TSLEEP(9)                    OpenBSD Kernel Manual                   TSLEEP(9)

NAME
     tsleep, msleep, wakeup - process context sleep and wakeup

SYNOPSIS
     #include <sys/param.h>
     #include <sys/proc.h>

     int
     tsleep(void *ident, int priority, const char *wmesg, int timo);

     int
     msleep(void *ident, struct mutex *mtx, int priority, const char *wmesg,
             int timo);

     void
     wakeup(void *ident);

DESCRIPTION
     These functions implement voluntary context switching.  tsleep() and
     msleep() are used throughout the kernel whenever processing in the cur-
     rent context cannot continue for any of the following reasons:

           o   The current process needs to await the results of a pending I/O
               operation.

           o   The current process needs resources (e.g. memory) which are
               temporarily unavailable.

           o   The current process wants access to data structures which are
               locked by other processes.

     The function wakeup() is used to notify sleeping processes of possible
     changes to the condition that caused them to go to sleep.  Typically, an
     awakened process will -- after it has acquired a context again -- retry
     the action that blocked its operation to see if the ``blocking'' condi-
     tion has cleared.

     The tsleep() function takes the following arguments:

     ident     An identifier of the ``wait channel'' representing the resource
               for which the current process needs to wait.  This typically is
               the virtual address of some kernel data structure related to
               the resource for which the process is contending.  The same
               identifier must be used in a call to wakeup() to get the pro-
               cess going again.  ident should not be NULL.

     priority  The process priority to be used when the process is awakened
               and put on the queue of runnable processes.  This mechanism is
               used to optimize ``throughput'' of processes executing in ker-
               nel mode.  If the flag PCATCH is OR'ed into priority the pro-
               cess checks for posted signals before and after sleeping.

     wmesg     A pointer to a character string indicating the reason a process
               is sleeping.  The kernel does not use the string, but makes it
               available (through the process structure field p_wmesg) for us-
               er level utilities such as ps(1).

     timo      If non-zero, the process will sleep for at most timo/hz sec-
               onds.  If this amount of time elapses and no wakeup(ident) has
               occurred, and no signal (if PCATCH was set) was posted,
               tsleep() will return EWOULDBLOCK.

     The msleep() function behaves just like tsleep(), but takes an additional
     argument:

     mtx       A mutex that will be unlocked when the process is safely on the
               sleep queue.  The mutex will be relocked at the end of msleep
               unless the PNORELOCK flag is set in the priority argument.

     The wakeup() function will mark all processes which are currently sleep-
     ing on the identifier ident as runnable.  Eventually, each of the pro-
     cesses will resume execution in the kernel context, causing a return from
     tsleep().  Note that processes returning from sleep should always re-
     evaluate the conditions that blocked them, since a call to wakeup() mere-
     ly signals a possible change to the blocking conditions.  For example,
     when two or more processes are waiting for an exclusive lock, only one of
     them will succeed in acquiring the lock when it is released.  All others
     will have to go back to sleep and wait for the next opportunity.

RETURN VALUES
     tsleep() and msleep() return 0 if they return as a result of a wakeup().
     If they return as a result of a signal, the return value is ERESTART if
     the signal has the SA_RESTART property (see sigaction(2)), and EINTR oth-
     erwise.  If they return as a result of a timeout, the return value is
     EWOULDBLOCK.

CODE REFERENCES
     These functions are implemented in the file sys/kern/kern_synch.c.

SEE ALSO
     hz(9), mi_switch(9), timeout(9)

OpenBSD 4.7                      June 26, 2008                               2
