alsa-lib 提供的 API 接口和 Android 等系统中使用的 tinyalsa 提供的 API 接口差异巨大，alsa-lib 的复杂度及支持的功能特性，与 tinyalsa 的有着数量级上的差异。插件机制及 dmix 和 dsnoop 等内置 PCM 插件是 alsa-lib 支持的众多高级特性的一部分。ALSA 音频内核设备驱动，在特定时刻只能运行在特定模式和一组参数下，通常只能支持由单个应用程序打开。dmix 和 dsnoop PCM 插件则提供了通过 alsa-lib API，多个应用程序共享音频硬件设备的能力，dmix PCM 插件将多个音频流混音之后送进硬件设备，dsnoop 插件将一个音频采集流分割为多个分别提供给不同的应用程序，它的工作方式与 dmix 相反，支持从多个客户端并发地读取共享采集缓冲区。这里简单分析 alsa-lib 的 PCM 插件 dsnoop 的实现。
alsa-lib 中的 PCM 设备
alsa-lib 支持的 PCM 设备类型众多，alsa-lib/include/pcm.h 中 PCM 设备类型的声明如下：
/** PCM handle */
typedef struct _snd_pcm snd_pcm_t;/** PCM type */
enum _snd_pcm_type {/** Kernel level PCM */SND_PCM_TYPE_HW = 0,/** Hooked PCM */SND_PCM_TYPE_HOOKS,/** One or more linked PCM with exclusive access to selectedchannels */SND_PCM_TYPE_MULTI,/** File writing plugin */SND_PCM_TYPE_FILE,/** Null endpoint PCM */SND_PCM_TYPE_NULL,/** Shared memory client PCM */SND_PCM_TYPE_SHM,/** INET client PCM (not yet implemented) */SND_PCM_TYPE_INET,/** Copying plugin */SND_PCM_TYPE_COPY,/** Linear format conversion PCM */SND_PCM_TYPE_LINEAR,/** A-Law format conversion PCM */SND_PCM_TYPE_ALAW,/** Mu-Law format conversion PCM */SND_PCM_TYPE_MULAW,/** IMA-ADPCM format conversion PCM */SND_PCM_TYPE_ADPCM,/** Rate conversion PCM */SND_PCM_TYPE_RATE,/** Attenuated static route PCM */SND_PCM_TYPE_ROUTE,/** Format adjusted PCM */SND_PCM_TYPE_PLUG,/** Sharing PCM */SND_PCM_TYPE_SHARE,/** Meter plugin */SND_PCM_TYPE_METER,/** Mixing PCM */SND_PCM_TYPE_MIX,/** Attenuated dynamic route PCM (not yet implemented) */SND_PCM_TYPE_DROUTE,/** Loopback server plugin (not yet implemented) */SND_PCM_TYPE_LBSERVER,/** Linear Integer <-> Linear Float format conversion PCM */SND_PCM_TYPE_LINEAR_FLOAT,/** LADSPA integration plugin */SND_PCM_TYPE_LADSPA,/** Direct Mixing plugin */SND_PCM_TYPE_DMIX,/** Jack Audio Connection Kit plugin */SND_PCM_TYPE_JACK,/** Direct Snooping plugin */SND_PCM_TYPE_DSNOOP,/** Direct Sharing plugin */SND_PCM_TYPE_DSHARE,/** IEC958 subframe plugin */SND_PCM_TYPE_IEC958,/** Soft volume plugin */SND_PCM_TYPE_SOFTVOL,/** External I/O plugin */SND_PCM_TYPE_IOPLUG,/** External filter plugin */SND_PCM_TYPE_EXTPLUG,/** Mmap-emulation plugin */SND_PCM_TYPE_MMAP_EMUL,SND_PCM_TYPE_LAST = SND_PCM_TYPE_MMAP_EMUL
};
这些 PCM 设备类型中，只有 SND_PCM_TYPE_HW 类型会与内核及音频硬件设备交互，会访问 PCM 设备文件，即 PCM 硬件设备。其它包括 SND_PCM_TYPE_DMIX 和 SND_PCM_TYPE_DSNOOP 等在内的 PCM 设备类型都是虚拟设备，它们为应用程序提供 PCM 设备操作接口，但并不直接访问 PCM 设备文件。
alsa-lib 用 snd_pcm_t 对象描述 PCM 设备，不同类型的 PCM 设备对应于不同的 snd_pcm_t 对象实现。在 alsa-lib 中，PCM 设备即 snd_pcm_t 对象，snd_pcm_t 对象即 PCM 设备。snd_pcm_t 类型的定义 (位于 alsa-lib/src/pcm/pcm_local.h) 如下：
typedef struct _snd_pcm_rbptr {snd_pcm_t *master;volatile snd_pcm_uframes_t *ptr;int fd;off_t offset;int link_dst_count;snd_pcm_t **link_dst;void *private_data;void (*changed)(snd_pcm_t *pcm, snd_pcm_t *src);
} snd_pcm_rbptr_t;typedef struct _snd_pcm_channel_info {unsigned int channel;void *addr; /* base address of channel samples */unsigned int first; /* offset to first sample in bits */unsigned int step; /* samples distance in bits */enum { SND_PCM_AREA_SHM, SND_PCM_AREA_MMAP, SND_PCM_AREA_LOCAL } type;union {struct {struct snd_shm_area *area;int shmid;} shm;struct {int fd;off_t offset;} mmap;} u;char reserved[64];
} snd_pcm_channel_info_t;typedef struct {int (*close)(snd_pcm_t *pcm);int (*nonblock)(snd_pcm_t *pcm, int nonblock); /* always locked */int (*async)(snd_pcm_t *pcm, int sig, pid_t pid);int (*info)(snd_pcm_t *pcm, snd_pcm_info_t *info);int (*hw_refine)(snd_pcm_t *pcm, snd_pcm_hw_params_t *params);int (*hw_params)(snd_pcm_t *pcm, snd_pcm_hw_params_t *params);int (*hw_free)(snd_pcm_t *pcm);int (*sw_params)(snd_pcm_t *pcm, snd_pcm_sw_params_t *params); /* always locked */int (*channel_info)(snd_pcm_t *pcm, snd_pcm_channel_info_t *info);void (*dump)(snd_pcm_t *pcm, snd_output_t *out);int (*mmap)(snd_pcm_t *pcm);int (*munmap)(snd_pcm_t *pcm);snd_pcm_chmap_query_t **(*query_chmaps)(snd_pcm_t *pcm);snd_pcm_chmap_t *(*get_chmap)(snd_pcm_t *pcm);int (*set_chmap)(snd_pcm_t *pcm, const snd_pcm_chmap_t *map);
} snd_pcm_ops_t;typedef struct {int (*status)(snd_pcm_t *pcm, snd_pcm_status_t *status); /* locked */int (*prepare)(snd_pcm_t *pcm); /* locked */int (*reset)(snd_pcm_t *pcm); /* locked */int (*start)(snd_pcm_t *pcm); /* locked */int (*drop)(snd_pcm_t *pcm); /* locked */int (*drain)(snd_pcm_t *pcm); /* need own locking */int (*pause)(snd_pcm_t *pcm, int enable); /* locked */snd_pcm_state_t (*state)(snd_pcm_t *pcm); /* locked */int (*hwsync)(snd_pcm_t *pcm); /* locked */int (*delay)(snd_pcm_t *pcm, snd_pcm_sframes_t *delayp); /* locked */int (*resume)(snd_pcm_t *pcm); /* need own locking */int (*link)(snd_pcm_t *pcm1, snd_pcm_t *pcm2);int (*link_slaves)(snd_pcm_t *pcm, snd_pcm_t *master);int (*unlink)(snd_pcm_t *pcm);snd_pcm_sframes_t (*rewindable)(snd_pcm_t *pcm); /* locked */snd_pcm_sframes_t (*rewind)(snd_pcm_t *pcm, snd_pcm_uframes_t frames); /* locked */snd_pcm_sframes_t (*forwardable)(snd_pcm_t *pcm); /* locked */snd_pcm_sframes_t (*forward)(snd_pcm_t *pcm, snd_pcm_uframes_t frames); /* locked */snd_pcm_sframes_t (*writei)(snd_pcm_t *pcm, const void *buffer, snd_pcm_uframes_t size); /* need own locking */snd_pcm_sframes_t (*writen)(snd_pcm_t *pcm, void **bufs, snd_pcm_uframes_t size); /* need own locking */snd_pcm_sframes_t (*readi)(snd_pcm_t *pcm, void *buffer, snd_pcm_uframes_t size); /* need own locking */snd_pcm_sframes_t (*readn)(snd_pcm_t *pcm, void **bufs, snd_pcm_uframes_t size); /* need own locking */snd_pcm_sframes_t (*avail_update)(snd_pcm_t *pcm); /* locked */snd_pcm_sframes_t (*mmap_commit)(snd_pcm_t *pcm, snd_pcm_uframes_t offset, snd_pcm_uframes_t size); /* locked */int (*htimestamp)(snd_pcm_t *pcm, snd_pcm_uframes_t *avail, snd_htimestamp_t *tstamp); /* locked */int (*poll_descriptors_count)(snd_pcm_t *pcm); /* locked */int (*poll_descriptors)(snd_pcm_t *pcm, struct pollfd *pfds, unsigned int space); /* locked */int (*poll_revents)(snd_pcm_t *pcm, struct pollfd *pfds, unsigned int nfds, unsigned short *revents); /* locked */int (*may_wait_for_avail_min)(snd_pcm_t *pcm, snd_pcm_uframes_t avail);int (*mmap_begin)(snd_pcm_t *pcm, const snd_pcm_channel_area_t **areas, snd_pcm_uframes_t *offset, snd_pcm_uframes_t *frames); /* locked */
} snd_pcm_fast_ops_t;struct _snd_pcm {void *open_func;char *name;snd_pcm_type_t type;snd_pcm_stream_t stream;int mode;long minperiodtime; /* in us */int poll_fd_count;int poll_fd;unsigned short poll_events;int setup: 1,compat: 1;snd_pcm_access_t access; /* access mode */snd_pcm_format_t format; /* SND_PCM_FORMAT_* */snd_pcm_subformat_t subformat; /* subformat */unsigned int channels; /* channels */unsigned int rate; /* rate in Hz */snd_pcm_uframes_t period_size;unsigned int period_time; /* period duration */snd_interval_t periods;snd_pcm_tstamp_t tstamp_mode; /* timestamp mode */snd_pcm_tstamp_type_t tstamp_type; /* timestamp type */unsigned int period_step;snd_pcm_uframes_t avail_min; /* min avail frames for wakeup */int period_event;snd_pcm_uframes_t start_threshold;snd_pcm_uframes_t stop_threshold;snd_pcm_uframes_t silence_threshold; /* Silence filling happens whennoise is nearest than this */snd_pcm_uframes_t silence_size; /* Silence filling size */snd_pcm_uframes_t boundary; /* pointers wrap point */unsigned int info; /* Info for returned setup */unsigned int msbits; /* used most significant bits */unsigned int rate_num; /* rate numerator */unsigned int rate_den; /* rate denominator */unsigned int hw_flags; /* actual hardware flags */snd_pcm_uframes_t fifo_size; /* chip FIFO size in frames */snd_pcm_uframes_t buffer_size;snd_interval_t buffer_time;unsigned int sample_bits;unsigned int frame_bits;snd_pcm_rbptr_t appl;snd_pcm_rbptr_t hw;snd_pcm_uframes_t min_align;unsigned int mmap_rw: 1; /* use always mmapped buffer */unsigned int mmap_shadow: 1; /* don't call actual mmap,* use the mmaped buffer of the slave*/unsigned int donot_close: 1; /* don't close this PCM */unsigned int own_state_check:1; /* plugin has own PCM state check */snd_pcm_channel_info_t *mmap_channels;snd_pcm_channel_area_t *running_areas;snd_pcm_channel_area_t *stopped_areas;const snd_pcm_ops_t *ops;const snd_pcm_fast_ops_t *fast_ops;snd_pcm_t *op_arg;snd_pcm_t *fast_op_arg;void *private_data;struct list_head async_handlers;
#ifdef THREAD_SAFE_APIint need_lock; /* true = this PCM (plugin) is thread-unsafe,* thus it needs a lock.*/int lock_enabled; /* thread-safety lock is enabled on the system;* it's set depending on $LIBASOUND_THREAD_SAFE.*/pthread_mutex_t lock;
#endif
};
snd_pcm_t 对象的 const snd_pcm_ops_t *ops 和 const snd_pcm_fast_ops_t *fast_ops 成员分别指向由众多函数指针组成的结构，它们决定着特定类型 PCM 设备各个操作的行为。
打开 dsnoop PCM 设备
alsa-lib 的 snd_pcm_open() 接口用于打开 PCM 设备，这个接口的声明 (位于 alsa-lib/include/pcm.h) 如下：
int snd_pcm_open(snd_pcm_t **pcm, const char *name, snd_pcm_stream_t stream, int mode);
snd_pcm_open() 接口各个参数的含义如下：
- pcmp：返回的
snd_pcm_t对象，PCM 句柄。 - name：PCM 句柄的 ASCII 标识符，用于指定要打开的 PCM 设备，如 dsnoop 或 dsnoop:1。
- stream：流的类型，播放流和录制流分别为
SND_PCM_STREAM_PLAYBACK和SND_PCM_STREAM_CAPTURE。 - mode：打开模式，可选值为
SND_PCM_NONBLOCK和SND_PCM_ASYNC。
arecord 命令可以指定通过 dsnoop 虚拟设备采集音频数据，如：
$ arecord -f S16_LE -r 48000 -c 2 -D dsnoop foobar.wav
arecord 命令支持设置 dsnoop 虚拟设备绑定到非默认的声卡 0，如绑定到硬件声卡 1 采集音频数据：
$ arecord -f S16_LE -r 48000 -c 2 -D dsnoop:1 foobar.wav
arecord 命令的 -D 参数，将在打开 PCM 设备时，作为 snd_pcm_open() 接口的 name 参数，指定要打开的 PCM 设备。
snd_pcm_open() 函数定义 (位于 alsa-lib/src/pcm/pcm.c) 如下：
/*** \brief Opens a PCM* \param pcmp Returned PCM handle* \param name ASCII identifier of the PCM handle* \param stream Wanted stream* \param mode Open mode (see #SND_PCM_NONBLOCK, #SND_PCM_ASYNC)* \return 0 on success otherwise a negative error code*/
int snd_pcm_open(snd_pcm_t **pcmp, const char *name, snd_pcm_stream_t stream, int mode)
{snd_config_t *top;int err;assert(pcmp && name);if (_snd_is_ucm_device(name)) {name = uc_mgr_alibcfg_by_device(&top, name);if (name == NULL)return -ENODEV;} else {err = snd_config_update_ref(&top);if (err < 0)return err;}err = snd_pcm_open_noupdate(pcmp, top, name, stream, mode, 0);snd_config_unref(top);return err;
}
UCM 设备是名称以 “_ucm” 开头的设备，这里忽略 UCM 设备的情况。snd_pcm_open() 函数的执行过程如下：
- 调用
snd_config_update_ref()函数更新 ALSA 顶层配置 snd_config 并获取它的引用。 - 调用
snd_pcm_open_noupdate()函数根据 ALSA 顶层配置打开 PCM 设备。 - 释放 ALSA 顶层配置 snd_config 的引用。
更新 ALSA 顶层配置的 snd_config_update_ref() 函数定义 (位于 alsa-lib/src/conf.c) 如下：
struct _snd_config {char *id;snd_config_type_t type;int refcount; /* default = 0 */union {long integer;long long integer64;char *string;double real;const void *ptr;struct {struct list_head fields;bool join;} compound;} u;struct list_head list;snd_config_t *parent;int hop;
};. . . . . .
const char *snd_config_topdir(void)
{static char *topdir;if (!topdir) {topdir = getenv("ALSA_CONFIG_DIR");if (!topdir || *topdir != '/' || strlen(topdir) >= PATH_MAX)topdir = ALSA_CONFIG_DIR;}return topdir;
}. . . . . .snd_config_t *snd_config = NULL;#ifndef DOC_HIDDEN
struct finfo {char *name;dev_t dev;ino64_t ino;time_t mtime;
};struct _snd_config_update {unsigned int count;struct finfo *finfo;
};
#endif /* DOC_HIDDEN */static snd_config_update_t *snd_config_global_update = NULL;. . . . . .
int snd_config_update_r(snd_config_t **_top, snd_config_update_t **_update, const char *cfgs)
{int err;const char *configs, *c;unsigned int k;size_t l;snd_config_update_t *local;snd_config_update_t *update;snd_config_t *top;assert(_top && _update);top = *_top;update = *_update;configs = cfgs;if (!configs) {configs = getenv(ALSA_CONFIG_PATH_VAR);if (!configs || !*configs) {const char *topdir = snd_config_topdir();char *s = alloca(strlen(topdir) +strlen("alsa.conf") + 2);sprintf(s, "%s/alsa.conf", topdir);configs = s;}}for (k = 0, c = configs; (l = strcspn(c, ": ")) > 0; ) {c += l;k++;if (!*c)break;c++;}if (k == 0) {local = NULL;goto _reread;}local = (snd_config_update_t *)calloc(1, sizeof(snd_config_update_t));if (!local)return -ENOMEM;local->count = k;local->finfo = calloc(local->count, sizeof(struct finfo));if (!local->finfo) {free(local);return -ENOMEM;}for (k = 0, c = configs; (l = strcspn(c, ": ")) > 0; ) {char name[l + 1];memcpy(name, c, l);name[l] = 0;err = snd_user_file(name, &local->finfo[k].name);if (err < 0)goto _end;c += l;k++;if (!*c)break;c++;}for (k = 0; k < local->count; ++k) {struct stat64 st;struct finfo *lf = &local->finfo[k];if (stat64(lf->name, &st) >= 0) {lf->dev = st.st_dev;lf->ino = st.st_ino;lf->mtime = st.st_mtime;} else {SNDERR("Cannot access file %s", lf->name);free(lf->name);memmove(&local->finfo[k], &local->finfo[k+1], sizeof(struct finfo) * (local->count - k - 1));k--;local->count--;}}if (!update)goto _reread;if (local->count != update->count)goto _reread;for (k = 0; k < local->count; ++k) {struct finfo *lf = &local->finfo[k];struct finfo *uf = &update->finfo[k];if (strcmp(lf->name, uf->name) != 0 ||lf->dev != uf->dev ||lf->ino != uf->ino ||lf->mtime != uf->mtime)goto _reread;}err = 0;_end:if (err < 0) {if (top) {snd_config_delete(top);*_top = NULL;}if (update) {snd_config_update_free(update);*_update = NULL;}}if (local)snd_config_update_free(local);return err;_reread:*_top = NULL;*_update = NULL;if (update) {snd_config_update_free(update);update = NULL;}if (top) {snd_config_delete(top);top = NULL;}err = snd_config_top(&top);if (err < 0)goto _end;if (!local)goto _skip;for (k = 0; k < local->count; ++k) {snd_input_t *in;err = snd_input_stdio_open(&in, local->finfo[k].name, "r");if (err >= 0) {err = snd_config_load(top, in);snd_input_close(in);if (err < 0) {SNDERR("%s may be old or corrupted: consider to remove or fix it", local->finfo[k].name);goto _end;}} else {SNDERR("cannot access file %s", local->finfo[k].name);}}_skip:err = snd_config_hooks(top, NULL);if (err < 0) {SNDERR("hooks failed, removing configuration");goto _end;}*_top = top;*_update = local;return 1;
}. . . . . .
/*** \brief Updates #snd_config and takes its reference.* \return 0 if #snd_config was up to date, 1 if #snd_config was* updated, otherwise a negative error code.** Unlike #snd_config_update, this function increases a reference counter* so that the obtained tree won't be deleted until unreferenced by* #snd_config_unref.** This function is supposed to be thread-safe.*/
int snd_config_update_ref(snd_config_t **top)
{int err;if (top)*top = NULL;snd_config_lock();err = snd_config_update_r(&snd_config, &snd_config_global_update, NULL);if (err >= 0) {if (snd_config) {if (top) {snd_config->refcount++;*top = snd_config;}} else {err = -ENODEV;}}snd_config_unlock();return err;
}
snd_config_update_ref() 函数根据从顶层配置文件读取的内容更新配置树，它是 snd_config_update_r() 函数的线程安全封装。snd_config_update_r() 函数的 cfgs 参数接受以冒号 (‘:’) 分割的顶层配置文件名列表，在snd_pcm_open()/snd_config_update_ref() 的场景中，cfgs 参数为空，取默认顶层配置文件。
snd_config_update_r() 函数按一定的优先级在多个目录中查找默认的 ALSA 顶层配置文件：
- 环境变量 ALSA_CONFIG_PATH 指向的 ALSA 顶层配置文件路径列表。
- 顶层配置目录下的 alsa.conf 文件。顶层配置目录按一定的优先级来查找：
- 环境变量 ALSA_CONFIG_DIR 指向的目录。
- /usr/share/alsa。
通常默认的 ALSA 顶层配置文件为 /usr/share/alsa/alsa.conf。对于 ALSA 顶层配置文件的路径，snd_config_update_r() 函数支持 “~/” 的形式，它会通过 snd_user_file() 函数获得用户目录路径，并进而获得配置文件的绝对路径。
alsa-lib 用 snd_config_update_t 对象描述 ALSA 顶层配置文件集，并用 finfo 对象描述其中的一个配置文件。snd_config_update_r() 函数在获得 ALSA 顶层配置文件路径列表之后，将它们转换为 snd_config_update_t 对象的表示，这会处理文件路径中的 “~/”，并过滤掉无法访问的文件。
snd_config_update_r() 函数的 _update 参数用来传入老的配置，并用来传出新加载的配置。snd_config_update_r() 函数对比老的配置的配置文件信息和新获取的配置文件的信息，如果两者存在差异（包括文件路径、文件所在的设备号、文件的 inode 号和最近修改时间等方面），则加载新的配置，否则释放新创建的 snd_config_update_t 对象并返回。
加载新配置时，如果通过 _top 和 _update 参数传入的老的配置已经存在，它们会先被释放掉。随后，通过 snd_config_top() 函数重新为 ALSA 顶层配置分配 snd_config_t 对象，通过 snd_input_stdio_open()/snd_config_load()/snd_input_close() 这组函数逐个加载配置文件，通过 snd_config_hooks() 函数处理 hooks。最后，通过传入的 _top 和 _update 参数返回加载的配置。
ALSA 配置文件支持声明 hook，即一个函数，如 ALSA 顶层配置文件 /usr/share/alsa/alsa.conf 中声明的 hook：
@hooks [{func loadfiles ["/etc/alsa/conf.d""/etc/asound.conf""~/.asoundrc"]errors false}
]
hooks 的 func 配置指定 hook 名称，而不直接对应 alsa-lib 内部的函数名，hook_func 用于指定 hook 的函数名，如 /usr/share/alsa/alsa.conf.d/pulse.conf 配置文件：
hook_func.pulse_load_if_running {lib "libasound_module_conf_pulse.so"func "conf_pulse_hook_load_if_running"
}@hooks [{func pulse_load_if_runningfiles ["/usr/share/alsa/pulse-alsa.conf"]errors false}
]
当没有为 hook 指定 hook 函数名时，hook 函数名将为 snd_config_hook_[func]，如 snd_config_hook_load，snd_config_hook_load() 函数将在 snd_config_hooks() 函数处理 hooks 过程中被调用。
如在 snd_config_update_ref() 函数中看到的，加载的 ALSA 顶层配置被保存在全局的 snd_config 和 snd_config_global_update 对象中。
ALSA 顶层配置文件的内容，如 /usr/share/alsa/alsa.conf 文件。关于 ALSA 配置文件更详细的信息，可以参考它的 WiKi 页面 Asoundrc。通常 /usr/share/alsa/alsa.conf 配置文件是 ALSA 配置文件的主入口点，它负责包含系统上 .asoundrc-format-type 格式文件的完整列表。
回到 snd_pcm_open() 函数，它在加载了 ALSA 顶层配置之后，调用 snd_pcm_open_noupdate() 函数打开 PCM 音频设备。snd_pcm_open_noupdate() 函数定义 (位于 alsa-lib/src/pcm/pcm.c) 如下：
static const char *const build_in_pcms[] = {"adpcm", "alaw", "copy", "dmix", "file", "hooks", "hw", "ladspa", "lfloat","linear", "meter", "mulaw", "multi", "null", "empty", "plug", "rate", "route", "share","shm", "dsnoop", "dshare", "asym", "iec958", "softvol", "mmap_emul",NULL
};static int snd_pcm_open_conf(snd_pcm_t **pcmp, const char *name,snd_config_t *pcm_root, snd_config_t *pcm_
