TSLEEP(9) OpenBSD Kernel Manual TSLEEP(9)NAME
tsleep, msleep, wakeup - process context sleep and wakeup
SYNOPSIS
#include <sys/param.h>
#include <sys/systm.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
current 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''
condition 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
process 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
kernel mode. If the flag PCATCH is OR'ed into priority the
process 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
user level utilities such as ps(1).
timo If non-zero, the process will sleep for at most timo/hz
seconds. 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
sleeping on the identifier ident as runnable. Eventually, each of the
processes 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()
merely 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 VALUEStsleep() 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
otherwise. 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 ALSOhz(9), mi_switch(9), timeout(9)OpenBSD 4.9 April 8, 2010 OpenBSD 4.9