参考：https://blog.csdn.net/haohenghai/article/details/24255451

alsa-lib如何解析asound.conf

https://www.xuebuyuan.com/1043778.html

一.打开代码流程分析

无论是在录音还是在放音，都要打开一个PCM流，具体对应的函数原型为：
int snd_pcm_open(snd_pcm_t **pcmp, const char *name, snd_pcm_stream_t stream, int mode)；

先解一下各个参数的函义：

pcmp: 是带回的pcm流
name是要找开的流即放音还是录音，放音对应AndroidPlayback_Speaker_normal，录音对应AndroidCapture。放音的字符串会因为当前使用设备和设备状况不一样而有所不同，不同之处在下划线后面那部分。录音则因为大多数板子成型后，都只有一个录音设备或者接口，因此录音串对应的就是AndroidCapture。对于linux就是放音的设备，比如hw:0,2
stream对应流的类型，是放音(0)还是录音(1).
mode是指设备打开模式，阻塞或非阻塞。

snd_pcm_open

//this function increases a reference counter so that the obtained tree won't be deleted until unreferenced by #snd_config_unref.

//这个函数增加了一个引用计数器，这样得到的树在未被引用之前不会被删除# snd_config_unref。

snd_config_update_ref(&top);

snd_pcm_open_noupdate(pcmp, top, name, stream, mode, 0);

if (snd_config_get_string(pcm_conf, &str) >= 0) //获取配置节点的字符串

snd_pcm_open_noupdate(pcmp, root, str, stream, mode, hop + 1);

else

snd_pcm_open_conf(pcmp, name, root, pcm_conf, stream, mode); //单独分析1

// 此函数将在配置树中查找对应的结点，然后将查找到的结点复制一份到pcm_conf中

snd_config_search_definition(root, "pcm", name, &pcm_conf);

//Decreases a reference counter of the given config tree, and eventually deletes the tree if all references are gone. This is the counterpart of #snd_config_unref.

//减少给定配置树的引用计数器，最终如果所有引用都已消失，则删除树。这是#snd_config_unref的对等项。

snd_config_unref(top);

单独分析1：

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_conf,

snd_pcm_stream_t stream, int mode)

//Searches for a node in a configuration tree.

* \code

* config {

* a 42 # "a"

* b { # "b"

* c "cee" # "b.c"

* d { # "b.d"

* e 2.71828 # "b.d.e"

* }

* \endcode

snd_config_search(pcm_conf, "type", &conf); //搜索config里面type的节点

snd_config_get_id(conf, &id); //获取ID

snd_config_get_string(conf, &str); //获取字符串

snd_config_search_definition(pcm_root, "pcm_type", str, &type_conf); //搜索pcm_type

snd_config_for_each(i, next, type_conf)

snd_config_t *n = snd_config_iterator_entry(i);

if (snd_config_get_id(n, &id) < 0) //看看是否有comment和open的字符

if (strcmp(id, "comment") == 0)

if (strcmp(id, "open") == 0)

if (!open_name) //如果没有指定打开的设备

buf = malloc(strlen(str) + 32); //分配空间

open_name = buf;

sprintf(buf, "_snd_pcm_%s_open", str); //这里是这个是str是前面type获得的

if (!lib) ////如果前面没有指定lib

const char *const *build_in = build_in_pcms; //赋值字符串数组build_in_pcms

while (*build_in) //如果存在

if (!strcmp(*build_in, str)) //如果与字符串数组中的字符相等就跳出

break;

build_in++;

if (*build_in == NULL)

buf1 = malloc(strlen(str) + sizeof(ALSA_PLUGIN_DIR) + 32); //分配空间

lib = buf1;

sprintf(buf1, "%s/libasound_module_pcm_%s.so", ALSA_PLUGIN_DIR, str); //赋值plugin的so的路径

//加载相关的so

open_func = snd_dlobj_cache_get(lib, open_name, SND_DLSYM_VERSION(SND_PCM_DLSYM_VERSION), 1);

//打开动态库，如果是默认的plugin就打开这个/usr/lib64/libasound.so.2，否则就打开额外添加的/usr/lib/alsa-lib/libasound_module_pcm_pmaplug.so的plugin库

dlobj = snd_dlopen(lib, RTLD_NOW);，

func = snd_dlsym(dlobj, name, version); //解析来自动态库的符号，找到函数的地址，如果有的符号没有就会报错

return dlsym(handle, name);

c->lib = lib ? strdup(lib) : NULL;

c->name = strdup(name);

list_add_tail(&c->list, &pcm_dlobj_list); //把找到的函数加入到pcm_dlobj_list这个链表

if (open_func) //如果加载成功

//打开这个plugin,可能调用的函数：_snd_pcm_asym_open， _snd_pcm_plug_open，等，还有执行SND_PCM_PLUGIN_DEFINE_FUNC,这些都是从so库里面得到的，获取到函数的地址

open_func(pcmp, name, pcm_root, pcm_conf, stream, mode);

if ((*pcmp)->open_func)

snd_dlobj_cache_put(open_func); //释放解析动态库的缓存

else

(*pcmp)->open_func = open_func; //赋值

if (err >= 0)

snd_config_search(pcm_root, "defaults.pcm.compat", &tmp); //搜索defaults.pcm.compat这个节点

if (err >= 0)

if (snd_config_get_integer(tmp, &i) >= 0)

(*pcmp)->compat = 1;

char *str = getenv("LIBASOUND_COMPAT");

if (str && *str) //如果环境变量有配置LIBASOUND_COMPAT，置1

(*pcmp)->compat = 1;

snd_config_search(pcm_root, "defaults.pcm.minperiodtime", &tmp); //获取defaults.pcm.minperiodtime

snd_config_get_integer(tmp, &(*pcmp)->minperiodtime); //最小周期时间

二.实例分析

1.asound.conf内容

pcm.7input {

type plug

slave {

pcm "hw:0,2"

format S16_LE

rate 16000

}

pcm.pma {

type pmaplug

slave {

pcm 7input

}

input_channels 8

loopback 1

local 1

}

pcm.speech_route {

type route

slave.pcm pma

slave.channels 8

ttable{

0.0 1

0.1 1

0.2 1

0.3 1

0.4 1

0.5 1

0.6 1

0.7 1

}

pcm.speech {

type asym

capture.pcm speech_route

playback.pcm default

}

2.添加打印后的log

第一步

ALSA lib pcm.c:2567:(snd_pcm_open_conf) cxw conf id =type, str = asym

ALSA lib pcm.c:2613:(snd_pcm_open_conf) open_name = _snd_pcm_asym_open //获取到type asym

ALSA lib pcm.c:2618:(snd_pcm_open_conf) open_name str = asym

............................................

ALSA lib pcm.c:2618:(snd_pcm_open_conf) open_name str = asym

ALSA lib pcm.c:2637:(snd_pcm_open_conf) open_name lib = (null), open_name = 428883088

ALSA lib pcm.c:2641:(snd_pcm_open_conf) open_name name = speech

ALSA lib pcm_asym.c:84:(_snd_pcm_asym_open) open_name _snd_pcm_asym_open //调用到_snd_pcm_asym_open 函数

第二步

ALSA lib pcm.c:2567:(snd_pcm_open_conf) cxw conf id =type, str = route

ALSA lib pcm.c:2613:(snd_pcm_open_conf) open_name = _snd_pcm_route_open //获取到type route

ALSA lib pcm.c:2618:(snd_pcm_open_conf) open_name str = route

................................

ALSA lib pcm.c:2618:(snd_pcm_open_conf) open_name str = route

ALSA lib pcm.c:2637:(snd_pcm_open_conf) open_name lib = (null), open_name = 428883248

ALSA lib pcm.c:2641:(snd_pcm_open_conf) open_name name = speech_route

ALSA lib pcm_route.c:1293:(_snd_pcm_route_open) open_name _snd_pcm_route_open //调用到_snd_pcm_route_open 函数

第三步

ALSA lib pcm.c:2567:(snd_pcm_open_conf) cxw conf id =type, str = pmaplug

ALSA lib pcm.c:2613:(snd_pcm_open_conf) open_name = _snd_pcm_pmaplug_open //获取到type pmaplug

ALSA lib pcm.c:2618:(snd_pcm_open_conf) open_name str = pmaplug

.................................

ALSA lib pcm.c:2618:(snd_pcm_open_conf) open_name str = pmaplug

ALSA lib pcm.c:2631:(snd_pcm_open_conf) open_name buf1 = /usr/lib/alsa-lib/libasound_module_pcm_pmaplug.so

//加载libasound_module_pcm_pmaplug.so库，并调用SND_PCM_PLUGIN_DEFINE_FUNC

ALSA lib pcm.c:2637:(snd_pcm_open_conf) open_name lib = /usr/lib/alsa-lib/libasound_module_pcm_pmaplug.so, open_name = 428882960

ALSA lib pcm.c:2641:(snd_pcm_open_conf) open_name name = pma

pmaplug() name: pma stream: 1 mode: 0

snd_pcm_extplug_create

第四步

ALSA lib pcm.c:2567:(snd_pcm_open_conf) cxw conf id =type, str = plug

ALSA lib pcm.c:2613:(snd_pcm_open_conf) open_name = _snd_pcm_plug_open //找到type plug

ALSA lib pcm.c:2618:(snd_pcm_open_conf) open_name str = plug

...................................

ALSA lib pcm.c:2618:(snd_pcm_open_conf) open_name str = plug

ALSA lib pcm.c:2637:(snd_pcm_open_conf) open_name lib = (null), open_name = 428956384

ALSA lib pcm.c:2641:(snd_pcm_open_conf) open_name name = 7input

第五步

ALSA lib pcm.c:2567:(snd_pcm_open_conf) cxw conf id =type, str = hw

ALSA lib pcm.c:2613:(snd_pcm_open_conf) open_name = _snd_pcm_hw_open

ALSA lib pcm.c:2618:(snd_pcm_open_conf) open_name str = hw

..........................................

ALSA lib pcm.c:2618:(snd_pcm_open_conf) open_name str = hw

ALSA lib pcm.c:2637:(snd_pcm_open_conf) open_name lib = (null), open_name = 429049088

ALSA lib pcm.c:2641:(snd_pcm_open_conf) open_name name = hw:0,2 //调用_snd_pcm_hw_open函数

分析结果：

代码介绍alsa.conf是一个递归的过程，根据type不同，调用不同的函数，逐步递归下去。

这是我们系统中snd_pcm_open的整个流程，虽然这里创建失败但是可以让我们清晰地认识流创建过程：它是一个分层的流建立过程。asym->route->pmaplug->plug->hw(pcm) 及asym->pmaplug->plug->hw函数层次调用过程。

三._snd_pcm_hw_open分析

Pcm_hw.c (z:\px30_linux\external\alsa-lib-1.1.5\src\pcm)

//Creates a new hw PCM

int _snd_pcm_hw_open(snd_pcm_t **pcmp, const char *name, snd_config_t *root ATTRIBUTE_UNUSED, snd_config_t *conf,

snd_pcm_stream_t stream, int mode)

snd_config_for_each(i, next, conf) //遍历

n = snd_config_iterator_entry(i);

if (snd_config_get_id(n, &id) < 0)

if (snd_pcm_conf_generic_id(id))

if (strcmp(id, "card") == 0) //获取card

if (strcmp(id, "device") == 0) //获取device

if (strcmp(id, "subdevice") == 0) //获取subdevice

。。。。。。。。。。

if (strcmp(id, "rate") == 0) //获取rate

if (strcmp(id, "format") == 0) //获取format

if (strcmp(id, "channels") == 0) //获取channels

//打开相应的hw stream

snd_pcm_hw_open(pcmp, name, card, device, subdevice, stream, mode | (nonblock ? SND_PCM_NONBLOCK : 0),

0, sync_ptr_ioctl);

snd_ctl_hw_open(&ctl, NULL, card, 0)) //单独分析1，打开控制节点

sprintf(filename, filefmt, card, device); //pcmC%iD%ic，配置pcm节点

fd = snd_open_device(filename, fmode); //打开stream

fd = open(filename, fmode);

ioctl(fd, SNDRV_PCM_IOCTL_INFO, &info) //获取信息

return snd_pcm_hw_open_fd(pcmp, name, fd, sync_ptr_ioctl);

hw = calloc(1, sizeof(snd_pcm_hw_t));

hw->version = ver;

hw->card = info.card;

hw->device = info.device;

hw->subdevice = info.subdevice;

hw->fd = fd;

/* no restriction */

hw->format = SND_PCM_FORMAT_UNKNOWN;

hw->rate = 0;

hw->channels = 0;

ret = snd_pcm_new(&pcm, SND_PCM_TYPE_HW, name, info.stream, mode); //创建一个snd_pcm_t结构体

pcm->ops = &snd_pcm_hw_ops;

pcm->fast_ops = &snd_pcm_hw_fast_ops;

pcm->private_data = hw;

pcm->poll_fd = fd;

pcm->poll_events = info.stream == SND_PCM_STREAM_PLAYBACK ? POLLOUT : POLLIN;

pcm->tstamp_type = tstamp_type;

map_status_and_control_data(pcm, !!sync_ptr_ioctl); //映射状态和控制数据

单独分析1

int snd_ctl_hw_open(snd_ctl_t **handle, const char *name, int card, int mode)

sprintf(filename, SNDRV_FILE_CONTROL, card); //赋值控制"controlC%i"

fd = snd_open_device(filename, fmode); //打开

fd = open(filename, fmode); //打开驱动节点

ioctl(fd, SNDRV_CTL_IOCTL_PVERSION, &ver) //控制的版本

hw->card = card;

hw->fd = fd;

hw->protocol = ver;

err = snd_ctl_new(&ctl, SND_CTL_TYPE_HW, name); //生成控制相关数据结构

ctl->ops = &snd_ctl_hw_ops;

ctl->private_data = hw;

ctl->poll_fd = fd;

四._snd_pcm_plug_open分析

int _snd_pcm_plug_open(snd_pcm_t **pcmp, const char *name, snd_config_t *root, snd_config_t *conf,

snd_pcm_stream_t stream, int mode)

snd_config_for_each(i, next, conf)

if (strcmp(id, "slave") == 0)

slave = n;

#ifdef BUILD_PCM_PLUGIN_ROUTE //可以设置路径相关的

if (strcmp(id, "ttable") == 0)

if (strcmp(id, "route_policy") == 0)

#ifdef BUILD_PCM_PLUGIN_RATE // //可以设置速率相关的

if (strcmp(id, "rate_converter") == 0)

//设置从设备的一些配置参数，这里会从alsa.conf里面获取

snd_pcm_slave_conf(root, slave, &sconf, 3, SND_PCM_HW_PARAM_FORMAT, SCONF_UNCHANGED, &sformat,

SND_PCM_HW_PARAM_CHANNELS, SCONF_UNCHANGED, &schannels, SND_PCM_HW_PARAM_RATE, SCONF_UNCHANGED, &srate);

err = snd_pcm_open_slave(&spcm, root, sconf, stream, mode, conf);

return snd_pcm_open_named_slave(pcmp, NULL, root, conf, stream, mode, parent_conf);

snd_pcm_open_conf(pcmp, name, root, conf, stream, mode); //前面有分析，获取相关的alsa.conf参数

//配置返回之后，就会到这里. 单独分析1

snd_pcm_plug_open(pcmp, name, sformat, schannels, srate, rate_converter, route_policy, ttable, ssize, cused, sused, spcm, 1);

单独分析1

int snd_pcm_plug_open(snd_pcm_t **pcmp, const char *name, snd_pcm_format_t sformat, int schannels, int srate,

const snd_config_t *rate_converter, enum snd_pcm_plug_route_policy route_policy, snd_pcm_route_ttable_entry_t *ttable,

unsigned int tt_ssize, unsigned int tt_cused, unsigned int tt_sused, snd_pcm_t *slave, int close_slave)

plug->sformat = sformat;

plug->schannels = schannels;

plug->srate = srate;

//将HW层的pcm流对象存放在PLUG层pcm流的私有数据成员中，slave就是那个指针

plug->gen.slave = plug->req_slave = slave;

plug->gen.close_slave = close_slave;

plug->route_policy = route_policy;

plug->ttable = ttable;

plug->tt_ssize = tt_ssize;

plug->tt_cused = tt_cused;

plug->tt_sused = tt_sused;

err = snd_pcm_new(&pcm, SND_PCM_TYPE_PLUG, name, slave->stream, slave->mode); //创建SND_PCM_TYPE_PLUG数据流

pcm->ops = &snd_pcm_plug_ops;

pcm->fast_ops = slave->fast_ops;

pcm->fast_op_arg = slave->fast_op_arg;

pcm->private_data = plug;

pcm->poll_fd = slave->poll_fd;

pcm->poll_events = slave->poll_events;

pcm->mmap_shadow = 1;

pcm->tstamp_type = slave->tstamp_type;

snd_pcm_link_hw_ptr(pcm, slave);

snd_pcm_link_appl_ptr(pcm, slave);

*pcmp = pcm;

五._snd_pcm_pmaplug_open 分析

因为有这个两个宏

#define SND_PCM_PLUGIN_ENTRY(name) _snd_pcm_##name##_open

SND_PCM_PLUGIN_DEFINE_FUNC(pmaplug) ->_snd_pcm_pmaplug_open

#define SND_PCM_PLUGIN_DEFINE_FUNC(plugin) \

int SND_PCM_PLUGIN_ENTRY(plugin) (snd_pcm_t **pcmp, const char *name,\

snd_config_t *root, snd_config_t *conf, \

snd_pcm_stream_t stream, int mode)

所有，下面这个就是

_snd_pcm_pmaplug_open(snd_pcm_t **pcmp, const char *name, snd_config_t *root, snd_config_t *conf, \

snd_pcm_stream_t stream, int mode)

snd_config_for_each(i, next, conf) //获取配置相关

if (strcmp(id, "slave") == 0)

slave = n;

(strcmp(id, "input_channels") == 0)

。。。。。。。

pmaplug->ext.version = SND_PCM_EXTPLUG_VERSION;

pmaplug->ext.name = "pmaplug"; //名字

pmaplug->ext.callback = &callbacks; //回调函数

pmaplug->ext.private_data = pmaplug; //私有

pmaplug->local = local; //私有的

pmaplug->loopback = loopback;

//私有的

//实例化，Create an extplug instance

snd_pcm_extplug_create(&pmaplug->ext, name, root, slave, stream, mode); //单独分析1

// mono out, user defined number in，设置

snd_pcm_extplug_set_param(&pmaplug->ext, SND_PCM_EXTPLUG_HW_CHANNELS, channels);

snd_pcm_extplug_set_slave_param(&pmaplug->ext, SND_PCM_EXTPLUG_HW_CHANNELS, channels);

// take double in and write signed 16 bit ints out, doubles are for PMA，设置

snd_pcm_extplug_set_param(&pmaplug->ext, SND_PCM_EXTPLUG_HW_FORMAT, SND_PCM_FORMAT_S16_LE);

snd_pcm_extplug_set_slave_param(&pmaplug->ext, SND_PCM_EXTPLUG_HW_FORMAT, SND_PCM_FORMAT_FLOAT64_LE);

*pcmp = pmaplug->ext.pcm;

//单独分析1

int snd_pcm_extplug_create(snd_pcm_extplug_t *extplug, const char *name,

snd_config_t *root, snd_config_t *slave_conf,

snd_pcm_stream_t stream, int mode)

snd_pcm_slave_conf(root, slave_conf, &sconf, 0);

snd_pcm_open_slave(&spcm, root, sconf, stream, mode, NULL); //之前有分析

ext = calloc(1, sizeof(*ext));

ext->data = extplug;

extplug->stream = stream;

ext->plug.read = snd_pcm_extplug_read_areas;

ext->plug.write = snd_pcm_extplug_write_areas;

ext->plug.undo_read = snd_pcm_plugin_undo_read_generic;

ext->plug.undo_write = snd_pcm_plugin_undo_write_generic;

//将plug层的pcm流对象存放在ext->plug层pcm流的私有数据成员中，spcm就是那个指针

ext->plug.gen.slave = spcm;

ext->plug.gen.close_slave = 1;

snd_pcm_new(&pcm, SND_PCM_TYPE_EXTPLUG, name, stream, mode); //创建一个SND_PCM_TYPE_EXTPLUG流

extplug->pcm = pcm;

pcm->ops = &snd_pcm_extplug_ops;

pcm->fast_ops = &snd_pcm_plugin_fast_ops;

pcm->private_data = ext;

pcm->poll_fd = spcm->poll_fd;

pcm->poll_events = spcm->poll_events;

snd_pcm_set_hw_ptr(pcm, &ext->plug.hw_ptr, -1, 0);

snd_pcm_set_appl_ptr(pcm, &ext->plug.appl_ptr, -1, 0);

六._snd_pcm_route_open分析

int _snd_pcm_route_open(snd_pcm_t **pcmp, const char *name, snd_config_t *root, snd_config_t *conf,

snd_pcm_stream_t stream, int mode)

snd_config_for_each(i, next, conf)

snd_config_t *n = snd_config_iterator_entry(i);

if (strcmp(id, "slave") == 0)

slave = n;

if (strcmp(id, "ttable") == 0)

tt = n;

err = snd_pcm_slave_conf(root, slave, &sconf, 2, SND_PCM_HW_PARAM_FORMAT, 0, &sformat,

SND_PCM_HW_PARAM_CHANNELS, 0, &schannels);

determine_chmap(tt, &tt_chmap); //决定通道的map

snd_pcm_open_slave(&spcm, root, sconf, stream, mode, conf);

if (tt_chmap)

find_matching_chmap(spcm, tt_chmap, &chmap, &schannels);

_snd_pcm_route_determine_ttable(tt, &csize, &ssize, chmap);

_snd_pcm_route_load_ttable(tt, ttable, csize, ssize, &cused, &sused, schannels, chmap);

snd_pcm_route_open(pcmp, name, sformat, schannels, ttable, ssize, cused, sused, spcm, 1); //单独分析1

单独分析1

int snd_pcm_route_open(snd_pcm_t **pcmp, const char *name, snd_pcm_format_t sformat, int schannels,

snd_pcm_route_ttable_entry_t *ttable, unsigned int tt_ssize, unsigned int tt_cused, unsigned int tt_sused,

snd_pcm_t *slave, int close_slave)

route = calloc(1, sizeof(snd_pcm_route_t));

snd_pcm_plugin_init(&route->plug);

route->sformat = sformat;

route->schannels = schannels;

route->plug.read = snd_pcm_route_read_areas;

route->plug.write = snd_pcm_route_write_areas;

route->plug.undo_read = snd_pcm_plugin_undo_read_generic;

route->plug.undo_write = snd_pcm_plugin_undo_write_generic;

route->plug.gen.slave = slave; //赋值slave指针

route->plug.gen.close_slave = close_slave;

route->plug.init = route_chmap_init;

snd_pcm_new(&pcm, SND_PCM_TYPE_ROUTE, name, slave->stream, slave->mode); //创建SND_PCM_TYPE_ROUTE的pcm

pcm->ops = &snd_pcm_route_ops;

pcm->fast_ops = &snd_pcm_plugin_fast_ops; //赋值操作函数

pcm->private_data = route;

pcm->poll_fd = slave->poll_fd;

pcm->poll_events = slave->poll_events;

pcm->tstamp_type = slave->tstamp_type;

snd_pcm_set_hw_ptr(pcm, &route->plug.hw_ptr, -1, 0);

snd_pcm_set_appl_ptr(pcm, &route->plug.appl_ptr, -1, 0);

err = route_load_ttable(&route->params, pcm->stream, tt_ssize, ttable, tt_cused, tt_sused);

七.总结

snd_pcm_open函数主要完成如下两步：
第一步：构建配置树
第二步：创建PCM流

由此可见，在整个过程中创建了三种类型的pcm流对象,EXTPLUG，PLUG,HW
_snd_pcm_asym_open -->_snd_pcm_route_open--->_snd_pcm_pmaplug_open--->_snd_pcm_plug_open--->_snd_pcm_hw_open层层递进分析配置树
_snd_pcm_asym_open <--....................................<---_snd_pcm_plug_open<--_snd_pcm_hw_open层层回溯。
回溯过程依次创建了HW pcm-->PLUG pcm-->EXTPLUG pcm-->ROUTE pcm及和它们区配的私有数据及相关操作函数集，而且可以通过ROUTE层的pcm流，查找到EXTPLUG ,plug,hw的pcm.它们都被寄存在上层的private_data成员中。