[FFmpeg-cvslog] avformat/aviobuf: Avoid allocating buffer when using dynamic buffer

Andreas Rheinhardt git at videolan.org
Thu Nov 28 20:23:08 EET 2019

ffmpeg | branch: master | Andreas Rheinhardt <andreas.rheinhardt at gmail.com> | Wed Nov 27 13:22:05 2019 +0100| [163bb9ac0af495a5cb95441bdb5c02170440d28c] | committer: James Almer

avformat/aviobuf: Avoid allocating buffer when using dynamic buffer

Up until now, using a dynamic buffer entailed at least three
allocations: One for the AVIOContext, one for the AVIOContext's opaque
(which, among other things, contains the small write buffer), and one
for the big buffer that is independently allocated that is returned when
calling avio_close_dyn_buf().

It is possible to avoid the third allocation if one doesn't use a
packetized dynamic buffer, if all the data written so far fit into the
write buffer and if one does not require the actual (big) buffer to have
an indefinite lifetime. This is done by making avio_get_dyn_buf() return
a pointer to the data in the write buffer if nothing has been written to
the main buffer yet. The dynamic buffer will then be freed using
ffio_free_dynamic_buffer (which needed to be modified not to call
avio_close_dyn_buf() internally).

So a typical use-case like:

size = avio_close_dyn_buf(dyn_pb, &buf);
do something with buf

can be converted to:

size = avio_get_dyn_buf(dyn_pb, &buf);
do something with buf

In more complex scenarios this can simplify freeing as well, because it
is now clear that freeing always has to be performed via

Of course, in case this saves an allocation it also saves a memcpy.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt at gmail.com>

> http://git.videolan.org/gitweb.cgi/ffmpeg.git/?a=commit;h=163bb9ac0af495a5cb95441bdb5c02170440d28c

 libavformat/aviobuf.c | 20 ++++++++++++++------
 1 file changed, 14 insertions(+), 6 deletions(-)

diff --git a/libavformat/aviobuf.c b/libavformat/aviobuf.c
index 2dfce8af8b..70e1d2ca10 100644
--- a/libavformat/aviobuf.c
+++ b/libavformat/aviobuf.c
@@ -1443,14 +1443,19 @@ int avio_get_dyn_buf(AVIOContext *s, uint8_t **pbuffer)
     DynBuffer *d;
-    if (!s) {
+    if (!s || s->error) {
         *pbuffer = NULL;
         return 0;
+    d = s->opaque;
+    if (!d->size) {
+        *pbuffer = d->io_buffer;
+        return FFMAX(s->buf_ptr, s->buf_ptr_max) - s->buffer;
+    }
-    d = s->opaque;
     *pbuffer = d->buffer;
     return d->size;
@@ -1488,12 +1493,15 @@ int avio_close_dyn_buf(AVIOContext *s, uint8_t **pbuffer)
 void ffio_free_dyn_buf(AVIOContext **s)
-    uint8_t *tmp;
+    DynBuffer *d;
     if (!*s)
-    avio_close_dyn_buf(*s, &tmp);
-    av_free(tmp);
-    *s = NULL;
+    d = (*s)->opaque;
+    av_free(d->buffer);
+    av_free(d);
+    avio_context_free(s);
 static int null_buf_write(void *opaque, uint8_t *buf, int buf_size)

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