0%

安卓 tinycap 的一个 bug

在安卓9.0之前,安卓的 tinycap 有一个 bug 是录音生成的文件头部标志错误,这个会导致生成的文件在某些播放器上无法识别。

从 WAV 说起

默认情况下,tinyucap 生成的录音文件是以 pcm 格式存储的。常见的文件扩展名是 .wav。文件格式定义如下

这个时候我们把 wav 文件当作二进制数据流来看就好。wav文件头部的44字节是这个文件的一些标志信息。这里我们不深究各个字节的含义。我们重点关注 SubChunk2Size 。这四个字节表示后边的 data 有多长。

tinycap

tinycap 的源码可以从这里下载https://android.googlesource.com/platform/external/tinyalsa/
下面我也贴出当前的 tinycap 的源码

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
/* tinycap.c
**
** Copyright 2011, The Android Open Source Project
**
** Redistribution and use in source and binary forms, with or without
** modification, are permitted provided that the following conditions are met:
** * Redistributions of source code must retain the above copyright
** notice, this list of conditions and the following disclaimer.
** * Redistributions in binary form must reproduce the above copyright
** notice, this list of conditions and the following disclaimer in the
** documentation and/or other materials provided with the distribution.
** * Neither the name of The Android Open Source Project nor the names of
** its contributors may be used to endorse or promote products derived
** from this software without specific prior written permission.
**
** THIS SOFTWARE IS PROVIDED BY The Android Open Source Project ``AS IS'' AND
** ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
** IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
** ARE DISCLAIMED. IN NO EVENT SHALL The Android Open Source Project BE LIABLE
** FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
** DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
** SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
** CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
** LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
** OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH
** DAMAGE.
*/

#include <tinyalsa/asoundlib.h>
#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>
#include <signal.h>
#include <string.h>
#include <time.h>

#define ID_RIFF 0x46464952
#define ID_WAVE 0x45564157
#define ID_FMT 0x20746d66
#define ID_DATA 0x61746164

#define FORMAT_PCM 1

struct wav_header {
uint32_t riff_id;
uint32_t riff_sz;
uint32_t riff_fmt;
uint32_t fmt_id;
uint32_t fmt_sz;
uint16_t audio_format;
uint16_t num_channels;
uint32_t sample_rate;
uint32_t byte_rate;
uint16_t block_align;
uint16_t bits_per_sample;
uint32_t data_id;
uint32_t data_sz;
};

int capturing = 1;

unsigned int capture_sample(FILE *file, unsigned int card, unsigned int device,
unsigned int channels, unsigned int rate,
enum pcm_format format, unsigned int period_size,
unsigned int period_count, unsigned int cap_time);

void sigint_handler(int sig __unused)
{
capturing = 0;
}

int main(int argc, char **argv)
{
FILE *file;
struct wav_header header;
unsigned int card = 0;
unsigned int device = 0;
unsigned int channels = 2;
unsigned int rate = 44100;
unsigned int bits = 16;
unsigned int frames;
unsigned int period_size = 1024;
unsigned int period_count = 4;
unsigned int cap_time = 0;
enum pcm_format format;

if (argc < 2) {
fprintf(stderr, "Usage: %s file.wav [-D card] [-d device]"
" [-c channels] [-r rate] [-b bits] [-p period_size]"
" [-n n_periods] [-T capture time]\n", argv[0]);
return 1;
}

file = fopen(argv[1], "wb");
if (!file) {
fprintf(stderr, "Unable to create file '%s'\n", argv[1]);
return 1;
}

/* parse command line arguments */
argv += 2;
while (*argv) {
if (strcmp(*argv, "-d") == 0) {
argv++;
if (*argv)
device = atoi(*argv);
} else if (strcmp(*argv, "-c") == 0) {
argv++;
if (*argv)
channels = atoi(*argv);
} else if (strcmp(*argv, "-r") == 0) {
argv++;
if (*argv)
rate = atoi(*argv);
} else if (strcmp(*argv, "-b") == 0) {
argv++;
if (*argv)
bits = atoi(*argv);
} else if (strcmp(*argv, "-D") == 0) {
argv++;
if (*argv)
card = atoi(*argv);
} else if (strcmp(*argv, "-p") == 0) {
argv++;
if (*argv)
period_size = atoi(*argv);
} else if (strcmp(*argv, "-n") == 0) {
argv++;
if (*argv)
period_count = atoi(*argv);
} else if (strcmp(*argv, "-T") == 0) {
argv++;
if (*argv)
cap_time = atoi(*argv);
}
if (*argv)
argv++;
}

header.riff_id = ID_RIFF;
header.riff_sz = 0;
header.riff_fmt = ID_WAVE;
header.fmt_id = ID_FMT;
header.fmt_sz = 16;
header.audio_format = FORMAT_PCM;
header.num_channels = channels;
header.sample_rate = rate;

switch (bits) {
case 32:
format = PCM_FORMAT_S32_LE;
break;
case 24:
format = PCM_FORMAT_S24_LE;
break;
case 16:
format = PCM_FORMAT_S16_LE;
break;
default:
fprintf(stderr, "%u bits is not supported.\n", bits);
fclose(file);
return 1;
}

header.bits_per_sample = pcm_format_to_bits(format);
header.byte_rate = (header.bits_per_sample / 8) * channels * rate;
header.block_align = channels * (header.bits_per_sample / 8);
header.data_id = ID_DATA;

/* leave enough room for header */
fseek(file, sizeof(struct wav_header), SEEK_SET);

/* install signal handler and begin capturing */
signal(SIGINT, sigint_handler);
signal(SIGHUP, sigint_handler);
signal(SIGTERM, sigint_handler);
frames = capture_sample(file, card, device, header.num_channels,
header.sample_rate, format,
period_size, period_count, cap_time);
printf("Captured %u frames\n", frames);

/* write header now all information is known */
header.data_sz = frames * header.block_align;
header.riff_sz = header.data_sz + sizeof(header) - 8;
fseek(file, 0, SEEK_SET);
fwrite(&header, sizeof(struct wav_header), 1, file);

fclose(file);

return 0;
}

unsigned int capture_sample(FILE *file, unsigned int card, unsigned int device,
unsigned int channels, unsigned int rate,
enum pcm_format format, unsigned int period_size,
unsigned int period_count, unsigned int cap_time)
{
struct pcm_config config;
struct pcm *pcm;
char *buffer;
unsigned int size;
unsigned int bytes_read = 0;
unsigned int frames = 0;
struct timespec end;
struct timespec now;

memset(&config, 0, sizeof(config));
config.channels = channels;
config.rate = rate;
config.period_size = period_size;
config.period_count = period_count;
config.format = format;
config.start_threshold = 0;
config.stop_threshold = 0;
config.silence_threshold = 0;

pcm = pcm_open(card, device, PCM_IN, &config);
if (!pcm || !pcm_is_ready(pcm)) {
fprintf(stderr, "Unable to open PCM device (%s)\n",
pcm_get_error(pcm));
return 0;
}

size = pcm_frames_to_bytes(pcm, pcm_get_buffer_size(pcm));
buffer = malloc(size);
if (!buffer) {
fprintf(stderr, "Unable to allocate %u bytes\n", size);
free(buffer);
pcm_close(pcm);
return 0;
}

printf("Capturing sample: %u ch, %u hz, %u bit\n", channels, rate,
pcm_format_to_bits(format));

clock_gettime(CLOCK_MONOTONIC, &now);
end.tv_sec = now.tv_sec + cap_time;
end.tv_nsec = now.tv_nsec;

while (capturing && !pcm_read(pcm, buffer, size)) {
if (fwrite(buffer, 1, size, file) != size) {
fprintf(stderr,"Error capturing sample\n");
break;
}
bytes_read += size;
if (cap_time) {
clock_gettime(CLOCK_MONOTONIC, &now);
if (now.tv_sec > end.tv_sec ||
(now.tv_sec == end.tv_sec && now.tv_nsec >= end.tv_nsec))
break;
}
}

frames = pcm_bytes_to_frames(pcm, bytes_read);
free(buffer);
pcm_close(pcm);
return frames;
}

我们看第44到58行

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
struct wav_header {
uint32_t riff_id;
uint32_t riff_sz;
uint32_t riff_fmt;
uint32_t fmt_id;
uint32_t fmt_sz;
uint16_t audio_format;
uint16_t num_channels;
uint32_t sample_rate;
uint32_t byte_rate;
uint16_t block_align;
uint16_t bits_per_sample;
uint32_t data_id;
uint32_t data_sz;
};

仔细看这里是和上图中的wav文件头一一对应的。

然后再看177到188行

1
2
3
4
5
6
7
8
9
10
11
12
frames = capture_sample(file, card, device, header.num_channels,
header.sample_rate, format,
period_size, period_count, cap_time);
printf("Captured %u frames\n", frames);

/* write header now all information is known */
header.data_sz = frames * header.block_align;
header.riff_sz = header.data_sz + sizeof(header) - 8;
fseek(file, 0, SEEK_SET);
fwrite(&header, sizeof(struct wav_header), 1, file);

fclose(file);

这里我们能看到最后通过fseek(file, 0, SEEK_SET);fwrite(&header, sizeof(struct wav_header), 1, file);把 header 信息写入了 file 。这里的file自然就是tinycap生成录音的目标文件。这里的 header 就是上边的结构体的一个实例。并且我们可以看到 header.data_sz = frames * header.block_align;frames = capture_sample(xxxxxxx。这里的frames和data的值有关。然后我们看一下下面的提交记录

我们可以看到这次修改记录中。原本的

1
2
3
free(buffer);
pcm_close(pcm);
return pcm_bytes_to_frames(pcm, bytes_read);

变成了

1
2
3
4
frames = pcm_bytes_to_frames(pcm, bytes_read);
free(buffer);
pcm_close(pcm);
return frames;

当然如果我们继续深入的看 pcm_close(pcm) 确实能出来这里做了一些内存释放的操作。这里就不展示相应的代码了。
到这里就已经明显了,修改之前,这个函数的返回值将是 0 ,这里也会导致生成的 wav 文件的头部用于标志 data 长度的 4 字节为 0,如果播放器按照标准的wav文件去读取,则会发现这是一个没有数据的wav文件。不过我们也能看到,这个bug已经在 2018.03.19 修复。所以本文开头才说这是安卓 9.0 之前的版本有的问题。