GLib -Event Loop

To allow multiple independent sets of events to be handled in different -threads, each source is associated with a GMainContext.

Each event source is given a priority. The default priority is G_PRIORITY_DEFAULT and -its value is 0. Values less than 0 denote higher priority and values greater -than zero denote lower priority. The events from higher priority sources are -processed earlier than events from lower priority sources.

The GMainLoop data type represents an event loop. GMainContext is -a parameter to GMainLoop. If GMainContext is -passed as NULL, then a main loop with the default context is created. After -the sources are added to GMainContext and a GMainLoop variable -is created, g_main_loop_run() is called. This checks continuously -for events from its sources and dispatches them. Finally when all the events -have been processed, g_main_loop_quit() must be called to -return from g_main_loop_run().

-Sources are associated with GMainContext and not with GMainLoop. -Events from sources will be checked and dispatched from all the GMainLoop the GMainContext is -associated with. -

Creating -an event loop

GLib provides ready-made functions for adding -the following sources either to the default context or to a custom context:

time-outs
I/O
child watch
idle source (events from idle sources are ready if none of the other -sources with priority > G_PRIORITY_DEFAULT are ready)

The following example code demonstrates the way to add a time-out -source to the default context. The program makes 10 calls to timeout_callback(), -which at the 10th call, calls g_main_loop_quit to make the -main loop return.

#include <glib.h> -gboolean timeout_callback(gpointer data) -{ - static int i = 0; - - i++; - g_print("timeout_callback called %d times\n", i); - if (10 == i) - { - g_main_loop_quit( (GMainLoop*)data ); - return FALSE; - } - - return TRUE; -} - -int main() -{ - GMainLoop *loop; - - loop = g_main_loop_new ( NULL , FALSE ); - - // add source to default context - g_timeout_add (100 , timeout_callback , loop); - g_main_loop_run (loop); - g_main_loop_unref(loop); - - return 0; -} -

The following code demonstrates the method to add the time-out -source to a different context than the default context:

#include <glib.h> -gboolean timeout_callback(gpointer data) -{ - static int i = 0; - - i++; - g_print("timeout_callback called %d times\n",i); - - if(10 == i) - { - g_main_loop_quit((GMainLoop*)data); - return FALSE; - } - - return TRUE; -} - -int main() -{ - GMainLoop *loop = NULL; - GMainContext *context; - GSource *source; - int id; - - //create a new time-out source - source = g_timeout_source_new(10); - - //create a context - context = g_main_context_new(); - - //attach source to context - id = g_source_attach(source,context); - - //create a main loop with context - loop = g_main_loop_new(context,FALSE); - - //set the callback for this source - g_source_set_callback (source,timeout_callback,loop,NULL); - - g_main_loop_run (loop); - - g_main_loop_unref (loop); - - return 0; -} -

The same process can be used for other default sources like -child watch, I/O, and idle source since they are GLib APIs that allow the -creation of a source directly.

Creating a -new source

The previous section applies only to four types of sources -( time-outs, I/O, child watch, and idle source ), for which GLib provides -ready-made functions. The program below demonstrates the creation of a new -source.

#include <glib.h> -gboolean callback(gpointer data) -{ - static int i = 0; - i++; - g_print ("timeout_callback called %d times\n",i); - if (10 == i) - { - g_main_loop_quit((GMainLoop*)data); - return FALSE; - } - - return TRUE; -} - -gboolean prepare(GSource *source,gint *timeout_) -{ - *timeout_ = -1; - return TRUE; -} - -gboolean check(GSource *source) -{ - return TRUE; -} - -gboolean dispatch(GSource *source,GSourceFunc callback,gpointer user_data) -{ - if (callback(user_data)) - return TRUE; - else - return FALSE; -} - -int main() -{ - GMainLoop *loop = NULL; - GMainContext *context; - GSource *source; - int id; - - //create a variable of type GSourceFuncs - GSourceFuncs SourceFuncs = - { - prepare, - check, - dispatch, - NULL - }; - - //create a new source - source = g_source_new (&SourceFuncs, sizeof(GSource)); - - //create a context - context = g_main_context_new (); - - //attach source to context - id = g_source_attach (source,context); - - //create a main loop with context - loop = g_main_loop_new (context,FALSE); - - //set the callback for this source - g_source_set_callback (source,callback,loop,NULL); - - g_main_loop_run (loop); - g_main_loop_unref (loop); - - return 0; -} -

The creation of a new source requires us to define at least -3 functions:

prepare(): Called before all the file descriptors -are polled. If the source can determine that it is ready here (without waiting -for the results of the poll() call), it should return TRUE. It can also return -a time-out value which should be the maximum time-out (in milliseconds) which -should be passed to the poll() call. The actual time-out used will be -1 if -all sources returned -1, or it will be the minimum of all the timeout_values -returned which were >= 0
check(): Called after all the file descriptors -are polled. The source should return TRUE if it is ready to be dispatched.
Time -may have passed since the previous prepare function was called, so the source -should be checked again.
dispatch() Called to dispatch the event source -after it has returned TRUE in either its prepare or check function. The dispatch -function is passed in a callback function and data. The callback function -may be NULL if the source was never connected to a callback using g_source_set_callback(). -The dispatch function should call the callback function with user_data and -the additional parameters that are needed for this type of event source.

Customizing -the main loop iteration

Single iteration of GMainContext can -be run in g_main_context_iteration (). When a more detailed -control of how the main loop runs is desired call the component function of g_main_context -iteration() directly.

The component functions of g_main_context -iteration() are listed below:

g_main_context_prepare()
g_main_context_query()
g_main_context_check()

+ + + + + +GLib +Event LoopThe GLib event loop manages all the sources of an event available +for GLib. These events can come from different kinds of sources like file +descriptors (plain file descriptors, sockets, or pipes), time-outs, or any +kind of source that can be added. +

To allow multiple independent sets of events to be handled in different +threads, each source is associated with a GMainContext.

Each event source is given a priority. The default priority is G_PRIORITY_DEFAULT and +its value is 0. Values less than 0 denote higher priority and values greater +than zero denote lower priority. The events from higher priority sources are +processed earlier than events from lower priority sources.

The GMainLoop data type represents an event loop. GMainContext is +a parameter to GMainLoop. If GMainContext is +passed as NULL, then a main loop with the default context is created. After +the sources are added to GMainContext and a GMainLoop variable +is created, g_main_loop_run() is called. This checks continuously +for events from its sources and dispatches them. Finally when all the events +have been processed, g_main_loop_quit() must be called to +return from g_main_loop_run().

+Sources are associated with GMainContext and not with GMainLoop. +Events from sources will be checked and dispatched from all the GMainLoop the GMainContext is +associated with. +

Creating +an event loop

GLib provides ready-made functions for adding +the following sources either to the default context or to a custom context:

time-outs
I/O
child watch
idle source (events from idle sources are ready if none of the other +sources with priority > G_PRIORITY_DEFAULT are ready)

The following example code demonstrates the way to add a time-out +source to the default context. The program makes 10 calls to timeout_callback(), +which at the 10th call, calls g_main_loop_quit to make the +main loop return.

#include <glib.h> +gboolean timeout_callback(gpointer data) +{ + static int i = 0; + + i++; + g_print("timeout_callback called %d times\n", i); + if (10 == i) + { + g_main_loop_quit( (GMainLoop*)data ); + return FALSE; + } + + return TRUE; +} + +int main() +{ + GMainLoop *loop; + + loop = g_main_loop_new ( NULL , FALSE ); + + // add source to default context + g_timeout_add (100 , timeout_callback , loop); + g_main_loop_run (loop); + g_main_loop_unref(loop); + + return 0; +} +

The following code demonstrates the method to add the time-out +source to a different context than the default context:

#include <glib.h> +gboolean timeout_callback(gpointer data) +{ + static int i = 0; + + i++; + g_print("timeout_callback called %d times\n",i); + + if(10 == i) + { + g_main_loop_quit((GMainLoop*)data); + return FALSE; + } + + return TRUE; +} + +int main() +{ + GMainLoop *loop = NULL; + GMainContext *context; + GSource *source; + int id; + + //create a new time-out source + source = g_timeout_source_new(10); + + //create a context + context = g_main_context_new(); + + //attach source to context + id = g_source_attach(source,context); + + //create a main loop with context + loop = g_main_loop_new(context,FALSE); + + //set the callback for this source + g_source_set_callback (source,timeout_callback,loop,NULL); + + g_main_loop_run (loop); + + g_main_loop_unref (loop); + + return 0; +} +

The same process can be used for other default sources like +child watch, I/O, and idle source since they are GLib APIs that allow the +creation of a source directly.

Creating a +new source

The previous section applies only to four types of sources +( time-outs, I/O, child watch, and idle source ), for which GLib provides +ready-made functions. The program below demonstrates the creation of a new +source.

#include <glib.h> +gboolean callback(gpointer data) +{ + static int i = 0; + i++; + g_print ("timeout_callback called %d times\n",i); + if (10 == i) + { + g_main_loop_quit((GMainLoop*)data); + return FALSE; + } + + return TRUE; +} + +gboolean prepare(GSource *source,gint *timeout_) +{ + *timeout_ = -1; + return TRUE; +} + +gboolean check(GSource *source) +{ + return TRUE; +} + +gboolean dispatch(GSource *source,GSourceFunc callback,gpointer user_data) +{ + if (callback(user_data)) + return TRUE; + else + return FALSE; +} + +int main() +{ + GMainLoop *loop = NULL; + GMainContext *context; + GSource *source; + int id; + + //create a variable of type GSourceFuncs + GSourceFuncs SourceFuncs = + { + prepare, + check, + dispatch, + NULL + }; + + //create a new source + source = g_source_new (&SourceFuncs, sizeof(GSource)); + + //create a context + context = g_main_context_new (); + + //attach source to context + id = g_source_attach (source,context); + + //create a main loop with context + loop = g_main_loop_new (context,FALSE); + + //set the callback for this source + g_source_set_callback (source,callback,loop,NULL); + + g_main_loop_run (loop); + g_main_loop_unref (loop); + + return 0; +} +

The creation of a new source requires us to define at least +3 functions:

prepare(): Called before all the file descriptors +are polled. If the source can determine that it is ready here (without waiting +for the results of the poll() call), it should return TRUE. It can also return +a time-out value which should be the maximum time-out (in milliseconds) which +should be passed to the poll() call. The actual time-out used will be -1 if +all sources returned -1, or it will be the minimum of all the timeout_values +returned which were >= 0
check(): Called after all the file descriptors +are polled. The source should return TRUE if it is ready to be dispatched.
Time +may have passed since the previous prepare function was called, so the source +should be checked again.
dispatch() Called to dispatch the event source +after it has returned TRUE in either its prepare or check function. The dispatch +function is passed in a callback function and data. The callback function +may be NULL if the source was never connected to a callback using g_source_set_callback(). +The dispatch function should call the callback function with user_data and +the additional parameters that are needed for this type of event source.

Customizing +the main loop iteration

Single iteration of GMainContext can +be run in g_main_context_iteration (). When a more detailed +control of how the main loop runs is desired call the component function of g_main_context +iteration() directly.

The component functions of g_main_context +iteration() are listed below:

g_main_context_prepare()
g_main_context_query()
g_main_context_check()

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