Anton Khirnov:
> There should be no good reason for the callers to access any of its
> contents.
>
> Define a new type for the internal struct that currently matches
> AVFifoBuffer. This will allow adding new private fields without waiting
> for the major bump and will be useful in the following commits.
>
> Unfortunately AVFifoBuffer fields cannot be marked as deprecated because
> it would trigger a warning wherever fifo.h is #included, due to
> inlined av_fifo_peek2().
> ---
> doc/APIchanges | 4 ++++
> libavutil/fifo.c | 23 +++++++++++++++++++----
> libavutil/fifo.h | 14 ++++++++++++--
> libavutil/tests/fifo.c | 2 +-
> libavutil/version.h | 1 +
> 5 files changed, 37 insertions(+), 7 deletions(-)
>
> diff --git a/doc/APIchanges b/doc/APIchanges
> index 8df0364e4c..21fa02ae9d 100644
> --- a/doc/APIchanges
> +++ b/doc/APIchanges
> @@ -14,6 +14,10 @@ libavutil: 2021-04-27
>
> API changes, most recent first:
>
> +2022-01-xx - xxxxxxxxxx - lavu fifo.h
> + Access to all AVFifoBuffer members is deprecated. The struct will
> + become an incomplete type in a future major libavutil version.
> +
> 2022-01-04 - 78dc21b123e - lavu 57.16.100 - frame.h
> Add AV_FRAME_DATA_DOVI_METADATA.
>
> diff --git a/libavutil/fifo.c b/libavutil/fifo.c
> index f2f046b1f3..aaade01333 100644
> --- a/libavutil/fifo.c
> +++ b/libavutil/fifo.c
> @@ -28,9 +28,24 @@
>
> #define FIFO_SIZE_MAX FFMIN3((uint64_t)INT_MAX, (uint64_t)UINT32_MAX,
> (uint64_t)SIZE_MAX)
>
> +#if FF_API_FIFO_PUBLIC
> +# define CTX_STRUCT_NAME FifoBuffer
> +#else
> +# define CTX_STRUCT_NAME AVFifoBuffer
This is invalid in pre-C11 (and will lead to compilation failures on old
GCC versions): Pre-C11, typedefs were subject to the ODR, yet you
already typedef AVFifoBuffer in fifo.h.
> +#endif
> +
> +typedef struct CTX_STRUCT_NAME {
> + // These fields must match then contents of AVFifoBuffer in fifo.h
the contents
> + // until FF_API_FIFO_PUBLIC is removed
The actual spec-compliant way for this is to add an AVFifoBuffer at the
start of this struct; it will also avoid the casts from FifoBuffer to
AVFifoBuffer. This will make the accesses to it a bit more cumbersome
and will mean more changes when FF_API_FIFO_PUBLIC is removed. (This
would not be an issue if we required support for anonymous structs
(mandatory in C11, supported by GCC and others long before that).)
> + uint8_t *buffer;
> + uint8_t *rptr, *wptr, *end;
> + uint32_t rndx, wndx;
> + /////////////////////////////////////////
> +} FifoBuffer;
> +
> AVFifoBuffer *av_fifo_alloc_array(size_t nmemb, size_t size)
> {
> - AVFifoBuffer *f;
> + FifoBuffer *f;
> void *buffer;
>
> if (nmemb > FIFO_SIZE_MAX / size)
> @@ -39,15 +54,15 @@ AVFifoBuffer *av_fifo_alloc_array(size_t nmemb, size_t
> size)
> buffer = av_realloc_array(NULL, nmemb, size);
> if (!buffer)
> return NULL;
> - f = av_mallocz(sizeof(AVFifoBuffer));
> + f = av_mallocz(sizeof(*f));
> if (!f) {
> av_free(buffer);
> return NULL;
> }
> f->buffer = buffer;
> f->end = f->buffer + nmemb * size;
> - av_fifo_reset(f);
> - return f;
> + av_fifo_reset((AVFifoBuffer*)f);
> + return (AVFifoBuffer*)f;
> }
>
> AVFifoBuffer *av_fifo_alloc(unsigned int size)
> diff --git a/libavutil/fifo.h b/libavutil/fifo.h
> index f4fd291e59..ca4e7fe060 100644
> --- a/libavutil/fifo.h
> +++ b/libavutil/fifo.h
> @@ -28,11 +28,21 @@
> #include "avutil.h"
> #include "attributes.h"
>
> -typedef struct AVFifoBuffer {
> +#if FF_API_FIFO_PUBLIC
> +/**
> + * The contents of the struct are private and should not be accessed by the
> + * callers in any way.
> + */
> +#endif
> +typedef struct AVFifoBuffer
> +#if FF_API_FIFO_PUBLIC
> +{
> uint8_t *buffer;
> uint8_t *rptr, *wptr, *end;
> uint32_t rndx, wndx;
> -} AVFifoBuffer;
> +}
> +#endif
> +AVFifoBuffer;
>
> /**
> * Initialize an AVFifoBuffer.
> diff --git a/libavutil/tests/fifo.c b/libavutil/tests/fifo.c
> index a17d913233..e5aa88d252 100644
> --- a/libavutil/tests/fifo.c
> +++ b/libavutil/tests/fifo.c
> @@ -18,7 +18,7 @@
>
> #include <stdio.h>
> #include <stdlib.h>
> -#include "libavutil/fifo.h"
> +#include "libavutil/fifo.c"
>
> int main(void)
> {
> diff --git a/libavutil/version.h b/libavutil/version.h
> index 953aac9d94..7c031f547e 100644
> --- a/libavutil/version.h
> +++ b/libavutil/version.h
> @@ -110,6 +110,7 @@
> #define FF_API_COLORSPACE_NAME (LIBAVUTIL_VERSION_MAJOR < 58)
> #define FF_API_AV_MALLOCZ_ARRAY (LIBAVUTIL_VERSION_MAJOR < 58)
> #define FF_API_FIFO_PEEK2 (LIBAVUTIL_VERSION_MAJOR < 58)
> +#define FF_API_FIFO_PUBLIC (LIBAVUTIL_VERSION_MAJOR < 58)
>
> /**
> * @}
>
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