Question

如何在默认声卡的主通道上收听音量变化？我希望通过dbus或回调或音量已更改的内容收到通知。

我已经尝试过查看ALSA和PulseAudio API，它们似乎只允许您设置和获取音量，但不能监听以查看音量的变化。

任何编程语言都可以。

Answer 1

使用ALSA API可以实现这一点。

如果您有控制设备，请致电snd_ctl_subscribe_events()以启用事件。然后使用snd_ctl_read()来读取事件;要等待它们，请使用阻止模式或poll()。如果事件的类型为SND_CTL_EVENT_ELEM，并且其事件位掩码包含SND_CTL_EVENT_MASK_VALUE，则该元素的值已更改。

有关示例，请参阅the implementation of amixer monitor。

Answer 2

修改：在第二个示例中，当音量低于5％或高于100％时，不会为我生成事件。就我所知，第一个例子完美无缺。

pactl subscribe会在音量变化时打印出有关接收器的数据。我现在正在做的是将输出传递给一个运行脚本的小型C程序。

run.sh：

pactl subscribe | grep --line-buffered "sink" | ./prog

或特定的水槽，例如3：

pactl subscribe | grep --line-buffered "sink #3" | ./prog

prog.c中：

#include <stdio.h>
#include <stdlib.h>

int main(int argc, char* argv[]){
    while(1){
        while(getchar() != '\n');
        system("./volume_notify.sh");
    }
}

当接收器的音量改变时，pactl将打印一行，这将导致程序运行该脚本。

-OR -

这是一个基于amixer monitor的示例，由CL引用。 while循环将在每次音量改变时进行迭代，因此将回调放在那里。

#include <stdio.h>
#include <alsa/asoundlib.h>

#define MAX_CARDS 256

int monitor_native(char const *name);
int open_ctl(const char *name, snd_ctl_t **ctlp);
void close_all(snd_ctl_t* ctls[], int ncards);

int main(int argc, char* argv[]){

    const char *ctl_name = "hw:0";

    while(monitor_native(ctl_name) == 1){
        //volume has been changed, do something
        system("~/.volume_notify.sh");
    }

    return 0;
}

int monitor_native(char const *name) {
    snd_ctl_t *ctls[MAX_CARDS];
    int ncards = 0;
    int i, err = 0;

    if (!name) {
        int card = -1;
        while (snd_card_next(&card) >= 0 && card >= 0) {
            char cardname[16];
            if (ncards >= MAX_CARDS) {
                fprintf(stderr, "alsactl: too many cards\n");
                close_all(ctls, ncards);
                return -E2BIG;
            }
            sprintf(cardname, "hw:%d", card);
            err = open_ctl(cardname, &ctls[ncards]);
            if (err < 0) {
                close_all(ctls, ncards);
                return err;
            }
            ncards++;
        }
    } else {
        err = open_ctl(name, &ctls[0]);
        if (err < 0) {
            close_all(ctls, ncards);
            return err;
        }
        ncards++;
    }

    for (;ncards > 0;) {
        pollfd* fds = new pollfd[ncards];

        for (i = 0; i < ncards; i++) {
            snd_ctl_poll_descriptors(ctls[i], &fds[i], 1);
        }

        err = poll(fds, ncards, -1);
        if (err <= 0) {
            err = 0;
            break;
        }

        for (i = 0; i < ncards; i++) {
            unsigned short revents;
            snd_ctl_poll_descriptors_revents(ctls[i], &fds[i], 1, &revents);
            if (revents & POLLIN) {
                snd_ctl_event_t *event;
                snd_ctl_event_alloca(&event);

                if (snd_ctl_read(ctls[i], event) < 0) {
                    continue;
                }
                if (snd_ctl_event_get_type(event) != SND_CTL_EVENT_ELEM) {
                    continue;
                }

                unsigned int mask = snd_ctl_event_elem_get_mask(event);
                if (mask & SND_CTL_EVENT_MASK_VALUE) {
                    close_all(ctls, ncards);
                    return 1;
                }
            }
        }
    }

    close_all(ctls, ncards);
    return 0;
}

int open_ctl(const char *name, snd_ctl_t **ctlp) {
    snd_ctl_t *ctl;
    int err;

    err = snd_ctl_open(&ctl, name, SND_CTL_READONLY);
    if (err < 0) {
        fprintf(stderr, "Cannot open ctl %s\n", name);
        return err;
    }
    err = snd_ctl_subscribe_events(ctl, 1);
    if (err < 0) {
        fprintf(stderr, "Cannot open subscribe events to ctl %s\n", name);
        snd_ctl_close(ctl);
        return err;
    }
    *ctlp = ctl;
    return 0;
}

void close_all(snd_ctl_t* ctls[], int ncards) {
    for (ncards -= 1; ncards >= 0; --ncards) {
        snd_ctl_close(ctls[ncards]);
    }
}

Answer 3

类似于@ sealj553的答案，但不需要C程序：

org.springframework.dao.DataAccessResourceFailureException: Operation on server 127.0.0.1:27017 failed; nested exception is com.mongodb.MongoException$Network: Operation on server 127.0.0.1:27017 failed
     at org.springframework.data.mongodb.core.MongoExceptionTranslator.translateExceptionIfPossible(MongoExceptionTranslator.java:59)
     at org.springframework.data.mongodb.core.MongoTemplate.potentiallyConvertRuntimeException(MongoTemplate.java:1920)
     at org.springframework.data.mongodb.core.MongoTemplate.execute(MongoTemplate.java:412)
     ...
 Caused by: com.mongodb.MongoException$Network: Operation on server 127.0.0.1:27017 failed
     at com.mongodb.DBTCPConnector.doOperation(DBTCPConnector.java:214)
     at com.mongodb.DBCollectionImpl.writeWithCommandProtocol(DBCollectionImpl.java:424)
     at com.mongodb.DBCollectionImpl.removeWithCommandProtocol(DBCollectionImpl.java:409)
     at com.mongodb.DBCollectionImpl.remove(DBCollectionImpl.java:223)
     at com.mongodb.DBCollectionImpl.remove(DBCollectionImpl.java:203)
     at com.mongodb.DBCollection.remove(DBCollection.java:274)
     at org.springframework.data.mongodb.core.MongoTemplate$12.doInCollection(MongoTemplate.java:1194)
     at org.springframework.data.mongodb.core.MongoTemplate$12.doInCollection(MongoTemplate.java:1178)
     at org.springframework.data.mongodb.core.MongoTemplate.execute(MongoTemplate.java:410)
     ... 8 more
 Caused by: java.net.SocketTimeoutException: Read timed out
     at java.net.SocketInputStream.socketRead0(Native Method)
     at java.net.SocketInputStream.socketRead(SocketInputStream.java:116)
     at java.net.SocketInputStream.read(SocketInputStream.java:170)
     at java.net.SocketInputStream.read(SocketInputStream.java:141)
     at java.io.BufferedInputStream.fill(BufferedInputStream.java:246)
     at java.io.BufferedInputStream.read1(BufferedInputStream.java:286)
     at java.io.BufferedInputStream.read(BufferedInputStream.java:345)
     at org.bson.io.Bits.readFully(Bits.java:48)
     at org.bson.io.Bits.readFully(Bits.java:35)
     at org.bson.io.Bits.readFully(Bits.java:30)
     at com.mongodb.Response.<init>(Response.java:42)
     at com.mongodb.DBCollectionImpl.receiveWriteCommandMessage(DBCollectionImpl.java:495)
     at com.mongodb.DBCollectionImpl.access$300(DBCollectionImpl.java:48)
     at com.mongodb.DBCollectionImpl$2.execute(DBCollectionImpl.java:437)
     at com.mongodb.DBCollectionImpl$2.execute(DBCollectionImpl.java:424)
     at com.mongodb.DBPort.doOperation(DBPort.java:164)
     at com.mongodb.DBTCPConnector.doOperation(DBTCPConnector.java:207)
     ... 16 more

观察ALSA / Pulseaudio中的音量变化

3 个答案: