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安卓 tinycap 的一个 bug

Posted on Edited on
Symbols count in article: Reading time ≈ NaN:aN

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

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/* 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

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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

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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);和 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; 而 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

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free(buffer);
pcm_close(pcm);
return pcm_bytes_to_frames(pcm, bytes_read);

变成了

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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。