SWI(9) BSD Kernel Developer's Manual SWI(9)NAME
swi_add, swi_sched — register and schedule software interrupt handlers
extern struct ithd *tty_ithd;
extern struct ithd *clk_ithd;
extern void *net_ih;
extern void *softclock_ih;
extern void *vm_ih;
swi_add(struct ithd **ithdp, const char *name, driver_intr_t handler,
void *arg, int pri, enum intr_type flags, void **cookiep);
swi_sched(void *cookie, int flags);
These functions are used to register and schedule software interrupt han‐
dlers. 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 pro‐
cessing 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 inter‐
rupt time, and that they can be run via a lightweight context switch.
The swi_add() function is used to register a new software interrupt han‐
dler. The ithdp argument is an optional pointer to a struct ithd
pointer. If this argument points to an existing software interrupt
thread, then this handler will be attached to that thread. Otherwise a
new thread will be created, and if ithdp is not NULL, then the pointer at
that address to will be modified to point to the newly created thread.
The 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 handler argument is the function that
will be executed when the handler is scheduled to run. The arg parameter
will be passed in as the only parameter to handler when the function is
executed. The pri value specifies the priority of this interrupt handler
relative to other software interrupt handlers. If an interrupt thread is
created, then this value is used as the vector, and the flags argument is
used to specify the attributes of a handler such as INTR_MPSAFE. The
cookiep argument points to a 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 swi_sched() function is used to schedule an interrupt handler and its
associated thread to run. The cookie argument specifies which software
interrupt handler should be scheduled to run. The flags argument speci‐
fies how and when the handler should be run and is a mask of one or more
of the following flags:
SWI_DELAY Specifies that the kernel should mark the specified handler as
needing to run, but the kernel should not schedule the soft‐
ware interrupt thread to run. Instead, handler will be exe‐
cuted 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 setdelayed() in ear‐
lier versions of FreeBSD.
The tty_ithd and clk_ithd variables contain pointers to the software
interrupt threads for the tty and clock software interrupts, respec‐
tively. tty_ithd is used to hang tty software interrupt handlers off of
the same thread. clk_ithd is used to hang delayed handlers off of the
clock software interrupt thread so that the functionality of setdelayed()
can be obtained in conjunction with SWI_DELAY. The net_ih, softclock_ih,
and vm_ih handler cookies are used to schedule software interrupt threads
to run for the networking stack, clock interrupt, and VM subsystem
The swi_add() function returns zero on success and non-zero on failure.
The swi_add() function will fail if:
[EAGAIN] The system-imposed limit on the total number of pro‐
cesses under execution would be exceeded. The limit
is given by the sysctl(3) MIB variable KERN_MAXPROC.
[EINVAL] The flags argument specifies either INTR_ENTROPY or
[EINVAL] The ithdp argument points to a hardware interrupt
[EINVAL] Either of the name or handler arguments are NULL.
[EINVAL] The INTR_EXCL flag is specified and the interrupt
thread pointed to by ithdp already has at least one
handler, or the interrupt thread already has an exclu‐
SEE ALSOithread(9), taskqueue(9)HISTORY
The swi_add() and swi_sched() functions first appeared in FreeBSD 5.0.
They replaced the register_swi() function which appeared in FreeBSD 3.0
and the setsoft*(), and schedsoft*() functions which date back to at
Most of the global variables described in this manual page should not be
global, or at the very least should not be declared in <sys/interrupt.h>.
BSD October 30, 2000 BSD