alsa - 观察 ALSA/Pulseaudio 中的音量变化

Question

如何在默认声卡上监听主 channel 的音量变化？我希望通过 dbus 或回调或其他方式收到音量已更改的通知。

我已经尝试查看 ALSA 和 PulseAudio API，它们似乎只允许您设置和获取音量，但不能收听音量变化。

任何编程语言都可以。

Answer 1

编辑: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。据我所知，第一个示例运行完美。

pactl subscribe 将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通过管道传输到一个将运行脚本的小型 C 程序。

运行.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

程序.c:

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

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

当水槽的体积改变时，pactl 会打印一行，这会导致程序运行脚本。

-或-

这是一个基于 amixer monitor 的示例，如 CL 所引用。每次音量变化时 while 循环都会迭代，因此请将您的回调放在那里。

#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]);
    }
}