When playing back, the PCM subchannel stops if the hardware consumes all the data in its buffer.
This can happen if the application can't produce data at the rate that the hardware is consuming data. A real-world example of this is when the application is preempted for a period of time by a higher-priority process. If this preemption continues long enough, all data in the buffer may be played before the application can add any more.
When this happens, the subchannel changes state to SND_PCM_STATUS_UNDERRUN. In this state, it doesn't accept any more data (i.e., snd_pcm_write() and snd_pcm_plugin_write() fail) and the subchannel doesn't restart playing.
The only ways to move out of this state are to close the subchannel or to reprepare the channel as you did before (see “Preparing the PCM subchannel,” earlier in this chapter). This forces the application to recognize and take action to get out of the underrun state; this is primarily for applications that want to synchronize audio with something else. Consider the difficulties involved with synchronization if the subchannel simply were to move back to the SND_PCM_STATUS_RUNNING state from underrun when more data became available.