diff --git a/libavcodec/assdec.c b/libavcodec/assdec.c
index 5a51703e86..d79065663b 100644
--- a/libavcodec/assdec.c
+++ b/libavcodec/assdec.c
@@ -29,10 +29,11 @@
 
 static av_cold int ass_decode_init(AVCodecContext *avctx)
 {
-    avctx->subtitle_header = av_malloc(avctx->extradata_size);
+    avctx->subtitle_header = av_malloc(avctx->extradata_size + 1);
     if (!avctx->subtitle_header)
         return AVERROR(ENOMEM);
     memcpy(avctx->subtitle_header, avctx->extradata, avctx->extradata_size);
+    avctx->subtitle_header[avctx->extradata_size] = 0;
     avctx->subtitle_header_size = avctx->extradata_size;
     avctx->priv_data = ff_ass_split(avctx->extradata);
     if(!avctx->priv_data)
diff --git a/libavcodec/assenc.c b/libavcodec/assenc.c
index 7ed21eedbc..50b89c00d6 100644
--- a/libavcodec/assenc.c
+++ b/libavcodec/assenc.c
@@ -28,11 +28,12 @@
 
 static av_cold int ass_encode_init(AVCodecContext *avctx)
 {
-    avctx->extradata = av_malloc(avctx->subtitle_header_size);
+    avctx->extradata = av_malloc(avctx->subtitle_header_size + 1);
     if (!avctx->extradata)
         return AVERROR(ENOMEM);
     memcpy(avctx->extradata, avctx->subtitle_header, avctx->subtitle_header_size);
     avctx->extradata_size = avctx->subtitle_header_size;
+    avctx->extradata[avctx->extradata_size] = 0;
     return 0;
 }
 
diff --git a/libavcodec/dvdsubenc.c b/libavcodec/dvdsubenc.c
index 2e0c37da4d..352272d311 100644
--- a/libavcodec/dvdsubenc.c
+++ b/libavcodec/dvdsubenc.c
@@ -20,6 +20,7 @@
  */
 #include "avcodec.h"
 #include "bytestream.h"
+#include "internal.h"
 #include "libavutil/avassert.h"
 #include "libavutil/bprint.h"
 #include "libavutil/imgutils.h"
@@ -408,9 +409,9 @@ static int dvdsub_init(AVCodecContext *avctx)
         av_bprintf(&extradata, " %06"PRIx32"%c",
                    dvdc->global_palette[i] & 0xFFFFFF, i < 15 ? ',' : '\n');
 
-    if ((ret = av_bprint_finalize(&extradata, (char **)&avctx->extradata)) < 0)
+    ret = ff_bprint_to_extradata(avctx, &extradata);
+    if (ret < 0)
         return ret;
-    avctx->extradata_size = extradata.len;
 
     return 0;
 }
diff --git a/libavcodec/internal.h b/libavcodec/internal.h
index 2d3433f434..386f6f3c26 100644
--- a/libavcodec/internal.h
+++ b/libavcodec/internal.h
@@ -201,4 +201,9 @@ int ff_codec_open2_recursive(AVCodecContext *avctx, const AVCodec *codec, AVDict
  */
 int ff_codec_close_recursive(AVCodecContext *avctx);
 
+/**
+ * Finalize buf into extradata and set its size appropriately.
+ */
+int ff_bprint_to_extradata(AVCodecContext *avctx, struct AVBPrint *buf);
+
 #endif /* AVCODEC_INTERNAL_H */
diff --git a/libavcodec/utils.c b/libavcodec/utils.c
index b3669950d5..a275899ff7 100644
--- a/libavcodec/utils.c
+++ b/libavcodec/utils.c
@@ -27,6 +27,7 @@
 
 #include "libavutil/avassert.h"
 #include "libavutil/avstring.h"
+#include "libavutil/bprint.h"
 #include "libavutil/channel_layout.h"
 #include "libavutil/crc.h"
 #include "libavutil/mathematics.h"
@@ -2684,3 +2685,21 @@ int avcodec_is_open(AVCodecContext *s)
 {
     return !!s->internal;
 }
+
+int ff_bprint_to_extradata(AVCodecContext *avctx, struct AVBPrint *buf)
+{
+    int ret;
+    char *str;
+
+    ret = av_bprint_finalize(buf, &str);
+    if (ret < 0)
+        return ret;
+    avctx->extradata = str;
+    /* Note: the string is NUL terminated (so extradata can be read as a
+     * string), but the ending character is not accounted in the size (in
+     * binary formats you are likely not supposed to mux that character). When
+     * extradata is copied, it is also padded with FF_INPUT_BUFFER_PADDING_SIZE
+     * zeros. */
+    avctx->extradata_size = buf->len;
+    return 0;
+}
diff --git a/libavformat/assdec.c b/libavformat/assdec.c
index 6c4614ec47..34229cb6f9 100644
--- a/libavformat/assdec.c
+++ b/libavformat/assdec.c
@@ -22,6 +22,7 @@
 #include "avformat.h"
 #include "internal.h"
 #include "subtitles.h"
+#include "libavcodec/internal.h"
 #include "libavutil/bprint.h"
 
 typedef struct ASSContext{
@@ -132,12 +133,9 @@ static int ass_read_header(AVFormatContext *s)
 
     av_bprint_finalize(&line, NULL);
 
-    av_bprint_finalize(&header, (char **)&st->codec->extradata);
-    if (!st->codec->extradata) {
-        res = AVERROR(ENOMEM);
+    res = ff_bprint_to_extradata(st->codec, &header);
+    if (res < 0)
         goto end;
-    }
-    st->codec->extradata_size = header.len + 1;
 
     ff_subtitles_queue_finalize(&ass->q);
 
diff --git a/libavformat/jacosubdec.c b/libavformat/jacosubdec.c
index 1c58e3bf25..153da4247a 100644
--- a/libavformat/jacosubdec.c
+++ b/libavformat/jacosubdec.c
@@ -28,6 +28,7 @@
 #include "avformat.h"
 #include "internal.h"
 #include "subtitles.h"
+#include "libavcodec/internal.h"
 #include "libavcodec/jacosub.h"
 #include "libavutil/avstring.h"
 #include "libavutil/bprint.h"
@@ -159,7 +160,7 @@ static int jacosub_read_header(AVFormatContext *s)
     JACOsubContext *jacosub = s->priv_data;
     int shift_set = 0; // only the first shift matters
     int merge_line = 0;
-    int i;
+    int i, ret;
 
     AVStream *st = avformat_new_stream(s, NULL);
     if (!st)
@@ -228,8 +229,9 @@ static int jacosub_read_header(AVFormatContext *s)
     }
 
     /* general/essential directives in the extradata */
-    av_bprint_finalize(&header, (char **)&st->codec->extradata);
-    st->codec->extradata_size = header.len + 1;
+    ret = ff_bprint_to_extradata(st->codec, &header);
+    if (ret < 0)
+        return ret;
 
     /* SHIFT and TIMERES affect the whole script so packet timing can only be
      * done in a second pass */
diff --git a/libavformat/samidec.c b/libavformat/samidec.c
index 85fd220f4a..bdde2f466f 100644
--- a/libavformat/samidec.c
+++ b/libavformat/samidec.c
@@ -27,6 +27,7 @@
 #include "avformat.h"
 #include "internal.h"
 #include "subtitles.h"
+#include "libavcodec/internal.h"
 #include "libavutil/avstring.h"
 #include "libavutil/bprint.h"
 #include "libavutil/intreadwrite.h"
@@ -91,13 +92,9 @@ static int sami_read_header(AVFormatContext *s)
         av_bprint_clear(&buf);
     }
 
-    st->codec->extradata_size = hdr_buf.len + 1;
-    av_bprint_finalize(&hdr_buf, (char **)&st->codec->extradata);
-    if (!st->codec->extradata) {
-        st->codec->extradata_size = 0;
-        res = AVERROR(ENOMEM);
+    res = ff_bprint_to_extradata(st->codec, &hdr_buf);
+    if (res < 0)
         goto end;
-    }
 
     ff_subtitles_queue_finalize(&sami->q);
 
diff --git a/libavformat/subviewerdec.c b/libavformat/subviewerdec.c
index 7691d82768..8ecc928817 100644
--- a/libavformat/subviewerdec.c
+++ b/libavformat/subviewerdec.c
@@ -27,6 +27,7 @@
 #include "avformat.h"
 #include "internal.h"
 #include "subtitles.h"
+#include "libavcodec/internal.h"
 #include "libavutil/avstring.h"
 #include "libavutil/bprint.h"
 #include "libavutil/intreadwrite.h"
@@ -99,12 +100,9 @@ static int subviewer_read_header(AVFormatContext *s)
                 av_bprintf(&header, "%s", line);
                 if (!strncmp(line, "[END INFORMATION]", 17) || !strncmp(line, "[SUBTITLE]", 10)) {
                     /* end of header */
-                    av_bprint_finalize(&header, (char **)&st->codec->extradata);
-                    if (!st->codec->extradata) {
-                        res = AVERROR(ENOMEM);
+                    res = ff_bprint_to_extradata(st->codec, &header);
+                    if (res < 0)
                         goto end;
-                    }
-                    st->codec->extradata_size = header.len + 1;
                 } else if (strncmp(line, "[INFORMATION]", 13)) {
                     /* assume file metadata at this point */
                     int i, j = 0;