swi_add(9) register and schedule software interrupt handlers

Other Alias

swi_remove, swi_sched


In sys/param.h In sys/bus.h In sys/interrupt.h Vt extern struct intr_event *tty_intr_event ; Vt extern struct intr_event *clk_intr_event ; Vt extern void *vm_ih ; Ft int Fo swi_add Fa struct intr_event **eventp Fa const char *name Fa driver_intr_t handler Fa void *arg Fa int pri Fa enum intr_type flags Fa void **cookiep Fc Ft int Fn swi_remove void *cookie Ft void Fn swi_sched void *cookie int flags


These functions are used to register and schedule software interrupt handlers. Software interrupt handlers are attached to a software interrupt thread, just as hardware interrupt handlers are attached to a hardware interrupt thread. Multiple handlers can be attached to the same thread. Software interrupt handlers can be used to queue up less critical processing inside of hardware interrupt handlers so that the work can be done at a later time. Software interrupt threads are different from other kernel threads in that they are treated as an interrupt thread. This means that time spent executing these threads is counted as interrupt time, and that they can be run via a lightweight context switch.

The Fn swi_add function is used to add a new software interrupt handler to a specified interrupt event. The Fa eventp argument is an optional pointer to a Vt struct intr_event pointer. If this argument points to an existing event that holds a list of interrupt handlers, then this handler will be attached to that event. Otherwise a new event will be created, and if Fa eventp is not NULL then the pointer at that address to will be modified to point to the newly created event. The Fa name argument is used to associate a name with a specific handler. This name is appended to the name of the software interrupt thread that this handler is attached to. The Fa handler argument is the function that will be executed when the handler is scheduled to run. The Fa arg parameter will be passed in as the only parameter to Fa handler when the function is executed. The Fa pri value specifies the priority of this interrupt handler relative to other software interrupt handlers. If an interrupt event is created, then this value is used as the vector, and the Fa flags argument is used to specify the attributes of a handler such as INTR_MPSAFE The Fa cookiep argument points to a Vt void * cookie. This cookie will be set to a value that uniquely identifies this handler, and is used to schedule the handler for execution later on.

The Fn swi_remove function is used to teardown an interrupt handler pointed to by the Fa cookie argument. It detaches the interrupt handler from the associated interrupt event and frees its memory.

The Fn swi_sched function is used to schedule an interrupt handler and its associated thread to run. The Fa cookie argument specifies which software interrupt handler should be scheduled to run. The Fa flags argument specifies how and when the handler should be run and is a mask of one or more of the following flags:

Specifies that the kernel should mark the specified handler as needing to run, but the kernel should not schedule the software interrupt thread to run. Instead, Fa handler will be executed the next time that the software interrupt thread runs after being scheduled by another event. Attaching a handler to the clock software interrupt thread and using this flag when scheduling a software interrupt handler can be used to implement the functionality performed by Fn setdelayed in earlier versions of Fx .

The tty_intr_event and clk_intr_event variables contain pointers to the software interrupt handlers for the tty and clock software interrupts, respectively. tty_intr_event is used to hang tty software interrupt handlers off of the same thread. clk_intr_event is used to hang delayed handlers off of the clock software interrupt thread so that the functionality of Fn setdelayed can be obtained in conjunction with SWI_DELAY The vm_ih handler cookie is used to schedule software interrupt threads to run for the VM subsystem.


The Fn swi_add and Fn swi_remove functions return zero on success and non-zero on failure.


The Fn swi_add function will fail if:

The system-imposed limit on the total number of processes under execution would be exceeded. The limit is given by the sysctl(3) MIB variable KERN_MAXPROC
The Fa flags argument specifies INTR_ENTROPY
The Fa eventp argument points to a hardware interrupt thread.
Either of the Fa name or Fa handler arguments are NULL
The INTR_EXCL flag is specified and the interrupt event pointed to by Fa eventp already has at least one handler, or the interrupt event already has an exclusive handler.

The Fn swi_remove function will fail if:

A software interrupt handler pointed to by Fa cookie is NULL


The Fn swi_add and Fn swi_sched functions first appeared in Fx 5.0 . They replaced the Fn register_swi function which appeared in Fx 3.0 and the Fn setsoft* , and Fn schedsoft* functions which date back to at least BSD 4.4 The Fn swi_remove function first appeared in Fx 6.1 .


Most of the global variables described in this manual page should not be global, or at the very least should not be declared in In sys/interrupt.h .