splvm(9) manipulate interrupt priorities

Other Alias

splbio, splclock, splhigh, splimp, splnet, splsoftclock, splsofttty, splstatclock, spltty, spl0, splx


In sys/types.h In sys/systm.h Ft intrmask_t Fn splbio void Ft intrmask_t Fn splclock void Ft intrmask_t Fn splhigh void Ft intrmask_t Fn splimp void Ft intrmask_t Fn splnet void Ft intrmask_t Fn splsoftclock void Ft intrmask_t Fn splsofttty void Ft intrmask_t Fn splstatclock void Ft intrmask_t Fn spltty void Ft void Fn spl0 void Ft void Fn splx intrmask_t ipl


Bf -symbolic This API is deprecated. Use mutexes to protect data structures instead. See mutex(9) for more information. The API is now a complete NOP. This man page documents historical behavior so you can understand the code locking that the spl did when converting code from versions of the kernel prior to Fx 5.0 . The examples in this man page are also obsolete and should not be viewed as documenting Fx 5.0 and newer. Ef

The Fn spl function family sets the interrupt priority ``level'' of the CPU. This prevents interrupt handlers of the blocked priority level from being run. This is used in the ``synchronous'' part of a driver (the part that runs on behalf of the user process) to examine or modify data areas that might be examined or modified by interrupt handlers.

Each driver that uses interrupts is normally assigned to an interrupt priority group by a keyword in its config line. For example:

device foo0 at isa? port 0x0815 irq 12 tty

assigns interrupt 12 to the ``tty'' priority group. The system automatically arranges for interrupts in the xxx group to be called at a priority >= spl Fn xxx

The function Fn splx sets the interrupt priority to an absolute value. The intent is that the value returned by the other functions should be saved in a local variable, and later passed to Fn splx in order to restore the previous priority.

The function Fn spl0 lowers the priority to a value where all interrupt handlers are unblocked, but ASTs (asynchronous system traps) remain blocked until the system is about to return to user mode.

The traditional assignment of the various device drivers to the interrupt priority groups can be roughly classified as:

Fn splnet
Software part of the network interface drivers.
Fn splimp
All network interface drivers.
Fn splbio
All buffered IO (i.e., disk and the like) drivers.
Fn spltty
Basically, all non-network communications devices, but effectively used for all drivers that are neither network nor disks.


All functions except Fn splx and Fn spl0 return the previous priority value.


This is a typical example demonstrating the usage:
struct foo_softc {
        int flags;
#define FOO_ASLEEP      1
#define FOO_READY       2
} foo_softc[NFOO];
        struct foo_softc *sc;
        int s, error;
        s = spltty();
        if (!(sc->flags & FOO_READY)) {
                /* Not ready, must sleep on resource. */
                sc->flags |= FOO_ASLEEP;
                error = tsleep(sc, PZERO, "foordy", 0);
                sc->flags &= ~FOO_ASLEEP;
        sc->flags &= ~FOO_READY;
        struct foo_softc *sc;
        sc->flags |= FOO_READY;
        if (sc->flags & FOO_ASLEEP)
                /* Somebody was waiting for us, awake him. */
Note that the interrupt handler should never reduce the priority level. It is automatically called as it had raised the interrupt priority to its own level, i.e., further interrupts of the same group are being blocked.


The interrupt priority levels appeared in a very early version of UNIX They have been traditionally known by number instead of by names, and were inclusive up to higher priority levels (i.e., priority 5 has been blocking everything up to level 5). This is no longer the case in Fx . The traditional name `level' for them is still reflected in the letter `l' of the respective functions and variables, although they are not really levels anymore, but rather different (partially inclusive) sets of functions to be blocked during some periods of the life of the system. The historical number scheme can be considered as a simple linearly ordered set of interrupt priority groups.

Fx 5.0 eliminated spl entirely in favor of locking primitives which scale to more than one processor.


This manual page was written by An Jörg Wunsch .