads, and automatically joins finished

          threads so they are not leaked.

      </item>

      <item>

          If it is not possible to use a GThreadPool (which is

          rarely the case):

        <list>

          <item>

              Use <link href="https://developer.gnome.org/glib/stable/glib-Threads.html#g-thread-try-new">g_thread_try_new()</link>

              to spawn threads, instead of

              <link href="https://developer.gnome.org/glib/stable/glib-Threads.html#g-thread-new">g_thread_new()</link>,

              so errors due to the system running out of threads can be handled

              gracefully rather than unconditionally aborting the program.

          </item>

          <item>

              Explicitly join threads using

              <link href="https://developer.gnome.org/glib/stable/glib-Threads.html#g-thread-join">g_thread_join()</link>

              to avoid leaking the thread resources.

          </item>

        </list>

      </item>

      <item>

          Use message passing to transfer data between threads, rather than

          manual locking with mutexes. GThreadPool explicitly

          supports this with

          <link href="https://developer.gnome.org/glib/stable/glib-Thread-Pools.html#g-thread-pool-push">g_thread_pool_push()</link>.

      </item>

      <item>

          If mutexes must be used:

        <list>

          <item>

              Isolate threading code as much as possible, keeping mutexes

              private within classes, and tightly bound to very specific class

              members.

          </item>

          <item>

              All mutexes should be clearly commented beside their declaration,

              indicating which other structures or variables they protect access

              to. Similarly, those variables should be commented saying that

              they should only be accessed with that mutex held.

          </item>

        </list>

      </item>

      <item>

          Be careful about interactions between main contexts and threads. For

          example,

          <link href="https://developer.gnome.org/glib/stable/glib-The-Main-Event-Loop.html#g-timeout-add-seconds">g_timeout_add_seconds()</link>

          adds a timeout to be executed in the global default main

          context, which is being run in the main thread, not

          necessarily the current thread. Getting this wrong can mean that

          work intended for a worker thread accidentally ends up being executed

          in the main thread anyway. (See also:

          <link xref="main-contexts#default-contexts"/>.)

      </item>

    </list>

  </section>

  <section id="debugging">

    <title>Depuração</title>

      Debugging threading issues is tricky, both because they are hard to

      reproduce, and because they are hard to reason about. This is one of the

      big reasons for avoiding using threads in the first place.

      However, if a threading issue does arise,

      <link xref="tooling#helgrind-and-drd">Valgrind’s drd and helgrind tools

      are useful</link>.

  </section>

</page>