GstIterator

GstIterator — Object to retrieve multiple elements in a threadsafe way.

Synopsis

#include <gst/gst.h>

struct              GstIterator;
enum                GstIteratorItem;
enum                GstIteratorResult;
void                (*GstIteratorDisposeFunction)       (gpointer owner);
GstIteratorResult   (*GstIteratorNextFunction)          (GstIterator *it,
                                                         gpointer *result);
GstIteratorItem     (*GstIteratorItemFunction)          (GstIterator *it,
                                                         gpointer item);
void                (*GstIteratorResyncFunction)        (GstIterator *it);
void                (*GstIteratorFreeFunction)          (GstIterator *it);
gboolean            (*GstIteratorFoldFunction)          (gpointer item,
                                                         GValue *ret,
                                                         gpointer user_data);
gpointer            (*GstCopyFunction)                  (gpointer object);
#define             GST_ITERATOR                        (it)
#define             GST_ITERATOR_LOCK                   (it)
#define             GST_ITERATOR_COOKIE                 (it)
#define             GST_ITERATOR_ORIG_COOKIE            (it)
GstIterator *       gst_iterator_new                    (guint size,
                                                         GType type,
                                                         GMutex *lock,
                                                         guint32 *master_cookie,
                                                         GstIteratorNextFunction next,
                                                         GstIteratorItemFunction item,
                                                         GstIteratorResyncFunction resync,
                                                         GstIteratorFreeFunction free);
GstIterator *       gst_iterator_new_list               (GType type,
                                                         GMutex *lock,
                                                         guint32 *master_cookie,
                                                         GList **list,
                                                         gpointer owner,
                                                         GstIteratorItemFunction item,
                                                         GstIteratorDisposeFunction free);
GstIterator *       gst_iterator_new_single             (GType type,
                                                         gpointer object,
                                                         GstCopyFunction copy,
                                                         GFreeFunc free);
GstIteratorResult   gst_iterator_next                   (GstIterator *it,
                                                         gpointer *elem);
void                gst_iterator_resync                 (GstIterator *it);
void                gst_iterator_free                   (GstIterator *it);
void                gst_iterator_push                   (GstIterator *it,
                                                         GstIterator *other);
GstIterator *       gst_iterator_filter                 (GstIterator *it,
                                                         GCompareFunc func,
                                                         gpointer user_data);
GstIteratorResult   gst_iterator_fold                   (GstIterator *it,
                                                         GstIteratorFoldFunction func,
                                                         GValue *ret,
                                                         gpointer user_data);
GstIteratorResult   gst_iterator_foreach                (GstIterator *it,
                                                         GFunc func,
                                                         gpointer user_data);
gpointer            gst_iterator_find_custom            (GstIterator *it,
                                                         GCompareFunc func,
                                                         gpointer user_data);

Description

A GstIterator is used to retrieve multiple objects from another object in a threadsafe way.

Various GStreamer objects provide access to their internal structures using an iterator.

In general, whenever calling a GstIterator function results in your code receiving a refcounted object, the refcount for that object will have been increased. Your code is responsible for unrefing that object after use.

The basic use pattern of an iterator is as follows:

Example 13. Using an iterator

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
it = _get_iterator(object);
done = FALSE;
while (!done) {
  switch (gst_iterator_next (it, &item)) {
    case GST_ITERATOR_OK:
      ... use/change item here...
      gst_object_unref (item);
      break;
    case GST_ITERATOR_RESYNC:
      ...rollback changes to items...
      gst_iterator_resync (it);
      break;
    case GST_ITERATOR_ERROR:
      ...wrong parameters were given...
      done = TRUE;
      break;
    case GST_ITERATOR_DONE:
      done = TRUE;
      break;
  }
}
gst_iterator_free (it);


Last reviewed on 2009-06-16 (0.10.24)

Details

struct GstIterator

struct GstIterator {
};

GstIterator base structure. The values of this structure are protected for subclasses, use the methods to use the GstIterator.


enum GstIteratorItem

typedef enum {
  GST_ITERATOR_ITEM_SKIP = 0,
  GST_ITERATOR_ITEM_PASS = 1,
  GST_ITERATOR_ITEM_END		= 2
} GstIteratorItem;

The result of a GstIteratorItemFunction.

GST_ITERATOR_ITEM_SKIP

Skip this item

GST_ITERATOR_ITEM_PASS

Return item

GST_ITERATOR_ITEM_END

Stop after this item.

enum GstIteratorResult

typedef enum {
  GST_ITERATOR_DONE = 0,
  GST_ITERATOR_OK = 1,
  GST_ITERATOR_RESYNC = 2,
  GST_ITERATOR_ERROR = 3
} GstIteratorResult;

The result of gst_iterator_next().

GST_ITERATOR_DONE

No more items in the iterator

GST_ITERATOR_OK

An item was retrieved

GST_ITERATOR_RESYNC

Datastructure changed while iterating

GST_ITERATOR_ERROR

An error happened

GstIteratorDisposeFunction ()

void                (*GstIteratorDisposeFunction)       (gpointer owner);

The function that will be called when a GList iterator is freed. The owner of the GList iterator can then clean up its resources.

owner :

the owner of the iterator

GstIteratorNextFunction ()

GstIteratorResult   (*GstIteratorNextFunction)          (GstIterator *it,
                                                         gpointer *result);

The function that will be called when the next element of the iterator should be retrieved.

Implementors of a GstIterator should implement this function and pass it to the constructor of the custom iterator. The function will be called with the iterator lock held.

it :

the iterator

result :

a pointer to hold the next item

Returns :

the result of the operation.

GstIteratorItemFunction ()

GstIteratorItem     (*GstIteratorItemFunction)          (GstIterator *it,
                                                         gpointer item);

The function that will be called after the next item of the iterator has been retrieved. This function will typically increase the refcount of the item or make a copy.

Implementors of a GstIterator should implement this function and pass it to the constructor of the custom iterator. The function will be called with the iterator lock held.

it :

the iterator

item :

the item being retrieved.

Returns :

the result of the operation.

GstIteratorResyncFunction ()

void                (*GstIteratorResyncFunction)        (GstIterator *it);

This function will be called whenever a concurrent update happened to the iterated datastructure. The implementor of the iterator should restart the iterator from the beginning and clean up any state it might have.

Implementors of a GstIterator should implement this function and pass it to the constructor of the custom iterator. The function will be called with the iterator lock held.

it :

the iterator

GstIteratorFreeFunction ()

void                (*GstIteratorFreeFunction)          (GstIterator *it);

This function will be called when the iterator is freed.

Implementors of a GstIterator should implement this function and pass it to the constructor of the custom iterator. The function will be called with the iterator lock held.

it :

the iterator

GstIteratorFoldFunction ()

gboolean            (*GstIteratorFoldFunction)          (gpointer item,
                                                         GValue *ret,
                                                         gpointer user_data);

A function to be passed to gst_iterator_fold().

item :

the item to fold

ret :

a GValue collecting the result

user_data :

data passed to gst_iterator_fold()

Returns :

TRUE if the fold should continue, FALSE if it should stop.

GstCopyFunction ()

gpointer            (*GstCopyFunction)                  (gpointer object);

A function to create a copy of some object or increase its reference count.

object :

The object to copy

Returns :

a copy of the object or the same object with increased reference count

Since 0.10.25


GST_ITERATOR()

#define GST_ITERATOR(it)		((GstIterator*)(it))

Macro to cast to a GstIterator

it :

the GstIterator value

GST_ITERATOR_LOCK()

#define GST_ITERATOR_LOCK(it)		(GST_ITERATOR(it)->lock)

Macro to get the lock protecting the datastructure being iterated.

it :

the GstIterator to get the lock of

GST_ITERATOR_COOKIE()

#define GST_ITERATOR_COOKIE(it)		(GST_ITERATOR(it)->cookie)

Macro to get the cookie of a GstIterator. The cookie of the iterator is the value of the master cookie when the iterator was created. Whenever the iterator is iterated, the value is compared to the value of the master cookie. If they are different, a concurrent modification happened to the iterator and a resync is needed.

it :

the GstIterator to get the cookie of

GST_ITERATOR_ORIG_COOKIE()

#define GST_ITERATOR_ORIG_COOKIE(it) (GST_ITERATOR(it)->master_cookie)

Macro to get a pointer to where the master cookie is stored. The master cookie protects the structure being iterated and gets updated whenever the datastructure changes.

it :

the GstIterator to get the master cookie of

gst_iterator_new ()

GstIterator *       gst_iterator_new                    (guint size,
                                                         GType type,
                                                         GMutex *lock,
                                                         guint32 *master_cookie,
                                                         GstIteratorNextFunction next,
                                                         GstIteratorItemFunction item,
                                                         GstIteratorResyncFunction resync,
                                                         GstIteratorFreeFunction free);

Create a new iterator. This function is mainly used for objects implementing the next/resync/free function to iterate a data structure.

For each item retrieved, the item function is called with the lock held. The free function is called when the iterator is freed.

size :

the size of the iterator structure

type :

GType of children

lock :

pointer to a GMutex.

master_cookie :

pointer to a guint32 that is changed when the items in the iterator changed.

next :

function to get next item

item :

function to call on each item retrieved

resync :

function to resync the iterator

free :

function to free the iterator

Returns :

the new GstIterator. MT safe.

gst_iterator_new_list ()

GstIterator *       gst_iterator_new_list               (GType type,
                                                         GMutex *lock,
                                                         guint32 *master_cookie,
                                                         GList **list,
                                                         gpointer owner,
                                                         GstIteratorItemFunction item,
                                                         GstIteratorDisposeFunction free);

Create a new iterator designed for iterating list.

The list you iterate is usually part of a data structure owner and is protected with lock.

The iterator will use lock to retrieve the next item of the list and it will then call the item function before releasing lock again.

The item function usualy makes sure that the item remains alive while lock is released and the application is using the item. The application is responsible for freeing/unreffing the item after usage as explained in gst_iterator_next().

When a concurrent update to the list is performed, usually by owner while holding lock, master_cookie will be updated. The iterator implementation will notice the update of the cookie and will return GST_ITERATOR_RESYNC to the user of the iterator in the next call to gst_iterator_next().

owner will be passed to the free function when the iterator is freed.

type :

GType of elements

lock :

pointer to a GMutex protecting the list.

master_cookie :

pointer to a guint32 that is incremented when the list is changed.

list :

pointer to the list

owner :

object owning the list

item :

function to call for each item

free :

function to call when the iterator is freed

Returns :

the new GstIterator for list. MT safe.

gst_iterator_new_single ()

GstIterator *       gst_iterator_new_single             (GType type,
                                                         gpointer object,
                                                         GstCopyFunction copy,
                                                         GFreeFunc free);

This GstIterator is a convenient iterator for the common case where a GstIterator needs to be returned but only a single object has to be considered. This happens often for the GstPadIterIntLinkFunction.

type :

GType of the passed object

object :

object that this iterator should return

copy :

Function that returns a copy of object or increases its refcount

free :

Function to be called for freeing object

Returns :

the new GstIterator for object.

Since 0.10.25


gst_iterator_next ()

GstIteratorResult   gst_iterator_next                   (GstIterator *it,
                                                         gpointer *elem);

Get the next item from the iterator in elem.

Only when this function returns GST_ITERATOR_OK, elem will contain a valid value. For iterators that return refcounted objects, the returned object will have its refcount increased and should therefore be unreffed after usage.

When this function returns GST_ITERATOR_DONE, no more elements can be retrieved from it.

A return value of GST_ITERATOR_RESYNC indicates that the element list was concurrently updated. The user of it should call gst_iterator_resync() to get the newly updated list.

A return value of GST_ITERATOR_ERROR indicates an unrecoverable fatal error.

it :

The GstIterator to iterate

elem :

pointer to hold next element

Returns :

The result of the iteration. Unref elem after usage if this is a refcounted object. MT safe.

gst_iterator_resync ()

void                gst_iterator_resync                 (GstIterator *it);

Resync the iterator. this function is mostly called after gst_iterator_next() returned GST_ITERATOR_RESYNC.

When an iterator was pushed on it, it will automatically be popped again with this function.

MT safe.

it :

The GstIterator to resync

gst_iterator_free ()

void                gst_iterator_free                   (GstIterator *it);

Free the iterator.

MT safe.

it :

The GstIterator to free

gst_iterator_push ()

void                gst_iterator_push                   (GstIterator *it,
                                                         GstIterator *other);

Pushes other iterator onto it. All calls performed on it are forwarded to other. If other returns GST_ITERATOR_DONE, it is popped again and calls are handled by it again.

This function is mainly used by objects implementing the iterator next function to recurse into substructures.

When gst_iterator_resync() is called on it, other will automatically be popped.

MT safe.

it :

The GstIterator to use

other :

The GstIterator to push

gst_iterator_filter ()

GstIterator *       gst_iterator_filter                 (GstIterator *it,
                                                         GCompareFunc func,
                                                         gpointer user_data);

Create a new iterator from an existing iterator. The new iterator will only return those elements that match the given compare function func. func should return 0 for elements that should be included in the iterator.

When this iterator is freed, it will also be freed.

it :

The GstIterator to filter

func :

the compare function to select elements. [scope call]

user_data :

user data passed to the compare function. [closure]

Returns :

a new GstIterator. MT safe. [transfer full]

gst_iterator_fold ()

GstIteratorResult   gst_iterator_fold                   (GstIterator *it,
                                                         GstIteratorFoldFunction func,
                                                         GValue *ret,
                                                         gpointer user_data);

Folds func over the elements of iter. That is to say, func will be called as func (object, ret, user_data) for each object in it. The normal use of this procedure is to accumulate the results of operating on the objects in ret. If object is a refcounted object its refcount will be increased before func is called, and it should be unrefed after use in func.

This procedure can be used (and is used internally) to implement the gst_iterator_foreach() and gst_iterator_find_custom() operations.

The fold will proceed as long as func returns TRUE. When the iterator has no more arguments, GST_ITERATOR_DONE will be returned. If func returns FALSE, the fold will stop, and GST_ITERATOR_OK will be returned. Errors or resyncs will cause fold to return GST_ITERATOR_ERROR or GST_ITERATOR_RESYNC as appropriate.

The iterator will not be freed.

it :

The GstIterator to fold over

func :

the fold function. [scope call]

ret :

the seed value passed to the fold function

user_data :

user data passed to the fold function. [closure]

Returns :

A GstIteratorResult, as described above. MT safe.

gst_iterator_foreach ()

GstIteratorResult   gst_iterator_foreach                (GstIterator *it,
                                                         GFunc func,
                                                         gpointer user_data);

Iterate over all element of it and call the given function func for each element. As in gst_iterator_fold(), the refcount of a refcounted object will be increased before func is called, and should be unrefed after use.

it :

The GstIterator to iterate

func :

the function to call for each element. [scope call]

user_data :

user data passed to the function. [closure]

Returns :

the result call to gst_iterator_fold(). The iterator will not be freed. MT safe.

gst_iterator_find_custom ()

gpointer            gst_iterator_find_custom            (GstIterator *it,
                                                         GCompareFunc func,
                                                         gpointer user_data);

Find the first element in it that matches the compare function func. func should return 0 when the element is found. As in gst_iterator_fold(), the refcount of a refcounted object will be increased before func is called, and should be unrefed after use in func unless it is the matching element.

The iterator will not be freed.

This function will return NULL if an error happened to the iterator.

it :

The GstIterator to iterate

func :

the compare function to use. [scope call]

user_data :

user data passed to the compare function. [closure]

Returns :

The element in the iterator that matches the compare function or NULL when no element matched. MT safe. [transfer full]

See Also

GstElement, GstBin