tinyalsa源码阅读解析

简介

tinyalsa的源代码包括两部分，tinyalsa库文件和小工具。
tinyalsa库文件的源代码有两个，是mixer.c pcm.c。其中mixer.c提供了控制接口。pcm.c提供了PCM播放、录音的接口。
tinyalsa工具包括了四个文件，tinyplay.c、tinycap.c、tinymix.c、tinypcminfo.c。每个文件对应一个可执行文件。tinyplay 是一个简单的播放器，可以播放WAV文件。tinycap是一个简单的录音程序，可以进行录音并保存到一个wav文件中。tinymix用于配置control，例如切换音频通道等等。tinypcminfo获取PCM的参数的示例，例如采样率的范围、通道数的范围等等。
**

PCM结构体说明

 struct pcm_config {
    unsigned int channels;
    unsigned int rate;
    unsigned int period_size;
    unsigned int period_count;
    enum pcm_format format;

    unsigned int start_threshold;
    unsigned int stop_threshold;
    unsigned int silence_threshold;
    int avail_min;
  };

.channels :通道数
.rate : 采样率
.period_size : 每次传输的数据长度。值越小，时延越小，cpu占用就越高。
.period_count: 缓冲区period的个数。缓冲区越多，发生XRUN的机会就越少。
.format : 定义数据格式，如数据宽度
.start_threshold : 缓冲区的数据超过该值时，硬件开始启动数据传输。
.stop_threshold : 缓冲区空闲区大于该值时，硬件停止传输。默认情况下，这个数 为整个缓冲区的大小，即整个缓冲区空了，就停止传输。
.avail_min : 缓冲区空闲区大于该值时，pcm_mmap_write()往缓冲写数据。

**

tinyalsa头文件asoundlib.h

**
该文件主要包含如下函数

/* PCM runtime states */                                                //PCM运行状态
#define        PCM_STATE_OPEN                0
#define        PCM_STATE_SETUP                1
#define        PCM_STATE_PREPARED        2
#define        PCM_STATE_RUNNING                3
#define        PCM_STATE_XRUN                4
#define        PCM_STATE_DRAINING        5
#define        PCM_STATE_PAUSED                6
#define        PCM_STATE_SUSPENDED        7
#define        PCM_STATE_DISCONNECTED        8
 
/* TLV header size*/                                                     //报头大小    
#define TLV_HEADER_SIZE (2 * sizeof(unsigned int))  
 
/* Bit formats */                                                         //位格式
enum pcm_format {                                             
    PCM_FORMAT_INVALID = -1,
    PCM_FORMAT_S16_LE = 0,  /* 16-bit signed */
    PCM_FORMAT_S32_LE,      /* 32-bit signed */
    PCM_FORMAT_S8,          /* 8-bit signed */
    PCM_FORMAT_S24_LE,      /* 24-bits in 4-bytes */
    PCM_FORMAT_S24_3LE,     /* 24-bits in 3-bytes */

    PCM_FORMAT_MAX,
};

/* Bitmask has 256 bits (32 bytes) in asound.h */                        //位掩码在asound.h中有32位
struct pcm_mask {
    unsigned int bits[32 / sizeof(unsigned int)];
};

/* Configuration for a stream */                                         //流的配置
struct pcm_config {
    unsigned int channels;
    unsigned int rate;
    unsigned int period_size;
    unsigned int period_count;
    enum pcm_format format;

   
    unsigned int start_threshold;
    unsigned int stop_threshold;
    unsigned int silence_threshold;
    unsigned int silence_size;

   
    int avail_min;
};

/* PCM parameters */                                           //PCM参数
enum pcm_param
{
    /* mask parameters */
    PCM_PARAM_ACCESS,
    PCM_PARAM_FORMAT,
    PCM_PARAM_SUBFORMAT,
    /* interval parameters */
    PCM_PARAM_SAMPLE_BITS,
    PCM_PARAM_FRAME_BITS,
    PCM_PARAM_CHANNELS,
    PCM_PARAM_RATE,
    PCM_PARAM_PERIOD_TIME,
    PCM_PARAM_PERIOD_SIZE,
    PCM_PARAM_PERIOD_BYTES,
    PCM_PARAM_PERIODS,
    PCM_PARAM_BUFFER_TIME,
    PCM_PARAM_BUFFER_SIZE,
    PCM_PARAM_BUFFER_BYTES,
    PCM_PARAM_TICK_TIME,
};

/* Mixer control types */                                       //混音控制类型
enum mixer_ctl_type {
    MIXER_CTL_TYPE_BOOL,
    MIXER_CTL_TYPE_INT,
    MIXER_CTL_TYPE_ENUM,
    MIXER_CTL_TYPE_BYTE,
    MIXER_CTL_TYPE_IEC958,
    MIXER_CTL_TYPE_INT64,
    MIXER_CTL_TYPE_UNKNOWN,

    MIXER_CTL_TYPE_MAX,
};

/* Open and close a stream */                                        //打开和关闭一个流
struct pcm *pcm_open(unsigned int card, unsigned int device,
                     unsigned int flags, struct pcm_config *config);
int pcm_close(struct pcm *pcm);
int pcm_is_ready(struct pcm *pcm);

/* Obtain the parameters for a PCM */                                       //获取PCM
struct pcm_params *pcm_params_get(unsigned int card, unsigned int device,
                                  unsigned int flags);
void pcm_params_free(struct pcm_params *pcm_params);                    //释放pcm

struct pcm_mask *pcm_params_get_mask(struct pcm_params *pcm_params,   //获得PCM掩码
                                     enum pcm_param param);
unsigned int pcm_params_get_min(struct pcm_params *pcm_params,              //获得参数最小值
                                enum pcm_param param);
void pcm_params_set_min(struct pcm_params *pcm_params,                     //设置参数最小值
                                enum pcm_param param, unsigned int val);
unsigned int pcm_params_get_max(struct pcm_params *pcm_params,             //获取参数最大值
                                enum pcm_param param);
void pcm_params_set_max(struct pcm_params *pcm_params,                   //设置参数最大值
                                enum pcm_param param, unsigned int val);


int pcm_params_to_string(struct pcm_params *params, char *string, unsigned int size);    //将pcm参数转换成字符串

/* Returns 1 if the pcm_format is present (format bit set) in
 * the pcm_params structure; 0 otherwise, or upon unrecognized format.
 */
int pcm_params_format_test(struct pcm_params *params, enum pcm_format format);       //pcm参数格式测试

/* Set and get config */
int pcm_get_config(struct pcm *pcm, struct pcm_config *config);          //获得配置
int pcm_set_config(struct pcm *pcm, struct pcm_config *config);               //设置配置

/* Returns a human readable reason for the last error */
const char *pcm_get_error(struct pcm *pcm);                                  //检查错误

/* Returns the sample size in bits for a PCM format.
 * As with ALSA formats, this is the storage size for the format, whereas the
 * format represents the number of significant bits. For example,
 * PCM_FORMAT_S24_LE uses 32 bits of storage.
 */
unsigned int pcm_format_to_bits(enum pcm_format format);                    //以位为单位返回pcm的格式

/* Returns the buffer size (int frames) that should be used for pcm_write. */
unsigned int pcm_get_buffer_size(struct pcm *pcm);                                //获得pcm缓冲区大小
unsigned int pcm_frames_to_bytes(struct pcm *pcm, unsigned int frames);    //PCM框架输出到字节
unsigned int pcm_bytes_to_frames(struct pcm *pcm, unsigned int bytes);         //PCM字节输出到框架

/* Returns the pcm latency in ms */
unsigned int pcm_get_latency(struct pcm *pcm);                 //以秒为单位获得PCM的时间时延


int pcm_get_htimestamp(struct pcm *pcm, unsigned int *avail,
                       struct timespec *tstamp);

/* Returns the subdevice on which the pcm has been opened */                //返回打开pcm的子设备
unsigned int pcm_get_subdevice(struct pcm *pcm);       

/* Write data to the fifo.
 * Will start playback on the first write or on a write that
 * occurs after a fifo underrun.
 */
int pcm_write(struct pcm *pcm, const void *data, unsigned int count);      //将PCM写入到队列中
int pcm_read(struct pcm *pcm, void *data, unsigned int count);             //从队列中取得PCM数据

/*
 * mmap() support.                                                             //内存映射的支持
 */
int pcm_mmap_write(struct pcm *pcm, const void *data, unsigned int count);    //pcm内存映射写操作
int pcm_mmap_read(struct pcm *pcm, void *data, unsigned int count);                 //pcm内存映射读操作
int pcm_mmap_begin(struct pcm *pcm, void **areas, unsigned int *offset,        //开始pcm内存映射
                   unsigned int *frames);
int pcm_mmap_commit(struct pcm *pcm, unsigned int offset, unsigned int frames);    //内存映射提交
int pcm_mmap_avail(struct pcm *pcm);                                                //利用内存映射

/* Returns current read/write position in the mmap buffer with associated time stamp.  
 */                                                                        //返回mmap缓冲区中当前读/写的位置和相关的时间戳。
int pcm_mmap_get_hw_ptr(struct pcm* pcm, unsigned int *hw_ptr, struct timespec *tstamp);

/* Prepare the PCM substream to be triggerable */                         //准备pcm子流为可触发
int pcm_prepare(struct pcm *pcm);
/* Start and stop a PCM channel that doesn't transfer data */              //启动和停止一个PCM通道不传输数据
int pcm_start(struct pcm *pcm);
int pcm_stop(struct pcm *pcm);

/* ioctl function for PCM driver */                                      //pcm驱动的ioctl函数
int pcm_ioctl(struct pcm *pcm, int request, ...);

/* Interrupt driven API */                                   
int pcm_wait(struct pcm *pcm, int timeout);                               //中断驱动API接口
int pcm_get_poll_fd(struct pcm *pcm);


int pcm_set_avail_min(struct pcm *pcm, int avail_min);

/*
 * MIXER API                     //-------------------------------混音的API------------------
 */

struct mixer;
struct mixer_ctl;
//-----------------------------------------------------------------------------------------------
/* Open and close a mixer */                                //打开和关闭一个混音器
struct mixer *mixer_open(unsigned int card);
void mixer_close(struct mixer *mixer);
//------------------------------------------------------------------------------------------
/* Get info about a mixer */
const char *mixer_get_name(struct mixer *mixer);     //获得混音器的信息
//---------------------------------------------------------------------------------------------------
/* Obtain mixer controls */                                     //获得混音控制器
unsigned int mixer_get_num_ctls(struct mixer *mixer);
struct mixer_ctl *mixer_get_ctl(struct mixer *mixer, unsigned int id);
struct mixer_ctl *mixer_get_ctl_by_name(struct mixer *mixer, const char *name);
//-----------------------------------------------------------------------------------------------
/* Get info about mixer controls */                            //得到混音控制器的信息
const char *mixer_ctl_get_name(struct mixer_ctl *ctl);
enum mixer_ctl_type mixer_ctl_get_type(struct mixer_ctl *ctl);
const char *mixer_ctl_get_type_string(struct mixer_ctl *ctl);
unsigned int mixer_ctl_get_num_values(struct mixer_ctl *ctl);
unsigned int mixer_ctl_get_num_enums(struct mixer_ctl *ctl);
const char *mixer_ctl_get_enum_string(struct mixer_ctl *ctl,
                                      unsigned int enum_id);

/* Some sound cards update their controls due to external events,
 * such as HDMI EDID byte data changing when an HDMI cable is
 * connected. This API allows the count of elements to be updated.
 */
void mixer_ctl_update(struct mixer_ctl *ctl);

/* Set and get mixer controls */
int mixer_ctl_get_percent(struct mixer_ctl *ctl, unsigned int id);
int mixer_ctl_set_percent(struct mixer_ctl *ctl, unsigned int id, int percent);

int mixer_ctl_get_value(struct mixer_ctl *ctl, unsigned int id);
int mixer_ctl_is_access_tlv_rw(struct mixer_ctl *ctl);
int mixer_ctl_get_array(struct mixer_ctl *ctl, void *array, size_t count);
int mixer_ctl_set_value(struct mixer_ctl *ctl, unsigned int id, int value);
int mixer_ctl_set_array(struct mixer_ctl *ctl, const void *array, size_t count);
int mixer_ctl_set_enum_by_string(struct mixer_ctl *ctl, const char *string);
//----------------------------------------------------------------------------------------------------------
/* Determine range of integer mixer controls */                    //确定混音器的控制range（范围）
int mixer_ctl_get_range_min(struct mixer_ctl *ctl);     //获得混音器控制的最小范围
int mixer_ctl_get_range_max(struct mixer_ctl *ctl);    //获得混音器控制的最大范围

int mixer_subscribe_events(struct mixer *mixer, int subscribe);  //混音订阅事件
int mixer_wait_event(struct mixer *mixer, int timeout);                  // 混音等待事件
int mixer_consume_event(struct mixer *mixer);                           //混音消费事件

#if defined(__cplusplus)
}  /* extern "C" */
#endif

#endif

**

mixer_open函数代码解析

**

//mixer_open函数的主要作用
/*
1-根据声卡号来拼凑 kcontrol 节点的字符串名字，并打开节点
2-通过 ioctrl 获取 所有 支持的Kcontrol 的字符串列表
3-分配 Mixer 的内存
4-获得 Mixer 结构体中的 card_info 信息
5-取出kontrol的id 存入 ei 中
6-获取到所有的 kcontrol list*/
struct mixer *mixer_open(unsigned int card)                                  //打开混音器
{
    struct snd_ctl_elem_list elist;
    struct snd_ctl_elem_id *eid = NULL;
    struct mixer *mixer = NULL;
    unsigned int n;
    int fd;
    char fn[256];

        //1.根据声卡号来拼凑kcontrol节点的字符串名字，并打开节点
    snprintf(fn, sizeof(fn), "/dev/snd/controlC%u", card);
    fd = open(fn, O_RDWR);                                                  //通过open打开声卡
    if (fd < 0)                     //如果小于0代表打开声卡失败
        return 0;

    //2.通过ioctrl获取所有支持的kcontrol的数量
    memset(&elist, 0, sizeof(elist));
    if (ioctl(fd, SNDRV_CTL_IOCTL_ELEM_LIST, &elist) < 0)       //ioctl驱动设备打开函数<0的话就代表打开失败
        goto fail;

    //3.分配mixer的内存，用于保存kernel kontrol 信息的结构体
    mixer = calloc(1, sizeof(*mixer));      //分配所需的内存空间，并且将内存设为0，返回的是一个指向该内存的指针
    if (!mixer)
        goto fail;
        //4.获得mixer结构体中的card_info信息
    mixer->ctl = calloc(elist.count, sizeof(struct mixer_ctl));                
    mixer->elem_info = calloc(elist.count, sizeof(struct snd_ctl_elem_info));
    if (!mixer->ctl || !mixer->elem_info)
        goto fail;

    if (ioctl(fd, SNDRV_CTL_IOCTL_CARD_INFO, &mixer->card_info) < 0)
        goto fail;

        //临时存储空间分配空间
    eid = calloc(elist.count, sizeof(struct snd_ctl_elem_id));
    if (!eid)
        goto fail;

    mixer->count = elist.count;
    mixer->fd = fd;
    elist.space = mixer->count;
    elist.pids = eid;
        //获取到所有的kcontrol list
    if (ioctl(fd, SNDRV_CTL_IOCTL_ELEM_LIST, &elist) < 0)
        goto fail;

    for (n = 0; n < mixer->count; n++) {
        struct mixer_ctl *ctl = mixer->ctl + n;

        ctl->mixer = mixer;
        ctl->info = mixer->elem_info + n;
        ctl->info->id.numid = eid[n].numid;
        strncpy((char *)ctl->info->id.name, (char *)eid[n].name,
                SNDRV_CTL_ELEM_ID_NAME_MAXLEN);
        ctl->info->id.name[SNDRV_CTL_ELEM_ID_NAME_MAXLEN - 1] = 0;
    }

    free(eid);
    return mixer;

fail:
    /* TODO: verify frees in failure case */
    if (eid)
        free(eid);
    if (mixer)
        mixer_close(mixer);
    else if (fd >= 0)
        close(fd);
    return 0;
}

**

tinycap.c代码解析

**

#include 
#include 
#include 
#include 
#include 
#include 
#include 

#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;
        //默认使用第一个是录音设备，双声道，44.1kHz踩样率，16bit
        //period为1024帧，count一共四个
    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;

        //传入参数检查
    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;
    }

        //根据传进来的录音文件参数，创建文件保存录音流
    file = fopen(argv[1], "wb");
    if (!file) {
        fprintf(stderr, "Unable to create file '%s'\n", argv[1]);
        return 1;
    }

        //解析命令行产生
    /* 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++;
    }

        //wav格式头文件结构填充
    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;


        //默认的是16bits
    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;


        //为头文件留足够的空间
    /* leave enough room for header */
    fseek(file, sizeof(struct wav_header), SEEK_SET);   //通过fseek进行文件大小偏移



        //安装信号捕捉程序并开始捕捉
    /* install signal handler and begin capturing */
    signal(SIGINT, sigint_handler);
    signal(SIGHUP, sigint_handler);
    signal(SIGTERM, sigint_handler);


        //开始录音操作
    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);

        //开始写文件
    fwrite(&header, sizeof(struct wav_header), 1, file);

    fclose(file);

    return 0;
}
//开始录制函数
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)
{

    //定义配置结构体
    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;



        //给config结构体初始化，并且根据参数赋值
    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;


        //打开pcm设备
    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;
    }

        //PCM帧变为字节，记录的时候是帧，要把帧变为字节数赋给size
    size = pcm_frames_to_bytes(pcm, pcm_get_buffer_size(pcm));


        //通过size开辟一片buffer大小
    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;
        }
    }

        //返回总共读取的帧数
    frames = pcm_bytes_to_frames(pcm, bytes_read);
    free(buffer);
    pcm_close(pcm);
    return frames;
}

**

tinymix.c源码解析

**

main()函数
首先来看函数执行流程
1. 处理传入的参数。其中的getopt_long()函数用于处理传入的参数

while (1) {
    int option_index = 0;
    int option_char = 0;

    option_char = getopt_long(argc, argv, tinymix_short_options,
                              tinymix_long_options, &option_index);
    if (option_char == -1)
        break;

    switch (option_char) {
    case 'D':
        card = atoi(optarg);
        break;
    case 'a':
        g_all_values = 1;
        break;
    case 't':
        g_tabs_only = 1;
        break;
    case 'v':
        g_value_only = 1;
        break;
    case 'h':
        usage();
        return 0;
    default:
        usage();
        return EINVAL;
    }
}

2. 打开对应声卡的混音器

mixer = mixer_open(card);

3. 参数为D a t v h以设置好的方式打印出所有控件类似1；主要是打印控件信息；对相关控件进行赋值

if (argc == optind) {
    printf("Mixer name: '%s'\n", mixer_get_name(mixer));
    tinymix_list_controls(mixer);
} else if (argc == optind + 1) {
    ret = tinymix_detail_control(mixer, argv[optind], !g_value_only, !g_value_only);
} else if (argc >= optind + 2) {
    ret = tinymix_set_value(mixer, argv[optind], &argv[optind + 1], argc - optind - 1);
}

4. 关闭设备

mixer_close(mixer);

tinymix_list_controls()函数
先来看看函数原型

static void tinymix_list_controls(struct mixer *mixer)

该函数内主要包含以下函数

/*拿到指定混音器控件数目*/
1. unsigned int mixer_get_num_ctls(struct mixer *mixer);
/*拿到指定混音器的指定控件*/
2. struct mixer_ctl *mixer_get_ctl(struct mixer *mixer, unsigned int id);
/*拿到控件名字*/
3. const char *mixer_ctl_get_name(struct mixer_ctl *ctl);
/*拿到控件类型*/
4. const char *mixer_ctl_get_type_string(struct mixer_ctl *ctl);
/*拿到空间具体数值*/
5. unsigned int mixer_ctl_get_num_values(struct mixer_ctl *ctl);
/*打印信息*/
6. static int tinymix_detail_control(struct mixer *mixer, const char *control,
                                     int prefix, int print_all)

这个函数最主要的部分是

if (g_tabs_only)
    printf("ctl\ttype\tnum\tname\tvalue");
else
    printf("ctl\ttype\tnum\t%-40s value\n", "name");
if (g_all_values)
    printf("\trange/values\n");
else
    printf("\n");
for (i = 0; i < num_ctls; i++) {
    ctl = mixer_get_ctl(mixer, i);

    name = mixer_ctl_get_name(ctl);
    type = mixer_ctl_get_type_string(ctl);
    num_values = mixer_ctl_get_num_values(ctl);
    if (g_tabs_only)
        printf("%d\t%s\t%d\t%s\t", i, type, num_values, name);
    else
        printf("%d\t%s\t%d\t%-40s ", i, type, num_values, name);
    tinymix_detail_control(mixer, name, 0, g_all_values);
}

这个函数主要作用还是打印信息
**

tinymix_set_value()函数

**
先来看看函数原型

static int tinymix_set_value(struct mixer *mixer, const char *control,
                             char **values, unsigned int num_values)

这个函数主要是用来设置参数，涉及到的函数主要是：

int mixer_ctl_set_value(struct mixer_ctl *ctl, unsigned int id, int value);

static void tinymix_set_byte_ctl(struct mixer_ctl *ctl, char **values,
                                 unsigned int num_values);

而tinymix_set_byte_ctl()函数又涉及到了

int mixer_ctl_set_array(struct mixer_ctl *ctl, const void *array, size_t count);

同样也是用于设置参数
**

tinyplay.c源码解析

**

#include 
#include 
#include 
#include 
#include 
#include 
#include 

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

struct riff_wave_header {
    uint32_t riff_id;
    uint32_t riff_sz;
    uint32_t wave_id;
};

struct chunk_header {
    uint32_t id;
    uint32_t sz;
};

struct chunk_fmt {
    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;
};

static int close = 0;

void play_sample(FILE *file, unsigned int card, unsigned int device, unsigned int channels,
                 unsigned int rate, unsigned int bits, unsigned int period_size,
                 unsigned int period_count, uint32_t data_sz);

void stream_close(int sig)
{
    /* allow the stream to be closed gracefully */
    signal(sig, SIG_IGN);
    close = 1;
}

int main(int argc, char **argv)
{
    FILE *file;
    struct riff_wave_header riff_wave_header;
    struct chunk_header chunk_header;
    struct chunk_fmt chunk_fmt;
    unsigned int device = 0;
    unsigned int card = 0;
    unsigned int period_size = 1024;
    unsigned int period_count = 4;
    char *filename;
    int more_chunks = 1;

        //确定传入参数目，没指定具体wav文件就打印帮助信息。
    if (argc < 2) {
        fprintf(stderr, "Usage: %s file.wav [-D card] [-d device] [-p period_size]"
                " [-n n_periods] \n", argv[0]);
        return 1;
    }

    filename = argv[1];

        //打开wav文件进行操作
    file = fopen(filename, "rb");
    if (!file) {
        fprintf(stderr, "Unable to open file '%s'\n", filename);
        return 1;
    }

        //读文件开头的信息，确认是wav或riff文件
    fread(&riff_wave_header, sizeof(riff_wave_header), 1, file);
    if ((riff_wave_header.riff_id != ID_RIFF) ||
        (riff_wave_header.wave_id != ID_WAVE)) {
        fprintf(stderr, "Error: '%s' is not a riff/wave file\n", filename);
        fclose(file);
        return 1;
    }
        /*进入循环，反复进行一下操作：
        读取一个块的头，判断接下来的信息是格式还是数据，如果是格式，那么就读取格式信息，
        并定位到格式信息末尾，如果是数据，则跳出循环，对于未知的块则定位到块头末尾*/

    do {
        fread(&chunk_header, sizeof(chunk_header), 1, file);

        switch (chunk_header.id) {
        case ID_FMT:
            fread(&chunk_fmt, sizeof(chunk_fmt), 1, file);
            /* If the format header is larger, skip the rest */
            if (chunk_header.sz > sizeof(chunk_fmt))
                fseek(file, chunk_header.sz - sizeof(chunk_fmt), SEEK_CUR);
            break;
        case ID_DATA:
            /* Stop looking for chunks */
            more_chunks = 0;
            chunk_header.sz = le32toh(chunk_header.sz);
            break;
        default:
            /* Unknown chunk, skip bytes */
            fseek(file, chunk_header.sz, SEEK_CUR);
        }
    } while (more_chunks);
        //匹配后面的参数并填入

    /* parse command line arguments */
    argv += 2;
    while (*argv) {
        if (strcmp(*argv, "-d") == 0) {
            argv++;
            if (*argv)
                device = atoi(*argv);
        }
        if (strcmp(*argv, "-p") == 0) {
            argv++;
            if (*argv)
                period_size = atoi(*argv);
        }
        if (strcmp(*argv, "-n") == 0) {
            argv++;
            if (*argv)
                period_count = atoi(*argv);
        }
        if (strcmp(*argv, "-D") == 0) {
            argv++;
            if (*argv)
                card = atoi(*argv);
        }
        if (*argv)
            argv++;
    }

        //开始播放音频文件

    play_sample(file, card, device, chunk_fmt.num_channels, chunk_fmt.sample_rate,
                chunk_fmt.bits_per_sample, period_size, period_count, chunk_header.sz);

    fclose(file);

    return 0;
}

int check_param(struct pcm_params *params, unsigned int param, unsigned int value,
                 char *param_name, char *param_unit)
{
    unsigned int min;
    unsigned int max;
    int is_within_bounds = 1;

    min = pcm_params_get_min(params, param);
    if (value < min) {
        fprintf(stderr, "%s is %u%s, device only supports >= %u%s\n", param_name, value,
                param_unit, min, param_unit);
        is_within_bounds = 0;
    }

    max = pcm_params_get_max(params, param);
    if (value > max) {
        fprintf(stderr, "%s is %u%s, device only supports <= %u%s\n", param_name, value,
                param_unit, max, param_unit);
        is_within_bounds = 0;
    }

    return is_within_bounds;
}

int sample_is_playable(unsigned int card, unsigned int device, unsigned int channels,
                        unsigned int rate, unsigned int bits, unsigned int period_size,
                        unsigned int period_count)
{
    struct pcm_params *params;
    int can_play;

    params = pcm_params_get(card, device, PCM_OUT);
    if (params == NULL) {
        fprintf(stderr, "Unable to open PCM device %u.\n", device);
        return 0;
    }

    can_play = check_param(params, PCM_PARAM_RATE, rate, "Sample rate", "Hz");
    can_play &= check_param(params, PCM_PARAM_CHANNELS, channels, "Sample", " channels");
    can_play &= check_param(params, PCM_PARAM_SAMPLE_BITS, bits, "Bitrate", " bits");
    can_play &= check_param(params, PCM_PARAM_PERIOD_SIZE, period_size, "Period size", " frames");
    can_play &= check_param(params, PCM_PARAM_PERIODS, period_count, "Period count", " periods");

    pcm_params_free(params);

    return can_play;
}

void play_sample(FILE *file, unsigned int card, unsigned int device, unsigned int channels,
                 unsigned int rate, unsigned int bits, unsigned int period_size,
                 unsigned int period_count, uint32_t data_sz)
{
    struct pcm_config config;
    struct pcm *pcm;
    char *buffer;
    unsigned int size, read_sz;
    int num_read;

        //填入pcm流的相关信息

    memset(&config, 0, sizeof(config));
    config.channels = channels;
    config.rate = rate;
    config.period_size = period_size;
    config.period_count = period_count;
    if (bits == 32)
        config.format = PCM_FORMAT_S32_LE;
    else if (bits == 24)
        config.format = PCM_FORMAT_S24_3LE;
    else if (bits == 16)
        config.format = PCM_FORMAT_S16_LE;
    config.start_threshold = 0;
    config.stop_threshold = 0;
    config.silence_threshold = 0;

        //通过验证相关信息的合法性判断能否播放

    if (!sample_is_playable(card, device, channels, rate, bits, period_size, period_count)) {
        return;
    }
        //打开pcm设备

    pcm = pcm_open(card, device, PCM_OUT, &config);
    if (!pcm || !pcm_is_ready(pcm)) {
        fprintf(stderr, "Unable to open PCM device %u (%s)\n",
                device, pcm_get_error(pcm));
        return;
    }
        //开个buffer保存数据

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

    printf("Playing sample: %u ch, %u hz, %u bit %u bytes\n", channels, rate, bits, data_sz);

    /* catch ctrl-c to shutdown cleanly */
    signal(SIGINT, stream_close);
        //循环写入数据到pcm设备

    do {
        read_sz = size < data_sz ? size : data_sz;
        num_read = fread(buffer, 1, read_sz, file);
        if (num_read > 0) {
            if (pcm_write(pcm, buffer, num_read)) {
                fprintf(stderr, "Error playing sample\n");
                break;
            }
            data_sz -= num_read;
        }
    } while (!close && num_read > 0 && data_sz > 0);

    free(buffer);
    pcm_close(pcm);
}