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SPL(9)                       OpenBSD Kernel Manual                      SPL(9)

NAME
     spl - modify system interrupt priority level

SYNOPSIS
     #include <machine/intr.h>

     int
     splhigh(void);

     int
     splserial(void);

     int
     splsched(void);

     int
     splclock(void);

     int
     splstatclock(void);

     int
     splvm(void);

     int
     spltty(void);

     int
     splsofttty(void);

     int
     splnet(void);

     int
     splbio(void);

     int
     splsoftnet(void);

     int
     splsoftclock(void);

     int
     spllowersoftclock(void);

     int
     spl0(void);

     void
     splx(int s);

     void
     splassert(int s);

DESCRIPTION
     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
     priority level.

     A spl function exists for each distinct priority level which can exist in
     the system.  These macros and the corresponding priority levels are used
     for various defined purposes, and may be divided into two main types:
     hard and soft.  Hard interrupts are generated by hardware devices, 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 macros are:

     splhigh()       blocks all hard and soft interrupts.  It is used for code
                     that cannot tolerate any interrupts, like hardware con-
                     text switching code and the ddb(4) in-kernel debugger.

     splserial()     blocks hard interrupts from serial interfaces.  Code run-
                     ning at this level may not access the tty subsystem.

     splsched()      blocks interrupts that may access scheduler data struc-
                     tures.  Code running at or above this level may not call
                     sleep(), tsleep(), or wakeup(), nor may it post signals.
                     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 called
                     splhigh() for blocking purposes can use sleep(),
                     tsleep(), or wakeup().

     splclock()      blocks the hardware clock interrupt.  It is used by
                     hardclock() to update kernel and process times, and must
                     be used by any other code that accesses time-related da-
                     ta.

     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 accesses
                     that data.  This level is identical to splclock() if
                     there is no separate statistics clock.

     splvm()         blocks hard interrupts from all devices that are allowed
                     to use the kernel malloc(9).  That includes all disk,
                     network, and tty device interrupts.

     spltty()        blocks hard interrupts from TTY devices.

     splsofttty()    blocks soft interrupts generated by serial devices.

     splnet()        blocks hard interrupts from network interfaces.

     splbio()        blocks hard interrupts from disks and other mass-storage
                     devices.

     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 clock inter-
                          rupt.

     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 other spl
     macros.

     The splassert() function checks that the system is running at a certain
     priority level.  The argument s should be one of these constants:

           IPL_STATCLOCK
           IPL_CLOCK
           IPL_VM
           IPL_BIO
           IPL_TTY
           IPL_NET
           IPL_SOFTNET
           IPL_SOFTCLOCK
           IPL_NONE

     The splassert() function is optional and is not necessarily implemented
     on all architectures nor enabled in all kernel configurations.  It checks
     the current system priority level to see if it's at least at the level
     specified in the argument s.  If possible, it also checks if it hasn't
     been called from an interrupt handler with a level higher than the one
     requested, which must be an error (if some code is protected from
     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 the
     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 3.4                     March 11, 1997                               3

NAME | SYNOPSIS | DESCRIPTION

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