On 31/01/2023 11:13, Tim Angus wrote:
In the write_header function of the ASS encoder, extradata is searched for a substring using the strstr function. strstr expects a null terminated C string as its first parameter, but extradata is (notionally) not one of these, meaning that the calls to strstr only happen to work because (presumably) they encounter a 0 byte somewhere downstream of extradata. I say notionally because often extradata will in fact be null terminated (which causes other problems, see parent commit).Signed-off-by: Tim Angus <t...@ngus.net> --- libavformat/assenc.c | 8 ++++---- 1 file changed, 4 insertions(+), 4 deletions(-) diff --git a/libavformat/assenc.c b/libavformat/assenc.c index 4c9ea6f982..f6f5293bb7 100644 --- a/libavformat/assenc.c +++ b/libavformat/assenc.c @@ -83,12 +83,12 @@ static int write_header(AVFormatContext *s) if (header_string[strlen(header_string) - 1] != '\n') avio_printf(s->pb, "\r\n");- av_free(header_string);- - ass->ssa_mode = !strstr(par->extradata, "\n[V4+ Styles]"); - if (!strstr(par->extradata, "\n[Events]")) + ass->ssa_mode = !strstr(header_string, "\n[V4+ Styles]"); + if (!strstr(header_string, "\n[Events]")) avio_printf(s->pb, "[Events]\r\nFormat: %s, Start, End, Style, Name, MarginL, MarginR, MarginV, Effect, Text\r\n", ass->ssa_mode ? "Marked" : "Layer"); + + av_free(header_string); }return 0;
Apparently extradata is padded with zeroes, so this is probably unnecessary, strictly speaking. Having said that, reasoning about why the code works is less obscure with the patch applied.
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