[FFmpeg-devel] [PATCH 1/8] lavu: add macros to help making future-proof structures

[Rémi Denis-Courmont]

Le perjantaina 28. huhtikuuta 2023, 12.55.01 EEST Nicolas George a écrit :
> Signed-off-by: Nicolas George <george at nsup.org>
> ---
>  libavutil/extendable.h | 59 ++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 59 insertions(+)
>  create mode 100644 libavutil/extendable.h
> 
> 
> FFReservedPadding is used by the WIP JSON writer.

A JSON writer that requires forced alignment is a poorly-written JSON parser. 
JSON has a very finite set of types that it can handle, so there should never 
be a need to do this kind of pointer arithmetic kludgery.

> diff --git a/libavutil/extendable.h b/libavutil/extendable.h
> new file mode 100644
> index 0000000000..79980fa202
> --- /dev/null
> +++ b/libavutil/extendable.h
> @@ -0,0 +1,59 @@
> +/*
> + * Copyright (c) 2021 The FFmpeg project
> + *
> + * This file is part of FFmpeg.
> + *
> + * FFmpeg is free software; you can redistribute it and/or
> + * modify it under the terms of the GNU Lesser General Public
> + * License as published by the Free Software Foundation; either
> + * version 2.1 of the License, or (at your option) any later version.
> + *
> + * FFmpeg is distributed in the hope that it will be useful,
> + * but WITHOUT ANY WARRANTY; without even the implied warranty of
> + * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
> + * Lesser General Public License for more details.
> + *
> + * You should have received a copy of the GNU Lesser General Public
> + * License along with FFmpeg; if not, write to the Free Software
> + * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301
> USA + */
> +
> +#ifndef AVUTIL_EXTENDABLE_H
> +#define AVUTIL_EXTENDABLE_H
> +
> +/**
> + * @defgroup ff_extendable FFExtendable
> + *
> + * Types and macros to help designing structures that can be allocated by
> + * the application, including on the stack, but will not break ABI when
> + * extendded.
> + *
> + * This should not be used outside FFmpeg.
> + *
> + * @{
> + */
> +
> +/**
> + * Define a value of type as a compound literal (hidden local variable)
> + * with the field self_size filled.
> + */
> +#define FF_NEW_SZ(type) ((type){ .self_size = sizeof(type) })
> +
> +/**
> + * Type suitable for paddign at the end of a structure, with maximum
> + * alignment.
> + */
> +typedef union FFReservedPadding {
> +    union {
> +        double d;
> +        void *p;
> +        void (*f)(void);
> +        intmax_t i;
> +    } dummy;
> +} FFReservedPadding;

This is reinventing standard max_align_t but with a more confusing name and 
less portable implementation... Indeed this is not about "padding", but about 
alignment.

> +
> +/**
> + * @}
> + */
> +
> +#endif /* AVUTIL_EXTENDABLE_H */


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