spl - modify system interrupt priority level
These functions raise and lower the system priority level.
They are used
by kernel code to block interrupts with priority less than
or equal to
the named level (i.e., spltty() blocks interrupts of priority less than
or equal to IPL_TTY). The code may then safely access variables and data
structures which are used by kernel code that runs at an
equal or lower
An spl function exists for each distinct priority level
which can exist
in the system. These macros and the corresponding priority
used for various defined purposes, and may be divided into
types: hard and soft. Hard interrupts are generated by
while soft interrupts are generated by callouts and called
from the kernel's
periodic timer interrupt service routine.
In order of highest to lowest priority, the priority-raising
splhigh() blocks all hard and soft interrupts. It is
used for code
that cannot tolerate any interrupts, like
switching code and the ddb(4) in-kernel
splserial() blocks hard interrupts from serial interfaces. Code running
at this level may not access the tty
splsched() blocks interrupts that may access scheduler
Code running at or above this level
may not call
sleep(), tsleep(), or wakeup(), nor may it
Note that "running" means invoked by an interrupt handler
that operates at this level or higher. Kernel code that
operates in the context of a process and has
splhigh() for blocking purposes can use
tsleep(), or wakeup().
splclock() blocks the hardware clock interrupt. It is
hardclock() to update kernel and process
times, and must
be used by any other code that accesses
splstatclock() blocks the hardware statistics clock interrupt. It is
used by statclock() to update kernel profiling and other
statistics, and must be used by any code
that data. This level is identical to
there is no separate statistics clock.
splvm() blocks hard interrupts from all devices that
to use the kernel malloc(9). That includes
network, and tty device interrupts.
spltty() blocks hard interrupts from TTY devices.
splsofttty() blocks soft interrupts generated by serial
splnet() blocks hard interrupts from network interfaces.
splbio() blocks hard interrupts from disks and other
splsoftnet() blocks soft network interrupts.
splsoftclock() blocks soft clock interrupts.
Two macros lower the system priority level. They are:
spllowersoftclock() unblocks all interrupts but the soft
spl0() unblocks all interrupts.
The splx() macro restores the system priority level to the
one encoded in
s, which must be a value previously returned by one of the
The splassert() function checks that the system is running
at a certain
priority level. The argument s should be one of these constants:
The splassert() function is optional and is not necessarily
on all architectures nor enabled in all kernel configurations. It checks
the current system priority level to see if it's at least at
specified in the argument s. If possible, it also checks if
been called from an interrupt handler with a level higher
than the one
requested, which must be an error (if some code is protected
IPL_SOFTNET interrupts, but accessed from an IPL_NET interrupt, it must
be a design error in the code).
The behavior of the splassert() function is controlled by
kern.splassert sysctl(8). Valid values for it are:
0 disable error checking
1 print a message if an error is detected
2 print a message and a stack trace if possible
3 like 2 but also drop into the kernel debugger
Any other value causes a system panic on errors.
OpenBSD 3.6 March 11, 1997
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