gstreamer/markdown/design/gstobject.md

# GstObject

The base class for the entire GStreamer hierarchy is the `GstObject`.

## Parentage

A pointer is available to store the current parent of the object. This
is one of the two fundamental requirements for a hierarchical system
such as GStreamer (for the other, read up on `GstBin`). Three functions
are provided: `_set_parent()`, `_get_parent()`, and `_unparent()`. The
third is required because there is an explicit check in `_set_parent()`:
an object must not already have a parent if you wish to set one. You
must unparent the object first. This allows for new additions later.

  - `GstObject`’s that can be parented: `GstElement` (inside a bin) `GstPad` (inside an element)

## Naming

- names of objects cannot be changed when they are parented
- names of objects should be unique across parent
    - `set_name()` can fail because of this
    - as can `gst_element_add_pad()`/`gst_bin_add_element()`
- `gst_object_set_name()` only changes the object’s name
- objects also have a name_prefix that is used to prefix the object
name during debugging and identification
- there are object-specific `set_name()` which also set the
name_prefix on the object. This is useful for debugging purposes to
give the object a more identifiable name. Typically a parent will
call `_set_name_prefix()` on children, taking a lock on them to do
so.

## Locking

The `GstObject` contains the necessary primitives to lock the object in a
thread-safe manner. This will be used to provide general thread-safety
as needed. However, this lock is generic, i.e. it covers the whole
object.

The object LOCK is a very lowlevel lock that should only be held to
access the object properties for short periods of code.

All members of the `GstObject` structure marked as `/**< public >**/ /*
with LOCK */` are protected by this lock. These members can only be
accessed for reading or writing while the lock is held. All members
should be copied or reffed if they are used after releasing the LOCK.

Note that this does **not** mean that no other thread can modify the
object at the same time that the lock is held. It only means that any
two sections of code that obey the lock are guaranteed to not be running
simultaneously. "The lock is voluntary and cooperative".

This lock will ideally be used for parentage, flags and naming, which is
reasonable, since they are the only possible things to protect in the
`GstObject`.

## Locking order

In parent-child situations the lock of the parent must always be taken
first before taking the lock of the child. It is NOT allowed to hold the
child lock before taking the parent lock.

This policy allows for parents to iterate their children and setting
properties on them.

Whenever a nested lock needs to be taken on objects not involved in a
parent-child relation (eg. pads), an explictic locking order has to be
defined.

## Path Generation

Due to the base nature of the `GstObject`, it becomes the only reasonable
place to put this particular function (`_get_path_string()`). It will
generate a string describing the parent hierarchy of a given `GstObject`.

## Flags

Each object in the GStreamer object hierarchy can have flags associated
with it, which are used to describe its state or a features.