GStreamer 1.0 Core Reference Manual: GstContext

## GstContext GstContext — Lightweight objects to represent element contexts |

## Functions

GstContext * | gst_context_new ()
GstContext * | gst_context_ref ()
void | gst_context_unref ()
GstContext * | gst_context_copy ()
const gchar * | gst_context_get_context_type ()
gboolean | gst_context_has_context_type ()
gboolean | gst_context_is_persistent ()
const GstStructure * | gst_context_get_structure ()
GstStructure * | gst_context_writable_structure ()
#define | gst_context_make_writable()
#define | gst_context_is_writable()
gboolean | gst_context_replace ()

## Types and Values

| GstContext

## Object Hierarchy

        GBoxed
        ╰── GstContext

## Includes

    #include <gst/gst.h>

## Description

GstContext is a container object used to store contexts like a device context, a display server connection and similar concepts that should be shared between multiple elements.

Applications can set a context on a complete pipeline by using `gst_element_set_context()`, which will then be propagated to all child elements. Elements can handle these in `GstElementClass.set_context()` and merge them with the context information they already have.

When an element needs a context it will do the following actions in this order until one step succeeds:

1. Check if the element already has a context

2. Query downstream with GST_QUERY_CONTEXT for the context

3. Query upstream with GST_QUERY_CONTEXT for the context

4. Post a GST_MESSAGE_NEED_CONTEXT message on the bus with the required context types and afterwards check if a usable context was set now

5. Create a context by itself and post a GST_MESSAGE_HAVE_CONTEXT message on the bus.

Bins will catch GST_MESSAGE_NEED_CONTEXT messages and will set any previously known context on the element that asks for it if possible. Otherwise the application should provide one if it can.

## Functions

### gst_context_new ()

    GstContext *
    gst_context_new (const gchar *context_type,
                     gboolean persistent);

Create a new context.

#### Parameters

context_type | Context type |
persistent | Persistent context |

#### Returns

The new context.

[transfer full]

Since 1.2

### gst_context_ref ()

    GstContext *
    gst_context_ref (GstContext *context);

Convenience macro to increase the reference count of the context.

#### Parameters

context | the context to ref |

#### Returns

`context` (for convenience when doing assignments)

### gst_context_unref ()

    void
    gst_context_unref (GstContext *context);

Convenience macro to decrease the reference count of the context, possibly freeing it.

#### Parameters

context | the context to unref |

### gst_context_copy ()

    GstContext *
    gst_context_copy (const GstContext *context);

Creates a copy of the context. Returns a copy of the context.

#### Parameters

context | the context to copy |

#### Returns

a new copy of `context` .

MT safe.

[transfer full]

### gst_context_get_context_type ()

    const gchar *
    gst_context_get_context_type (const GstContext *context);

Get the type of `context` .

#### Parameters

context | The GstContext. |

#### Returns

The type of the context.

Since 1.2

### gst_context_has_context_type ()

    gboolean
    gst_context_has_context_type (const GstContext *context,
                                  const gchar *context_type);

Checks if `context` has `context_type` .

#### Parameters

context | The GstContext. |
context_type | Context type to check. |

#### Returns

`TRUE` if `context` has `context_type` .

Since 1.2

### gst_context_is_persistent ()

    gboolean
    gst_context_is_persistent (const GstContext *context);

Check if `context` is persistent.

#### Parameters

context | The GstContext. |

#### Returns

`TRUE` if the context is persistent.

Since 1.2

### gst_context_get_structure ()

    const GstStructure *
    gst_context_get_structure (const GstContext *context);

Access the structure of the context.

#### Parameters

context | The GstContext. |

#### Returns

The structure of the context. The structure is still owned by the context, which means that you should not modify it, free it and that the pointer becomes invalid when you free the context.

[transfer none]

Since 1.2

### gst_context_writable_structure ()

    GstStructure *
    gst_context_writable_structure (GstContext *context);

Get a writable version of the structure.

#### Parameters

context | The GstContext. |

#### Returns

The structure of the context. The structure is still owned by the event, which means that you should not free it and that the pointer becomes invalid when you free the event. This function checks if `context` is writable.

Since 1.2

### gst_context_make_writable()

    #define         gst_context_make_writable(context)  GST_CONTEXT_CAST (gst_mini_object_make_writable (GST_MINI_OBJECT_CAST (context)))

Checks if a context is writable. If not, a writable copy is made and returned.

#### Parameters

context | the context to make writable. | [transfer full]

#### Returns

a context (possibly a duplicate) that is writable.

MT safe.

[transfer full]

### gst_context_is_writable()

    #define         gst_context_is_writable(context)     gst_mini_object_is_writable (GST_MINI_OBJECT_CAST (context))

Tests if you can safely write into a context's structure or validly modify the seqnum and timestamp fields.

#### Parameters

context | a GstContext |

### gst_context_replace ()

    gboolean
    gst_context_replace (GstContext **old_context,
                         GstContext *new_context);

Modifies a pointer to a GstContext to point to a different GstContext. The modification is done atomically (so this is useful for ensuring thread safety in some cases), and the reference counts are updated appropriately (the old context is unreffed, the new one is reffed).

Either `new_context` or the GstContext pointed to by `old_context` may be `NULL`.

#### Parameters

old_context | pointer to a pointer to a GstContext to be replaced. | [inout][transfer full]
new_context | pointer to a GstContext that will replace the context pointed to by `old_context` . | [allow-none][transfer none]

#### Returns

`TRUE` if `new_context` was different from `old_context`

## Types and Values

### GstContext

    typedef struct _GstContext GstContext;

## See Also

GstMiniObject, GstElement