The functions tsleep and wakeup handle event-based process blocking. If a process must wait for an external event, it is put on sleep by tsleep. The parameter ident is an arbitrary address that uniquely identifies the event on which the process is being asleep. All processes sleeping on a single ident are woken up later by wakeup, often called from inside an interrupt routine, to indicate that the resource the process was blocking on is available now.
The parameter wmesg is a string describing the sleep condition for tools like ps(1). Due to the limited space of those programs to display arbitrary strings, this message should not be longer than 6 characters.
The wakeup_one function is used to make the first process in the queue that is sleeping on the parameter ident runnable. This can prevent the system from becoming saturated when a large number of processes are sleeping on the same address, but only one of them can actually do any useful work when made runnable.
The tsleep function is the general sleep call. Suspends the current process until a wakeup is performed on the specified identifier. The process will then be made runnable with the specified priority. Sleeps at most timo / hz seconds (0 means no timeout). If the Giant lock is not held, then timo must be non-zero. If priority includes the PCATCH flag, signals are checked before and after sleeping, else signals are not checked. Returns 0 if awakened, EWOULDBLOCK if the timeout expires. If PCATCH is set and a signal needs to be delivered, ERESTART is returned if the current system call should be restarted if possible, and EINTR is returned if the system call should be interrupted by the signal (return EINTR).
The msleep function is a variation on tsleep. The parameter mtx is a mutex which will be released before sleeping and reacquired before msleep returns. If priority includes the PDROP flag, the mtx parameter will not be reacquired before returning. The mutex is used to ensure that a condition can be checked atomically, and that the current process can be suspended without missing a change to the condition, or an associated wakeup.
ps(1), malloc(9), mi_switch(9)