SEMAPHORE(9F)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
Solaris DDI specific (Solaris 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 This is obsolete and should be NULL.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 neces‐
sary, 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 with‐
out decrementing the value if a signal (that is, from kill(2)) is pend‐
ing for the thread.
The sema_tryp() function will decrement the semaphore value only if it
is greater than zero, and will not block.
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.
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 net‐
work 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.
SEE ALSOkill(2), condvar(9F), mutex(9F)
Writing Device Drivers
May 7, 1997 SEMAPHORE(9F)
