SWI(9) BSD Kernel Developer's Manual SWI(9)
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 handlers. Software interrupt handlers are attached to a software inter-
rupt 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
The swi_add() function is used to register a new software interrupt handler. 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 vec-
tor, 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
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 specifies 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 software
interrupt thread to run. Instead, 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 setdelayed() in earlier 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 respectively.
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 processes 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 INTR_FAST.
[EINVAL] The ithdp argument points to a hardware interrupt thread.
[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 exclusive handler.
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 least 4.4BSD.
Most of the global variables described in this manual page should not be global, or at the very least should not be declared in
October 30, 2000 BSD