我在一个 linux 嵌入式设备上工作，每次我开始播放音频流时，对 snd_pcm_start 的调用都会返回 -77：这意味着“文件描述符处于错误状态”。
#define EBADFD 77 /* File descriptor in bad state */
我用一些 printf 重建了 alsa lib，我发现使用命令 aplay 不会产生该错误。
即使出现这个错误，我也可以播放声音，但每 10 次我就会听到一种粗糙的、金属的、类似机器人的声音。
问题的原因可能是什么？我按照打开声音设备时发现的所有指示进行操作。
这里是我的代码的相关部分：
在捕获/播放线程之前，我以这种方式启动音频，这里出现错误：
void alsa_device_start(AlsaDevice *dev)
{
int i;
int rval = 0;
short pcm[dev->period*dev->channels];
for (i=0;i<dev->period*dev->channels;i++)
pcm[i] = 0;
alsa_device_write(dev, pcm, dev->period);
alsa_device_write(dev, pcm, dev->period);
rval = snd_pcm_start(dev->capture_handle); //<<-- returns -77
}
在我初始化设备之前，如下所示：
AlsaDevice *alsa_device_open(pj_pool_t* pool, char *device_name, unsigned int rate, int channels, int period)
{
int dir;
int err;
snd_pcm_hw_params_t *hw_params;
snd_pcm_sw_params_t *sw_params;
snd_pcm_uframes_t period_size = period;
snd_pcm_uframes_t buffer_size = 2*period;
static snd_output_t *jcd_out;
hw_params = NULL;
sw_params = NULL;
//AlsaDevice *dev = pj_pool_alloc(pool, sizeof(AlsaDevice)); //malloc(sizeof(*dev));
AlsaDevice *dev = malloc(sizeof(*dev));
if (!dev)
return NULL;
dev->device_name = pj_pool_alloc(pool, 1+strlen(device_name)); //malloc(1+strlen(device_name));
if (!dev->device_name)
{
free(dev);
return NULL;
}
strcpy(dev->device_name, device_name);
dev->channels = channels;
dev->period = period;
dev->read_buff = pj_pool_alloc(pool, period * sizeof(short));
dev->read_buff_tmp = pj_pool_alloc(pool, period * sizeof(short));
err = snd_output_stdio_attach(&jcd_out, stdout, 0);
if ((err = snd_pcm_open (&dev->capture_handle, dev->device_name, SND_PCM_STREAM_CAPTURE, 0)) < 0) {
PJ_LOG (4,(THIS_FILE, "cannot open audio device %s (%s)\n", dev->device_name,
snd_strerror (err)));
assert(0);
}
if ((err = snd_pcm_hw_params_malloc(&hw_params)) < 0) {
PJ_LOG (4,(THIS_FILE, "cannot allocate hardware parameter structure (%s)\n",
snd_strerror (err)));
assert(0);
}
if ((err = snd_pcm_hw_params_any (dev->capture_handle, hw_params)) < 0) {
PJ_LOG (4,(THIS_FILE, "cannot initialize hardware parameter structure (%s)\n",
snd_strerror (err)));
assert(0);
}
if ((err = snd_pcm_hw_params_set_access (dev->capture_handle, hw_params, SND_PCM_ACCESS_RW_INTERLEAVED)) < 0) {
PJ_LOG (4,(THIS_FILE, "cannot set access type (%s)\n",
snd_strerror (err)));
assert(0);
}
if ((err = snd_pcm_hw_params_set_format (dev->capture_handle, hw_params, SND_PCM_FORMAT_S16_LE)) < 0) {
PJ_LOG (4,(THIS_FILE, "cannot set sample format (%s)\n",
snd_strerror (err)));
assert(0);
}
if ((err = snd_pcm_hw_params_set_rate_near (dev->capture_handle, hw_params, &rate, 0)) < 0) {
PJ_LOG (4,(THIS_FILE, "cannot set sample rate (%s)\n",
snd_strerror (err)));
assert(0);
}
/*fprintf (stderr, "rate = %d\n", rate);*/
if ((err = snd_pcm_hw_params_set_channels (dev->capture_handle, hw_params, channels)) < 0) {
PJ_LOG (4,(THIS_FILE, "cannot set channel count (%s)\n",
snd_strerror (err)));
assert(0);
}
period_size = period;
dir = 0;
if ((err = snd_pcm_hw_params_set_period_size_near (dev->capture_handle, hw_params, &period_size, &dir)) < 0) {
PJ_LOG (4,(THIS_FILE, "cannot set period size (%s)\n",
snd_strerror (err)));
assert(0);
}
if ((err = snd_pcm_hw_params_set_periods (dev->capture_handle, hw_params, ALSA_PERIODS, 0)) < 0) {
PJ_LOG (4,(THIS_FILE, "cannot set number of periods (%s)\n",
snd_strerror (err)));
assert(0);
}
buffer_size = period_size * 2;
dir=0;
if ((err = snd_pcm_hw_params_set_buffer_size_near (dev->capture_handle, hw_params, &buffer_size)) < 0) {
PJ_LOG (4,(THIS_FILE, "cannot set buffer time (%s)\n",
snd_strerror (err)));
assert(0);
}
if ((err = snd_pcm_hw_params (dev->capture_handle, hw_params)) < 0) {
PJ_LOG (4,(THIS_FILE, "cannot set capture parameters (%s)\n",
snd_strerror (err)));
assert(0);
}
/*snd_pcm_dump_setup(dev->capture_handle, jcd_out);*/
snd_pcm_hw_params_free (hw_params);
hw_params = NULL; //ermes
if ((err = snd_pcm_sw_params_malloc (&sw_params)) < 0) {
PJ_LOG (4,(THIS_FILE, "cannot allocate software parameters structure (%s)\n",
snd_strerror (err)));
assert(0);
}
if ((err = snd_pcm_sw_params_current (dev->capture_handle, sw_params)) < 0) {
PJ_LOG (4,(THIS_FILE, "cannot initialize software parameters structure (%s)\n",
snd_strerror (err)));
assert(0);
}
if ((err = snd_pcm_sw_params_set_avail_min (dev->capture_handle, sw_params, period)) < 0) {
PJ_LOG (4,(THIS_FILE, "cannot set minimum available count (%s)\n",
snd_strerror (err)));
assert(0);
}
if ((err = snd_pcm_sw_params (dev->capture_handle, sw_params)) < 0) {
PJ_LOG (4,(THIS_FILE, "cannot set software parameters (%s)\n",
snd_strerror (err)));
assert(0);
}
if (sw_params) snd_pcm_sw_params_free(sw_params); //ermes
sw_params = NULL; //ermes
if ((err = snd_pcm_open (&dev->playback_handle, dev->device_name, SND_PCM_STREAM_PLAYBACK, 0)) < 0) {
PJ_LOG (4,(THIS_FILE, "cannot open audio device %s (%s)\n",
dev->device_name,
snd_strerror (err)));
assert(0);
}
if ((err = snd_pcm_hw_params_malloc (&hw_params)) < 0) {
PJ_LOG (4,(THIS_FILE, "cannot allocate hardware parameter structure (%s)\n",
snd_strerror (err)));
assert(0);
}
if ((err = snd_pcm_hw_params_any (dev->playback_handle, hw_params)) < 0) {
PJ_LOG (4,(THIS_FILE, "cannot initialize hardware parameter structure (%s)\n",
snd_strerror (err)));
assert(0);
}
if ((err = snd_pcm_hw_params_set_access (dev->playback_handle, hw_params, SND_PCM_ACCESS_RW_INTERLEAVED)) < 0) {
PJ_LOG (4,(THIS_FILE, "cannot set access type (%s)\n",
snd_strerror (err)));
assert(0);
}
if ((err = snd_pcm_hw_params_set_format (dev->playback_handle, hw_params, SND_PCM_FORMAT_S16_LE)) < 0) {
PJ_LOG (4,(THIS_FILE, "cannot set sample format (%s)\n",
snd_strerror (err)));
assert(0);
}
if ((err = snd_pcm_hw_params_set_rate_near (dev->playback_handle, hw_params, &rate, 0)) < 0) {
PJ_LOG (4,(THIS_FILE, "cannot set sample rate (%s)\n",
snd_strerror (err)));
assert(0);
}
if ((err = snd_pcm_hw_params_set_channels (dev->playback_handle, hw_params, channels)) < 0) {
PJ_LOG (4,(THIS_FILE, "cannot set channel count (%s)\n",
snd_strerror (err)));
assert(0);
}
period_size = period;
dir = 0;
if ((err = snd_pcm_hw_params_set_period_size_near (dev->playback_handle, hw_params, &period_size, &dir)) < 0) {
PJ_LOG (4,(THIS_FILE, "cannot set period size (%s)\n",
snd_strerror (err)));
assert(0);
}
if ((err = snd_pcm_hw_params_set_periods (dev->playback_handle, hw_params, ALSA_PERIODS, 0)) < 0) {
PJ_LOG (4,(THIS_FILE, "cannot set number of periods (%s)\n",
snd_strerror (err)));
assert(0);
}
buffer_size = period_size * 2;
dir=0;
if ((err = snd_pcm_hw_params_set_buffer_size_near (dev->playback_handle, hw_params, &buffer_size)) < 0) {
PJ_LOG (4,(THIS_FILE, "cannot set buffer time (%s)\n",
snd_strerror (err)));
assert(0);
}
if ((err = snd_pcm_hw_params (dev->playback_handle, hw_params)) < 0) {
PJ_LOG (4,(THIS_FILE, "cannot set playback parameters (%s)\n",
snd_strerror (err)));
assert(0);
}
/*snd_pcm_dump_setup(dev->playback_handle, jcd_out);*/
snd_pcm_hw_params_free (hw_params);
hw_params = NULL; //ermes
if ((err = snd_pcm_sw_params_malloc (&sw_params)) < 0) {
PJ_LOG (4,(THIS_FILE, "cannot allocate software parameters structure (%s)\n",
snd_strerror (err)));
assert(0);
}
if ((err = snd_pcm_sw_params_current (dev->playback_handle, sw_params)) < 0) {
PJ_LOG (4,(THIS_FILE, "cannot initialize software parameters structure (%s)\n",
snd_strerror (err)));
assert(0);
}
if ((err = snd_pcm_sw_params_set_avail_min (dev->playback_handle, sw_params, period)) < 0) {
PJ_LOG (4,(THIS_FILE, "cannot set minimum available count (%s)\n",
snd_strerror (err)));
assert(0);
}
if ((err = snd_pcm_sw_params_set_start_threshold (dev->playback_handle, sw_params, 2*period)) < 0) {
PJ_LOG (4,(THIS_FILE, "cannot set start mode (%s)\n",
snd_strerror (err)));
assert(0);
}
if ((err = snd_pcm_sw_params (dev->playback_handle, sw_params)) < 0) {
PJ_LOG (4,(THIS_FILE, "cannot set software parameters (%s)\n",
snd_strerror (err)));
assert(0);
}
if (sw_params) snd_pcm_sw_params_free(sw_params);
sw_params = NULL;
snd_pcm_link(dev->capture_handle, dev->playback_handle);
if ((err = snd_pcm_prepare (dev->capture_handle)) < 0) {
PJ_LOG (4,(THIS_FILE, "cannot prepare audio interface for use (%s)\n",
snd_strerror (err)));
assert(0);
}
if ((err = snd_pcm_prepare (dev->playback_handle)) < 0) {
PJ_LOG (4,(THIS_FILE, "cannot prepare audio interface for use (%s)\n",
snd_strerror (err)));
assert(0);
}
dev->readN = snd_pcm_poll_descriptors_count(dev->capture_handle);
dev->writeN = snd_pcm_poll_descriptors_count(dev->playback_handle);
dev->read_fd = pj_pool_alloc(pool, dev->readN*sizeof(*dev->read_fd));
if (snd_pcm_poll_descriptors(dev->capture_handle, dev->read_fd, dev->readN) != dev->readN)
{
PJ_LOG (4,(THIS_FILE, "cannot obtain capture file descriptors (%s)\n",
snd_strerror (err)));
assert(0);
}
dev->write_fd = pj_pool_alloc(pool, dev->writeN*sizeof(*dev->read_fd)); // malloc(dev->writeN*sizeof(*dev->read_fd));
if (snd_pcm_poll_descriptors(dev->playback_handle, dev->write_fd, dev->writeN) != dev->writeN)
{
PJ_LOG (4,(THIS_FILE, "cannot obtain playback file descriptors (%s)\n",
snd_strerror (err)));
assert(0);
}
return dev;
}