FFmpeg
mpc8.c
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1 /*
2  * Musepack SV8 demuxer
3  * Copyright (c) 2007 Konstantin Shishkov
4  *
5  * This file is part of FFmpeg.
6  *
7  * FFmpeg is free software; you can redistribute it and/or
8  * modify it under the terms of the GNU Lesser General Public
9  * License as published by the Free Software Foundation; either
10  * version 2.1 of the License, or (at your option) any later version.
11  *
12  * FFmpeg is distributed in the hope that it will be useful,
13  * but WITHOUT ANY WARRANTY; without even the implied warranty of
14  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
15  * Lesser General Public License for more details.
16  *
17  * You should have received a copy of the GNU Lesser General Public
18  * License along with FFmpeg; if not, write to the Free Software
19  * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
20  */
21 
22 #include "libavcodec/get_bits.h"
23 #include "libavcodec/unary.h"
24 #include "apetag.h"
25 #include "avformat.h"
26 #include "internal.h"
27 #include "avio_internal.h"
28 
29 /// Two-byte MPC tag
30 #define MKMPCTAG(a, b) ((a) | ((b) << 8))
31 
32 #define TAG_MPCK MKTAG('M','P','C','K')
33 
34 /// Reserved MPC tags
36  TAG_STREAMHDR = MKMPCTAG('S','H'),
37  TAG_STREAMEND = MKMPCTAG('S','E'),
38 
40 
42  TAG_SEEKTABLE = MKMPCTAG('S','T'),
43 
45  TAG_ENCINFO = MKMPCTAG('E','I'),
46 };
47 
48 static const int mpc8_rate[8] = { 44100, 48000, 37800, 32000, -1, -1, -1, -1 };
49 
50 typedef struct MPCContext {
51  int ver;
52  int64_t header_pos;
53  int64_t samples;
54 
55  int64_t apetag_start;
56 } MPCContext;
57 
58 static inline int64_t bs_get_v(const uint8_t **bs)
59 {
60  uint64_t v = 0;
61  int br = 0;
62  int c;
63 
64  do {
65  c = **bs; (*bs)++;
66  v <<= 7;
67  v |= c & 0x7F;
68  br++;
69  if (br > 10)
70  return -1;
71  } while (c & 0x80);
72 
73  return v - br;
74 }
75 
76 static int mpc8_probe(const AVProbeData *p)
77 {
78  const uint8_t *bs = p->buf + 4;
79  const uint8_t *bs_end = bs + p->buf_size;
80  int64_t size;
81 
82  if (p->buf_size < 16)
83  return 0;
84  if (AV_RL32(p->buf) != TAG_MPCK)
85  return 0;
86  while (bs < bs_end + 3) {
87  int header_found = (bs[0] == 'S' && bs[1] == 'H');
88  if (bs[0] < 'A' || bs[0] > 'Z' || bs[1] < 'A' || bs[1] > 'Z')
89  return 0;
90  bs += 2;
91  size = bs_get_v(&bs);
92  if (size < 2)
93  return 0;
94  if (size >= bs_end - bs + 2)
95  return AVPROBE_SCORE_EXTENSION - 1; // seems to be valid MPC but no header yet
96  if (header_found) {
97  if (size < 11 || size > 28)
98  return 0;
99  if (!AV_RL32(bs)) //zero CRC is invalid
100  return 0;
101  return AVPROBE_SCORE_MAX;
102  } else {
103  bs += size - 2;
104  }
105  }
106  return 0;
107 }
108 
109 static inline int64_t gb_get_v(GetBitContext *gb)
110 {
111  uint64_t v = 0;
112  int bits = 0;
113  while(get_bits1(gb) && bits < 64-7){
114  v <<= 7;
115  v |= get_bits(gb, 7);
116  bits += 7;
117  }
118  v <<= 7;
119  v |= get_bits(gb, 7);
120 
121  return v;
122 }
123 
124 static void mpc8_get_chunk_header(AVIOContext *pb, int *tag, int64_t *size)
125 {
126  int64_t pos;
127  pos = avio_tell(pb);
128  *tag = avio_rl16(pb);
129  *size = ffio_read_varlen(pb);
130  pos -= avio_tell(pb);
131  if (av_sat_add64(*size, pos) != (uint64_t)*size + pos) {
132  *size = -1;
133  } else
134  *size += pos;
135 }
136 
137 static void mpc8_parse_seektable(AVFormatContext *s, int64_t off)
138 {
139  MPCContext *c = s->priv_data;
140  int tag;
141  int64_t size, pos, ppos[2];
142  uint8_t *buf;
143  int i, t, seekd, ret;
144  GetBitContext gb;
145 
146  if (s->nb_streams == 0) {
147  av_log(s, AV_LOG_ERROR, "No stream added before parsing seek table\n");
148  return;
149  }
150 
151  avio_seek(s->pb, off, SEEK_SET);
152  mpc8_get_chunk_header(s->pb, &tag, &size);
153  if(tag != TAG_SEEKTABLE){
154  av_log(s, AV_LOG_ERROR, "No seek table at given position\n");
155  return;
156  }
157  if (size > INT_MAX/10 || size<=0) {
158  av_log(s, AV_LOG_ERROR, "Bad seek table size\n");
159  return;
160  }
162  return;
163  ret = avio_read(s->pb, buf, size);
164  if (ret != size) {
165  av_log(s, AV_LOG_ERROR, "seek table truncated\n");
166  av_free(buf);
167  return;
168  }
169  memset(buf+size, 0, AV_INPUT_BUFFER_PADDING_SIZE);
170 
171  init_get_bits(&gb, buf, size * 8);
172  size = gb_get_v(&gb);
173  if(size > UINT_MAX/4 || size > c->samples/1152){
174  av_log(s, AV_LOG_ERROR, "Seek table is too big\n");
175  av_free(buf);
176  return;
177  }
178  seekd = get_bits(&gb, 4);
179  for(i = 0; i < 2; i++){
180  pos = gb_get_v(&gb) + c->header_pos;
181  ppos[1 - i] = pos;
182  av_add_index_entry(s->streams[0], pos, i, 0, 0, AVINDEX_KEYFRAME);
183  }
184  for(; i < size; i++){
185  if (get_bits_left(&gb) < 13) {
186  av_free(buf);
187  return;
188  }
189  t = get_unary(&gb, 1, 33) << 12;
190  t += get_bits(&gb, 12);
191  if(t & 1)
192  t = -(t & ~1);
193  pos = (t >> 1) + (uint64_t)ppos[0]*2 - ppos[1];
194  av_add_index_entry(s->streams[0], pos, (int64_t)i << seekd, 0, 0, AVINDEX_KEYFRAME);
195  ppos[1] = ppos[0];
196  ppos[0] = pos;
197  }
198  av_free(buf);
199 }
200 
201 static void mpc8_handle_chunk(AVFormatContext *s, int tag, int64_t chunk_pos, int64_t size)
202 {
203  AVIOContext *pb = s->pb;
204  int64_t pos, off;
205 
206  switch(tag){
207  case TAG_SEEKTBLOFF:
208  pos = avio_tell(pb);
209  off = ffio_read_varlen(pb);
210  if (pos > INT64_MAX - size || off < 0 || off > INT64_MAX - chunk_pos)
211  return;
212  pos += size;
213  mpc8_parse_seektable(s, chunk_pos + off);
214  avio_seek(pb, pos, SEEK_SET);
215  break;
216  default:
217  avio_skip(pb, size);
218  }
219 }
220 
222 {
223  MPCContext *c = s->priv_data;
224  AVIOContext *pb = s->pb;
225  AVStream *st;
226  int tag = 0, ret;
227  int64_t size, pos;
228 
229  c->header_pos = avio_tell(pb);
230  if(avio_rl32(pb) != TAG_MPCK){
231  av_log(s, AV_LOG_ERROR, "Not a Musepack8 file\n");
232  return AVERROR_INVALIDDATA;
233  }
234 
235  while(!avio_feof(pb)){
236  pos = avio_tell(pb);
238  if (size < 0) {
239  av_log(s, AV_LOG_ERROR, "Invalid chunk length\n");
240  return AVERROR_INVALIDDATA;
241  }
242  if(tag == TAG_STREAMHDR)
243  break;
245  }
246  if(tag != TAG_STREAMHDR){
247  av_log(s, AV_LOG_ERROR, "Stream header not found\n");
248  return AVERROR_INVALIDDATA;
249  }
250  pos = avio_tell(pb);
251  avio_skip(pb, 4); //CRC
252  c->ver = avio_r8(pb);
253  if(c->ver != 8){
254  avpriv_report_missing_feature(s, "Stream version %d", c->ver);
255  return AVERROR_PATCHWELCOME;
256  }
257  c->samples = ffio_read_varlen(pb);
258  ffio_read_varlen(pb); //silence samples at the beginning
259 
260  st = avformat_new_stream(s, NULL);
261  if (!st)
262  return AVERROR(ENOMEM);
266 
267  if ((ret = ff_get_extradata(s, st->codecpar, pb, 2)) < 0)
268  return ret;
269 
270  st->codecpar->channels = (st->codecpar->extradata[1] >> 4) + 1;
271  st->codecpar->sample_rate = mpc8_rate[st->codecpar->extradata[0] >> 5];
272  avpriv_set_pts_info(st, 64, 1152 << (st->codecpar->extradata[1]&3)*2, st->codecpar->sample_rate);
273  st->start_time = 0;
274  st->duration = c->samples / (1152 << (st->codecpar->extradata[1]&3)*2);
275  size -= avio_tell(pb) - pos;
276  if (size > 0)
277  avio_skip(pb, size);
278 
279  if (pb->seekable & AVIO_SEEKABLE_NORMAL) {
280  int64_t pos = avio_tell(s->pb);
281  c->apetag_start = ff_ape_parse_tag(s);
282  avio_seek(s->pb, pos, SEEK_SET);
283  }
284 
285  return 0;
286 }
287 
289 {
290  MPCContext *c = s->priv_data;
291  int tag, ret;
292  int64_t pos, size;
293 
294  while(!avio_feof(s->pb)){
295  pos = avio_tell(s->pb);
296 
297  /* don't return bogus packets with the ape tag data */
298  if (c->apetag_start && pos >= c->apetag_start)
299  return AVERROR_EOF;
300 
301  mpc8_get_chunk_header(s->pb, &tag, &size);
302  if (size < 0 || size > INT_MAX)
303  return -1;
304  if(tag == TAG_AUDIOPACKET){
305  if ((ret = av_get_packet(s->pb, pkt, size)) < 0)
306  return ret;
307  pkt->stream_index = 0;
308  pkt->duration = 1;
309  return 0;
310  }
311  if(tag == TAG_STREAMEND)
312  return AVERROR_EOF;
314  }
315  return AVERROR_EOF;
316 }
317 
318 static int mpc8_read_seek(AVFormatContext *s, int stream_index, int64_t timestamp, int flags)
319 {
320  AVStream *st = s->streams[stream_index];
321  int index = av_index_search_timestamp(st, timestamp, flags);
322 
323  if(index < 0) return -1;
324  if (avio_seek(s->pb, st->internal->index_entries[index].pos, SEEK_SET) < 0)
325  return -1;
327  return 0;
328 }
329 
330 
332  .name = "mpc8",
333  .long_name = NULL_IF_CONFIG_SMALL("Musepack SV8"),
334  .priv_data_size = sizeof(MPCContext),
339 };
AVCodecParameters::extradata
uint8_t * extradata
Extra binary data needed for initializing the decoder, codec-dependent.
Definition: codec_par.h:74
ff_mpc8_demuxer
const AVInputFormat ff_mpc8_demuxer
Definition: mpc8.c:331
get_bits_left
static int get_bits_left(GetBitContext *gb)
Definition: get_bits.h:850
AVERROR
Filter the word “frame” indicates either a video frame or a group of audio as stored in an AVFrame structure Format for each input and each output the list of supported formats For video that means pixel format For audio that means channel sample they are references to shared objects When the negotiation mechanism computes the intersection of the formats supported at each end of a all references to both lists are replaced with a reference to the intersection And when a single format is eventually chosen for a link amongst the remaining all references to the list are updated That means that if a filter requires that its input and output have the same format amongst a supported all it has to do is use a reference to the same list of formats query_formats can leave some formats unset and return AVERROR(EAGAIN) to cause the negotiation mechanism toagain later. That can be used by filters with complex requirements to use the format negotiated on one link to set the formats supported on another. Frame references ownership and permissions
avformat_new_stream
AVStream * avformat_new_stream(AVFormatContext *s, const AVCodec *c)
Add a new stream to a media file.
Definition: utils.c:4375
AVCodecParameters::codec_type
enum AVMediaType codec_type
General type of the encoded data.
Definition: codec_par.h:56
TAG_AUDIOPACKET
@ TAG_AUDIOPACKET
Definition: mpc8.c:39
ff_get_extradata
int ff_get_extradata(AVFormatContext *s, AVCodecParameters *par, AVIOContext *pb, int size)
Allocate extradata with additional AV_INPUT_BUFFER_PADDING_SIZE at end which is always set to 0 and f...
Definition: utils.c:3283
TAG_ENCINFO
@ TAG_ENCINFO
Definition: mpc8.c:45
AVERROR_EOF
#define AVERROR_EOF
End of file.
Definition: error.h:57
mpc8_read_seek
static int mpc8_read_seek(AVFormatContext *s, int stream_index, int64_t timestamp, int flags)
Definition: mpc8.c:318
apetag.h
index
fg index
Definition: ffmpeg_filter.c:168
mpc8_get_chunk_header
static void mpc8_get_chunk_header(AVIOContext *pb, int *tag, int64_t *size)
Definition: mpc8.c:124
AVStream::internal
AVStreamInternal * internal
An opaque field for libavformat internal usage.
Definition: avformat.h:1009
MPCContext
Definition: mpc.h:54
mpc8_read_packet
static int mpc8_read_packet(AVFormatContext *s, AVPacket *pkt)
Definition: mpc8.c:288
AVPacket::duration
int64_t duration
Duration of this packet in AVStream->time_base units, 0 if unknown.
Definition: packet.h:391
AVProbeData::buf_size
int buf_size
Size of buf except extra allocated bytes.
Definition: avformat.h:449
mpc8_parse_seektable
static void mpc8_parse_seektable(AVFormatContext *s, int64_t off)
Definition: mpc8.c:137
init_get_bits
static int init_get_bits(GetBitContext *s, const uint8_t *buffer, int bit_size)
Initialize GetBitContext.
Definition: get_bits.h:660
ff_ape_parse_tag
int64_t ff_ape_parse_tag(AVFormatContext *s)
Read and parse an APE tag.
Definition: apetag.c:108
av_malloc
#define av_malloc(s)
Definition: tableprint_vlc.h:31
AVINDEX_KEYFRAME
#define AVINDEX_KEYFRAME
Definition: avformat.h:785
AVPROBE_SCORE_MAX
#define AVPROBE_SCORE_MAX
maximum score
Definition: avformat.h:458
get_bits
static unsigned int get_bits(GetBitContext *s, int n)
Read 1-25 bits.
Definition: get_bits.h:380
AVCodecParameters::channels
int channels
Audio only.
Definition: codec_par.h:166
read_seek
static int read_seek(AVFormatContext *ctx, int stream_index, int64_t timestamp, int flags)
Definition: libcdio.c:149
av_add_index_entry
int av_add_index_entry(AVStream *st, int64_t pos, int64_t timestamp, int size, int distance, int flags)
Add an index entry into a sorted list.
Definition: utils.c:1941
GetBitContext
Definition: get_bits.h:62
avio_tell
static av_always_inline int64_t avio_tell(AVIOContext *s)
ftell() equivalent for AVIOContext.
Definition: avio.h:538
AVStream::duration
int64_t duration
Decoding: duration of the stream, in stream time base.
Definition: avformat.h:890
avio_rl16
unsigned int avio_rl16(AVIOContext *s)
Definition: aviobuf.c:717
pkt
AVPacket * pkt
Definition: movenc.c:59
AV_LOG_ERROR
#define AV_LOG_ERROR
Something went wrong and cannot losslessly be recovered.
Definition: log.h:180
AVInputFormat
Definition: avformat.h:626
mpc8_read_header
static int mpc8_read_header(AVFormatContext *s)
Definition: mpc8.c:221
s
#define s(width, name)
Definition: cbs_vp9.c:257
TAG_SEEKTABLE
@ TAG_SEEKTABLE
Definition: mpc8.c:42
TAG_STREAMEND
@ TAG_STREAMEND
Definition: mpc8.c:37
AVInputFormat::name
const char * name
A comma separated list of short names for the format.
Definition: avformat.h:631
AVProbeData::buf
unsigned char * buf
Buffer must have AVPROBE_PADDING_SIZE of extra allocated bytes filled with zero.
Definition: avformat.h:448
AVMEDIA_TYPE_AUDIO
@ AVMEDIA_TYPE_AUDIO
Definition: avutil.h:202
bits
uint8_t bits
Definition: vp3data.h:141
AVIndexEntry::timestamp
int64_t timestamp
Timestamp in AVStream.time_base units, preferably the time from which on correctly decoded frames are...
Definition: avformat.h:779
get_bits.h
AVFormatContext
Format I/O context.
Definition: avformat.h:1107
avpriv_update_cur_dts
void avpriv_update_cur_dts(AVFormatContext *s, AVStream *ref_st, int64_t timestamp)
Update cur_dts of all streams based on the given timestamp and AVStream.
Definition: utils.c:1855
internal.h
AVStream::codecpar
AVCodecParameters * codecpar
Codec parameters associated with this stream.
Definition: avformat.h:994
read_header
static int read_header(FFV1Context *f)
Definition: ffv1dec.c:527
NULL
#define NULL
Definition: coverity.c:32
AVERROR_PATCHWELCOME
#define AVERROR_PATCHWELCOME
Not yet implemented in FFmpeg, patches welcome.
Definition: error.h:64
read_probe
static int read_probe(const AVProbeData *pd)
Definition: jvdec.c:55
gb_get_v
static int64_t gb_get_v(GetBitContext *gb)
Definition: mpc8.c:109
get_bits1
static unsigned int get_bits1(GetBitContext *s)
Definition: get_bits.h:499
AVProbeData
This structure contains the data a format has to probe a file.
Definition: avformat.h:446
MPCContext::samples
int64_t samples
Definition: mpc8.c:53
TAG_MPCK
#define TAG_MPCK
Definition: mpc8.c:32
c
Undefined Behavior In the C some operations are like signed integer dereferencing freed accessing outside allocated Undefined Behavior must not occur in a C it is not safe even if the output of undefined operations is unused The unsafety may seem nit picking but Optimizing compilers have in fact optimized code on the assumption that no undefined Behavior occurs Optimizing code based on wrong assumptions can and has in some cases lead to effects beyond the output of computations The signed integer overflow problem in speed critical code Code which is highly optimized and works with signed integers sometimes has the problem that often the output of the computation does not c
Definition: undefined.txt:32
AVPROBE_SCORE_EXTENSION
#define AVPROBE_SCORE_EXTENSION
score for file extension
Definition: avformat.h:456
AVCodecParameters::sample_rate
int sample_rate
Audio only.
Definition: codec_par.h:170
get_unary
static int get_unary(GetBitContext *gb, int stop, int len)
Get unary code of limited length.
Definition: unary.h:46
avio_rl32
unsigned int avio_rl32(AVIOContext *s)
Definition: aviobuf.c:733
AVIOContext
Bytestream IO Context.
Definition: avio.h:161
NULL_IF_CONFIG_SMALL
#define NULL_IF_CONFIG_SMALL(x)
Return NULL if CONFIG_SMALL is true, otherwise the argument without modification.
Definition: internal.h:116
AVIOContext::seekable
int seekable
A combination of AVIO_SEEKABLE_ flags or 0 when the stream is not seekable.
Definition: avio.h:260
TAG_SEEKTBLOFF
@ TAG_SEEKTBLOFF
Definition: mpc8.c:41
avpriv_set_pts_info
void avpriv_set_pts_info(AVStream *s, int pts_wrap_bits, unsigned int pts_num, unsigned int pts_den)
Set the time base and wrapping info for a given stream.
Definition: utils.c:4800
size
int size
Definition: twinvq_data.h:10344
avpriv_report_missing_feature
void avpriv_report_missing_feature(void *avc, const char *msg,...) av_printf_format(2
Log a generic warning message about a missing feature.
avio_r8
int avio_r8(AVIOContext *s)
Definition: aviobuf.c:607
AVStreamInternal::index_entries
AVIndexEntry * index_entries
Only used if the format does not support seeking natively.
Definition: internal.h:256
ffio_read_varlen
uint64_t ffio_read_varlen(AVIOContext *bc)
Definition: aviobuf.c:893
unary.h
i
int i
Definition: input.c:406
avio_internal.h
av_get_packet
int av_get_packet(AVIOContext *s, AVPacket *pkt, int size)
Allocate and read the payload of a packet and initialize its fields with default values.
Definition: utils.c:282
tag
uint32_t tag
Definition: movenc.c:1594
ret
ret
Definition: filter_design.txt:187
read_packet
static int read_packet(void *opaque, uint8_t *buf, int buf_size)
Definition: avio_reading.c:42
AVStream
Stream structure.
Definition: avformat.h:847
MPCContext::header_pos
int64_t header_pos
Definition: mpc8.c:52
avio_seek
int64_t avio_seek(AVIOContext *s, int64_t offset, int whence)
fseek() equivalent for AVIOContext.
Definition: aviobuf.c:243
pos
unsigned int pos
Definition: spdifenc.c:412
avformat.h
AV_INPUT_BUFFER_PADDING_SIZE
#define AV_INPUT_BUFFER_PADDING_SIZE
Definition: defs.h:40
AV_CODEC_ID_MUSEPACK8
@ AV_CODEC_ID_MUSEPACK8
Definition: codec_id.h:456
av_sat_add64
#define av_sat_add64
Definition: common.h:138
AV_RL32
uint64_t_TMPL AV_WL64 unsigned int_TMPL AV_RL32
Definition: bytestream.h:92
MKMPCTAG
#define MKMPCTAG(a, b)
Two-byte MPC tag.
Definition: mpc8.c:30
AVIO_SEEKABLE_NORMAL
#define AVIO_SEEKABLE_NORMAL
Seeking works like for a local file.
Definition: avio.h:40
MPCContext::apetag_start
int64_t apetag_start
Definition: mpc8.c:55
avio_read
int avio_read(AVIOContext *s, unsigned char *buf, int size)
Read size bytes from AVIOContext into buf.
Definition: aviobuf.c:616
AVIndexEntry::pos
int64_t pos
Definition: avformat.h:778
AVPacket::stream_index
int stream_index
Definition: packet.h:375
avio_skip
int64_t avio_skip(AVIOContext *s, int64_t offset)
Skip given number of bytes forward.
Definition: aviobuf.c:327
mpc8_handle_chunk
static void mpc8_handle_chunk(AVFormatContext *s, int tag, int64_t chunk_pos, int64_t size)
Definition: mpc8.c:201
AVCodecParameters::bits_per_coded_sample
int bits_per_coded_sample
The number of bits per sample in the codedwords.
Definition: codec_par.h:102
av_free
#define av_free(p)
Definition: tableprint_vlc.h:34
AVCodecParameters::codec_id
enum AVCodecID codec_id
Specific type of the encoded data (the codec used).
Definition: codec_par.h:60
AVPacket
This structure stores compressed data.
Definition: packet.h:350
TAG_STREAMHDR
@ TAG_STREAMHDR
Definition: mpc8.c:36
TAG_REPLAYGAIN
@ TAG_REPLAYGAIN
Definition: mpc8.c:44
flags
#define flags(name, subs,...)
Definition: cbs_av1.c:561
MPCContext::ver
int ver
Definition: mpc.c:40
av_log
#define av_log(a,...)
Definition: tableprint_vlc.h:28
AVERROR_INVALIDDATA
#define AVERROR_INVALIDDATA
Invalid data found when processing input.
Definition: error.h:61
AVStream::start_time
int64_t start_time
Decoding: pts of the first frame of the stream in presentation order, in stream time base.
Definition: avformat.h:880
mpc8_probe
static int mpc8_probe(const AVProbeData *p)
Definition: mpc8.c:76
bs_get_v
static int64_t bs_get_v(const uint8_t **bs)
Definition: mpc8.c:58
MPCPacketTags
MPCPacketTags
Reserved MPC tags.
Definition: mpc8.c:35
av_index_search_timestamp
int av_index_search_timestamp(AVStream *st, int64_t timestamp, int flags)
Get the index for a specific timestamp.
Definition: utils.c:2058
mpc8_rate
static const int mpc8_rate[8]
Definition: mpc8.c:48
avio_feof
int avio_feof(AVIOContext *s)
Similar to feof() but also returns nonzero on read errors.
Definition: aviobuf.c:354