illumos: manual page: sema

SEMAPHORE(9F) Kernel Functions for Drivers SEMAPHORE(9F)

NAME

semaphore, sema_init, sema_destroy, sema_p, sema_p_sig, sema_v, sema_tryp -
semaphore functions

SYNOPSIS

#include <sys/ksynch.h>

void
sema_init(ksema_t *sp, uint_t val, char *name, ksema_type_t type,
void *arg);

void
sema_destroy(ksema_t *sp);

void
sema_p(ksema_t *sp);

void
sema_v(ksema_t *sp);

int
sema_p_sig(ksema_t *sp);

int
sema_tryp(ksema_t *sp);

INTERFACE LEVEL

illumos DDI specific (illumos DDI).

PARAMETERS

sp A pointer to a semaphore, type ksema_t.

val Initial value for semaphore.

name Descriptive string. This is obsolete and should be NULL.
(Non-NULL strings are legal, but they are a waste of kernel
memory.)

type Variant type of the semaphore. Currently, only SEMA_DRIVER is
supported.

arg Type-specific argument; should be NULL.

DESCRIPTION

These functions implement counting semaphores as described by Dijkstra. A
semaphore has a value which is atomically decremented by sema_p() and
atomically incremented by sema_v(). The value must always be greater than
or equal to zero. If sema_p() is called and the value is zero, the calling
thread is blocked until another thread performs a sema_v() operation on the
semaphore.

Semaphores are initialized by calling sema_init(). The argument, val,
gives the initial value for the semaphore. The semaphore storage is
provided by the caller but more may be dynamically allocated, if necessary,
by sema_init(). For this reason, sema_destroy() should be called before
deallocating the storage containing the semaphore.

The sema_p_sig() function decrements the semaphore, as does sema_p().
However, if the semaphore value is zero, sema_p_sig() will return without
decrementing the value if a signal (that is, from kill(2)) is pending for
the thread.

The sema_tryp() function will decrement the semaphore value only if it is
greater than zero, and will not block.

CONTEXT

These functions can be called from user, interrupt, or kernel context,
except for sema_init() and sema_destroy(), which can be called from user or
kernel context only. None of these functions can be called from a high-
level interrupt context. In most cases, sema_v() and sema_p() should not
be called from any interrupt context.

If sema_p() is used from interrupt context, lower-priority interrupts will
not be serviced during the wait. This means that if the thread that will
eventually perform the sema_v() becomes blocked on anything that requires
the lower-priority interrupt, the system will hang.

For example, the thread that will perform the sema_v() may need to first
allocate memory. This memory allocation may require waiting for paging I/O
to complete, which may require a lower-priority disk or network interrupt
to be serviced. In general, situations like this are hard to predict, so
it is advisable to avoid waiting on semaphores or condition variables in an
interrupt context.

Similar to many other synchronization mechanisms, semaphores should not be
used in any code path that requires synchronization while handling system
panic, at which time many of the semaphore operations become no-ops.

RETURN VALUES

0 sema_tryp() could not decrement the semaphore value because it was
zero.

1 sema_p_sig() was not able to decrement the semaphore value and
detected a pending signal.