Allow output formats without any streams.

Required for future metadata format.

Originally committed as revision 26100 to svn://svn.ffmpeg.org/ffmpeg/trunk

ffmpeg.c

@@ -1990,7 +1990,7 @@ static int transcode(AVFormatContext **output_files,
     nb_ostreams = 0;
     for(i=0;i<nb_output_files;i++) {
         os = output_files[i];
-        if (!os->nb_streams) {
+        if (!os->nb_streams && !(os->oformat->flags & AVFMT_NOSTREAMS)) {
             dump_format(output_files[i], i, output_files[i]->filename, 1);
             fprintf(stderr, "Output file #%d does not contain any stream\n", i);
             ret = AVERROR(EINVAL);

libavformat/avformat.h

@@ -323,6 +323,7 @@ typedef struct AVFormatParameters {
 #define AVFMT_TS_DISCONT    0x0200 /**< Format allows timestamp discontinuities. Note, muxers always require valid (monotone) timestamps */
 #define AVFMT_VARIABLE_FPS  0x0400 /**< Format allows variable fps. */
 #define AVFMT_NODIMENSIONS  0x0800 /**< Format does not need width/height */
+#define AVFMT_NOSTREAMS     0x1000 /**< Format does not require any streams */
 
 typedef struct AVOutputFormat {
     const char *name;

libavformat/utils.c

@@ -2709,7 +2709,7 @@ int av_write_header(AVFormatContext *s)
     AVStream *st;
 
     // some sanity checks
-    if (s->nb_streams == 0) {
+    if (s->nb_streams == 0 && !(s->oformat->flags & AVFMT_NOSTREAMS)) {
         av_log(s, AV_LOG_ERROR, "no streams\n");
         return AVERROR(EINVAL);
     }
@@ -2777,7 +2777,7 @@ int av_write_header(AVFormatContext *s)
 #endif
 
     /* set muxer identification string */
-    if (!(s->streams[0]->codec->flags & CODEC_FLAG_BITEXACT)) {
+    if (s->nb_streams && !(s->streams[0]->codec->flags & CODEC_FLAG_BITEXACT)) {
         av_metadata_set2(&s->metadata, "encoder", LIBAVFORMAT_IDENT, 0);
     }