#include <stdio.h>
#include <time.h>
#include <errno.h>
#include <unistd.h>
#include <ctype.h>
#include <inttypes.h>
#include <stdlib.h>
#define ID3v2_HEADER_SIZE 10
#define MAX_BUF_SIZE 100000000
//#define MAX_BUF_SIZE 2519178
#define MPA_STEREO 0
#define MPA_JSTEREO 1
#define MPA_DUAL 2
#define MPA_MONO 3
#define AV_RB32(x) ((((const uint8_t*)(x))[0] << 24) | \
(((const uint8_t*)(x))[1] << 16) | \
(((const uint8_t*)(x))[2] << 8) | \
((const uint8_t*)(x))[3])
#define FFMAX(a,b) ((a) > (b) ? (a) : (b))
#define MPA_DECODE_HEADER \
int frame_size; \
int error_protection; \
int layer; \
int sample_rate; \
int sample_rate_index; /* between 0 and 8 */ \
int bit_rate; \
int nb_channels; \
int mode; \
int mode_ext; \
int lsf;
typedef struct MPADecodeHeader {
MPA_DECODE_HEADER
} MPADecodeHeader;
static unsigned char const_buf[MAX_BUF_SIZE];
//const char *filename = "/tmp/MrsLeta.mp3";
//const char *filename = "/tmp/312178.mp3";
const char *filename = "/tmp/test.mp3";
const uint16_t ff_mpa_bitrate_tab[2][3][15] = {
{ {0, 32, 64, 96, 128, 160, 192, 224, 256, 288, 320, 352, 384, 416, 448 },
{0, 32, 48, 56, 64, 80, 96, 112, 128, 160, 192, 224, 256, 320, 384 },
{0, 32, 40, 48, 56, 64, 80, 96, 112, 128, 160, 192, 224, 256, 320 } },
{ {0, 32, 48, 56, 64, 80, 96, 112, 128, 144, 160, 176, 192, 224, 256},
{0, 8, 16, 24, 32, 40, 48, 56, 64, 80, 96, 112, 128, 144, 160},
{0, 8, 16, 24, 32, 40, 48, 56, 64, 80, 96, 112, 128, 144, 160}
}
};
const uint16_t ff_mpa_freq_tab[3] = { 44100, 48000, 32000 };
int ff_id3v2_tag_len (unsigned char* buf);
int ff_int_buf(unsigned char *buf);
int ff_mpa_decode_header( uint32_t head, int *sample_rate, int *channels, int *frame_size, int *bit_rate);
static inline int ff_mpa_check_header(uint32_t header);
int ff_id3v2_match(const uint8_t *buf);
int main(void)
{
int size = ff_int_buf(const_buf);
int len = ff_id3v2_tag_len(const_buf);
unsigned char *buf0,*buf2,*buf,*end;
int max_frames = 0, first_frames = 0;
int fsize, frames, sample_rate;
uint32_t header;
buf0 = const_buf ;
if(ff_id3v2_match(buf0)) {
buf0 += ff_id3v2_tag_len(buf0);
}
buf = buf0;
end = buf + size - sizeof(uint32_t);
for(; buf < end; buf= buf2+1) {
buf2 = buf;
for(frames = 0; buf2 < end; frames++) {
header = AV_RB32(buf2);
fsize = ff_mpa_decode_header(header, &sample_rate, &sample_rate, &sample_rate, &sample_rate);
if(fsize < 0)
break;
buf2 += fsize;
}
max_frames = FFMAX(max_frames, frames);
}
return 0;
}
int ff_int_buf(unsigned char *buf)
{
FILE *f;
f = fopen(filename, "r");
if (!f) {
fprintf(stderr, "Cannot read file '%s' for pass-2 encoding: %s\n", filename, strerror(errno));
return -1;
}
int size = fread(const_buf, 1, MAX_BUF_SIZE, f);
const_buf[size] = '\0';
fclose(f);
return size;
}
int ff_id3v2_tag_len (unsigned char* buf)
{
int len = ((buf[6] & 0x7f) << 21) +
((buf[7] & 0x7f) << 14) +
((buf[8] & 0x7f) << 7) +
(buf[9] & 0x7f) +
ID3v2_HEADER_SIZE;
if (buf[5] & 0x10)
len += ID3v2_HEADER_SIZE;
return len;
}
int ff_mpa_decode_header( uint32_t head, int *sample_rate, int *channels, int *frame_size, int *bit_rate
)
{
MPADecodeHeader s1, *s = &s1;
if (ff_mpa_check_header(head) != 0)
return -1;
if (ff_mpegaudio_decode_header(s, head) != 0) {
return -1;
}
switch(s->layer) {
case 1:
*frame_size = 384;
break;
case 2:
*frame_size = 1152;
break;
default:
case 3:
if (s->lsf)
*frame_size = 576;
else
*frame_size = 1152;
break;
}
*sample_rate = s->sample_rate;
*channels = s->nb_channels;
*bit_rate = s->bit_rate;
return s->frame_size;
}
/* fast header check for resync */
static inline int ff_mpa_check_header(uint32_t header){
/* header */
if ((header & 0xffe00000) != 0xffe00000)
return -1;
/* layer check */
if ((header & (3<<17)) == 0)
return -1;
/* bit rate */
if ((header & (0xf<<12)) == 0xf<<12)
return -1;
/* frequency */
if ((header & (3<<10)) == 3<<10)
return -1;
return 0;
}
int ff_mpegaudio_decode_header(MPADecodeHeader *s, uint32_t header)
{
int sample_rate, frame_size, mpeg25, padding;
int sample_rate_index, bitrate_index;
if (header & (1<<20)) {
s->lsf = (header & (1<<19)) ? 0 : 1;
mpeg25 = 0;
} else {
s->lsf = 1;
mpeg25 = 1;
}
s->layer = 4 - ((header >> 17) & 3);
/* extract frequency */
sample_rate_index = (header >> 10) & 3;
sample_rate = ff_mpa_freq_tab[sample_rate_index] >> (s->lsf + mpeg25);
sample_rate_index += 3 * (s->lsf + mpeg25);
s->sample_rate_index = sample_rate_index;
s->error_protection = ((header >> 16) & 1) ^ 1;
s->sample_rate = sample_rate;
bitrate_index = (header >> 12) & 0xf;
padding = (header >> 9) & 1;
//extension = (header >> 8) & 1;
s->mode = (header >> 6) & 3;
s->mode_ext = (header >> 4) & 3;
//copyright = (header >> 3) & 1;
//original = (header >> 2) & 1;
//emphasis = header & 3;
if (s->mode == MPA_MONO)
s->nb_channels = 1;
else
s->nb_channels = 2;
if (bitrate_index != 0) {
frame_size = ff_mpa_bitrate_tab[s->lsf][s->layer - 1][bitrate_index];
s->bit_rate = frame_size * 1000;
switch(s->layer) {
case 1:
frame_size = (frame_size * 12000) / sample_rate;
frame_size = (frame_size + padding) * 4;
break;
case 2:
frame_size = (frame_size * 144000) / sample_rate;
frame_size += padding;
break;
default:
case 3:
frame_size = (frame_size * 144000) / (sample_rate << s->lsf);
frame_size += padding;
break;
}
s->frame_size = frame_size;
} else {
/* if no frame size computed, signal it */
return 1;
}
#if defined(DEBUG)
dprintf(s->avctx, "layer%d, %d Hz, %d kbits/s, ",
s->layer, s->sample_rate, s->bit_rate);
if (s->nb_channels == 2) {
if (s->layer == 3) {
if (s->mode_ext & MODE_EXT_MS_STEREO)
dprintf(s->avctx, "ms-");
if (s->mode_ext & MODE_EXT_I_STEREO)
dprintf(s->avctx, "i-");
}
dprintf(s->avctx, "stereo");
} else {
dprintf(s->avctx, "mono");
}
dprintf(s->avctx, "\n");
#endif
return 0;
}
int ff_id3v2_match(const uint8_t *buf)
{
return buf[0] == 'I' &&
buf[1] == 'D' &&
buf[2] == '3' &&
buf[3] != 0xff &&
buf[4] != 0xff &&
(buf[6] & 0x80) == 0 &&
(buf[7] & 0x80) == 0 &&
(buf[8] & 0x80) == 0 &&
(buf[9] & 0x80) == 0;
}文件格式详细说明参考：
http://blog.csdn.net/sunshine1314/article/details/2514322
经验证，MP3文件可以在任意位置切割，切割后的文件是依然可以播放的。
个人认为，通过代码学习文件格式比阅读文档要深刻快速，当然文档的表达能力也不能被忽略。