avformat/matroskaenc: Don't write \0 unnecessarily

Writing the duration SimpleTag is special: It's size is
reserved in advance via an EBML Void element (if seekable)
and this reserved space is overwritten when writing the trailer;
it does not use put_ebml_string().

The string to write is created via snprintf on a buffer
of size 20; this buffer is then written via put_ebml_binary()
with a size of 20.

EBML strings need not be zero-terminated; if not, they
are implicitly terminated by the element's length field.
snprintf() always zero-terminates the buffer, i.e.
the last byte can be discarded when using an EBML string.
This patch does this.

The FATE changes are as expected: One byte saved for every
track; the only exception is the matroska-qt-mode test:
An additional byte is saved because an additional byte
could be saved from the enclosing Tags length field.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

1 files changed, 10 insertions, 5 deletions

diff --git a/libavformat/matroskaenc.c b/libavformat/matroskaenc.c
index 7cbae47e42..e813ef86cf 100644
--- a/libavformat/matroskaenc.c
+++ b/libavformat/matroskaenc.c
@@ -264,8 +264,12 @@ typedef struct MatroskaMuxContext {
 /** 4 * (1-byte EBML ID, 1-byte EBML size, 8-byte uint max) */
 #define MAX_CUETRACKPOS_SIZE 40
 
-/** 2 + 1 Simpletag header, 2 + 1 + 8 Name "DURATION", 23B for TagString */
-#define DURATION_SIMPLETAG_SIZE (2 + 1 + (2 + 1 + 8) + 23)
+/** DURATION_STRING_LENGTH must be <= 112 or the containing
+ * simpletag will need more than one byte for its length field. */
+#define DURATION_STRING_LENGTH 19
+
+/** 2 + 1 Simpletag header, 2 + 1 + 8 Name "DURATION", rest for TagString */
+#define DURATION_SIMPLETAG_SIZE (2 + 1 + (2 + 1 + 8) + (2 + 1 + DURATION_STRING_LENGTH))
 
 /** Seek preroll value for opus */
 #define OPUS_SEEK_PREROLL 80000000
@@ -3239,7 +3243,7 @@ after_cues:
 
             if (track->duration_offset > 0) {
                 double duration_sec = track->duration * av_q2d(st->time_base);
-                char duration_string[20] = "";
+                char duration_string[DURATION_STRING_LENGTH + 1] = "";
                 ebml_master simpletag;
 
                 av_log(s, AV_LOG_DEBUG, "stream %d end duration = %" PRIu64 "\n", i,
@@ -3250,11 +3254,12 @@ after_cues:
                                               2 + 1 + 8 + 23);
                 put_ebml_string(tags_bc, MATROSKA_ID_TAGNAME, "DURATION");
 
-                snprintf(duration_string, 20, "%02d:%02d:%012.9f",
+                snprintf(duration_string, sizeof(duration_string), "%02d:%02d:%012.9f",
                          (int) duration_sec / 3600, ((int) duration_sec / 60) % 60,
                          fmod(duration_sec, 60));
 
-                put_ebml_binary(tags_bc, MATROSKA_ID_TAGSTRING, duration_string, 20);
+                put_ebml_binary(tags_bc, MATROSKA_ID_TAGSTRING,
+                                duration_string, DURATION_STRING_LENGTH);
                 end_ebml_master(tags_bc, simpletag);
             }
         }