Create a handle and open a connection to the preferred audio interface
#include <sys/asoundlib.h> int snd_pcm_open_preferred( snd_pcm_t **handle, int *rcard, int *rdevice, int mode );
- A pointer to a location where snd_pcm_open_preferred() can store a handle for the audio interface. You'll need this handle when you call the other snd_pcm_* functions.
- If non-NULL, this must be a pointer to a location where snd_pcm_open_preferred() can store the number of the card that it opened.
- If non-NULL, this must be a pointer to a location where snd_pcm_open_preferred() can store the number of the audio device that it opened.
- One of:
- SND_PCM_OPEN_PLAYBACK — open the playback channel (direction).
- SND_PCM_OPEN_CAPTURE — open the capture channel (direction).
You can OR this flag with any of the above:
SND_PCM_OPEN_NONBLOCK — force the mode to be
This affects any reading from or writing to the device that you do later;
you can query the device any time without blocking.
You can change the blocking setup later by calling snd_pcm_nonblock_mode().
Use the -l asound option to qcc to link against this library.
The snd_pcm_open_preferred() function is an extension to the snd_pcm_open() function that attempts to open the user-selected default (or preferred) device for the system.
If you use this function, your application will be more flexible than if you use snd_pcm_open().
In a system where more than one PCM device exists, the user may set a preference for one of these devices. This function attempts to open that device and return a PCM handle to it. The function returns the card and device numbers if the rcard and rdevice arguments aren't NULL.
Here's the search order to find the preferred device:
- Read /etc/system/config/audio/preferences.
The format of this file is as follows, with a tab character between the fields:
pcmPreferredp card_number device_number pcmPreferredc card_number device_number
- If this file doesn't exist or has no entry, check PCM device 0 of card 0 for a software mixing overlay device. If this overlay device is found, it's opened.
- Open the default device 0 of card 0.
If all of the above fail, you don't have an audio system running.
Zero on success, or a negative value on error.
- Invalid mode.
- Search permission is denied on a component of the path prefix, or the device exists and the permissions specified are denied.
- The open() operation was interrupted by a signal.
- Too many file descriptors are currently in use by this process.
- Too many files are currently open in the system.
- The named device doesn't exist.
- The audio driver version is incompatible with the client library that the application uses.
- No memory available for data structures.
See the example in " Opening your PCM device " in the Playing and Capturing Audio Data chapter.
|Thread||Read the Caveats|
This function is not thread safe if handle (snd_pcm_t) is used across multiple threads.
Successfully opening a PCM channel doesn't guarantee that there are enough audio resources free to handle your application. Audio resources (e.g. subchannels) are allocated when you configure the channel by calling snd_pcm_channel_params() or snd_pcm_plugin_params().
Last modified: 2014-06-24