Waiting on Signals

In the qotd_rw() and qotd_ioctl() routines, the cv_wait_sig(9F) calls wait until the condition variable is signaled to proceed or until a signal(3C) is received. Either the cv_signal(9F) function or the cv_broadcast(9F) function signals the cv condition variable to proceed.

A thread can wait on a condition variable until either the condition variable is signaled or a signal(3C) is received by the process. The cv_wait(9F) function waits until the condition variable is signaled but ignores signal(3C) signals. This driver uses the cv_wait_sig(9F) function instead of the cv_wait(9F) function because this driver responds if a signal is received by the process performing the operation. If a signal(3C) is taken by the process, this driver returns an interrupt error and does not complete the operation. The cv_wait_sig(9F) function usually is preferred to the cv_wait(9F) function because this implementation offers the user program more precise response. The signal(3C) causes an effect closer to the point at which the process was executing when the signal(3C) was received.

In some cases, you cannot use the cv_wait_sig(9F) function because your driver cannot be interrupted by a signal(3C). For example, you cannot use the cv_wait_sig(9F) function during a DMA transfer that will result in an interrupt later. In this case, if you abandon the cv_wait_sig(9F) call, you have nowhere to put the data when the DMA transfer is finished, and your driver will panic.