GstMeta buffers

[John P Poet]

In the example for creating a new meta info type:

https://gstreamer.freedesktop.org/data/doc/gstreamer/head/pwg/html/section-allocation-meta.html

It says

Lastly, you implement a gst_buffer_add_*_meta() that adds the metadata

implementation to a buffer and sets the values of the metadata.

And the example just show assigning values to local variables in the
structure. It does not show any memory allocation, so I assume that is not
necessary. In that structure I would like it to hold a c++ class:

struct _VANCMeta {
GstMeta meta;
AncillaryPacket packet;
};

AncillaryPacket would have various methods as well as data. Will that
work, or do I need to use more "base" types in my metadata structure?
There are only a few data elements in there, so just having those elements
in the VANCMeta struct would be easy, but it would be nice to have the
methods automatically available by whatever reads the metadata.

If I do need to just have 'simple' data types, and one of those types is an
array of uint16_t, is there a preferred way of allocating the memory for
it? I assume I would use a GArray rather than a GstBuffer, right? Or
would I just use a basic C style array of uint16_t?

struct _VANCMeta {
GstMeta meta;

// primary data id.
int data_id;
// data block number (type 1), or secondary data id (type 2).
int dbn_sdid;
// number of words of data.
int data_count;
// actual data.
uint16_t* buf;
};

I would like to understand how the memory is deallocated after the life of
the struct is over. Who is responsible for that deallocation, and how do
they know how to deallocate the memory?

Thanks,

John

[John P Poet]

I forgot to ask another related question. When reading Ancillary data
embedded in the frame for a Decklink source, there can be multiple VANC
packets in a a single frame. Do I need to combine those into a single
GstMeta object, or can I have several _VANCMeta objects attached to a video
frame buffer? If I can have several, do I need to do anything special to
maintain serialization, so they are parsed in the correct order?

Thanks,

John

Post by John P Poet
https://gstreamer.freedesktop.org/data/doc/gstreamer/head/pwg/html/section-allocation-meta.html
It says

Lastly, you implement a gst_buffer_add_*_meta() that adds the metadata

implementation to a buffer and sets the values of the metadata.
And the example just show assigning values to local variables in the
structure. It does not show any memory allocation, so I assume that is not
struct _VANCMeta {
GstMeta meta;
AncillaryPacket packet;
};
AncillaryPacket would have various methods as well as data. Will that
work, or do I need to use more "base" types in my metadata structure?
There are only a few data elements in there, so just having those elements
in the VANCMeta struct would be easy, but it would be nice to have the
methods automatically available by whatever reads the metadata.
If I do need to just have 'simple' data types, and one of those types is
an array of uint16_t, is there a preferred way of allocating the memory for
it? I assume I would use a GArray rather than a GstBuffer, right? Or
would I just use a basic C style array of uint16_t?
struct _VANCMeta {
GstMeta meta;
// primary data id.
int data_id;
// data block number (type 1), or secondary data id (type 2).
int dbn_sdid;
// number of words of data.
int data_count;
// actual data.
uint16_t* buf;
};
I would like to understand how the memory is deallocated after the life of
the struct is over. Who is responsible for that deallocation, and how do
they know how to deallocate the memory?
Thanks,
John

[Sebastian Dröge]

I forgot to ask another related question.   When reading Ancillary
data embedded in the frame for a Decklink source, there can be
multiple VANC packets in a a single frame.  Do I need to combine
those into a single GstMeta object, or can I have several _VANCMeta
objects attached to a video frame buffer?  If I can have several, do
I need to do anything special to maintain serialization, so they are
parsed in the correct order?

You can add multiple metas of the same type. It's basically like a
linked list, and adding a new one is appending it.

[Sebastian Dröge]

Post by John P Poet
https://gstreamer.freedesktop.org/data/doc/gstreamer/head/pwg/html/se
ction-allocation-meta.html
It says

Lastly, you implement a gst_buffer_add_*_meta() that adds the

metadata implementation to a buffer and sets the values of the
metadata.
And the example just show assigning values to local variables in the
structure.  It does not show any memory allocation, so I assume that
is not necessary.  In that structure I would like it to hold a c++
struct _VANCMeta {
  GstMeta       meta;
  AncillaryPacket packet;
};
AncillaryPacket would have various methods as well as data.  Will
that work, or do I need to use more "base" types in my metadata
structure?  There are only a few data elements in there, so just
having those elements in the VANCMeta struct would be easy, but it
would be nice to have the methods automatically available by whatever
reads the metadata.

You can use a C++ class but you will have to store it as a pointer, and
make sure that you call delete on it when the meta is freed.

Storing C++ classes directly in C structs causes memory leaks as their
destructor can't be called.

Post by John P Poet
If I do need to just have 'simple' data types, and one of those types
is an array of uint16_t, is there a preferred way of allocating the
memory for it?  I assume I would use a GArray rather than a
GstBuffer, right?   Or would I just use a basic C style array of
uint16_t?

You could use either of those 3 options, whichever is easier to use in
your case.

Post by John P Poet
I would like to understand how the memory is deallocated after the
life of the struct is over.  Who is responsible for that
deallocation, and how do they know how to deallocate the memory?

You're responsible for deallocating everything that you allocated
yourself in the free_function you passed to gst_meta_register().

Also make sure to initialize all your fields in the init_func().

[John P Poet]

Post by Sebastian Dröge
You can use a C++ class but you will have to store it as a pointer, and
make sure that you call delete on it when the meta is freed.
Storing C++ classes directly in C structs causes memory leaks as their
destructor can't be called.

The problem I forgot about with using C++, is there is no direct way to
call a C++ method from C. Can I rename:

gst-libav/ext/libav/gstavvidenc.c
to
gst-libav/ext/libav/gstavvidenc.cpp

?

Assuming the answer is no, means that I need to change my C++ methods to
just be C functions. It is probably time for me to get a better handle on
glib (GObject) stuff. Until I started working on this gstreamer stuff, I
had not written straight C in a couple of decades. Do you happen to know
of a good guide to "GObject for C++ programers"?

Thanks,

John

[Sebastian Dröge]

Post by John P Poet

Post by Sebastian Dröge
You can use a C++ class but you will have to store it as a pointer, and
make sure that you call delete on it when the meta is freed.
Storing C++ classes directly in C structs causes memory leaks as their
destructor can't be called.

The problem I forgot about with using C++, is there is no direct way
 gst-libav/ext/libav/gstavvidenc.c
to
 gst-libav/ext/libav/gstavvidenc.cpp
?  

What do you want to add there? You *could* rename it of course, but
that's nothing we would ever accept upstream.

Post by John P Poet
Assuming the answer is no, means that I need to change my C++ methods
to just be C functions.

That's indeed the preferred way :)

Post by John P Poet
  It is probably time for me to get a better handle on glib (GObject)
stuff.  Until I started working on this gstreamer stuff, I had not
written straight C in a couple of decades.  Do you happen to know of
a good guide to "GObject for C++ programers"?

The tutorial in the GObject docs is quite good IMHO:
https://developer.gnome.org/gobject/stable/pt02.html

In my experience, the biggest obstacles for people who (only) know C++
is to accept that writing GObject C code requires writing a lot of
boilerplate code (basically similar to what the C++ compiler does
behind your back), that the compiler does not know about GObject and
thus can't help you much, and that there is only single inheritance but
multiple implementations of interfaces (basically like in Java, C#,
etc)