SEMAPHORE(9F) Kernel Functions for Drivers SEMAPHORE(9F)
SYNOPSIS #include <sys/ksynch.h> void sema_init
, uint_t val
, char *name
, ksema_type_t type
, void *arg
); void sema_destroy
); void sema_p
); void sema_v
); int sema_p_sig
); int sema_tryp
illumos DDI specific (illumos DDI).
A pointer to a semaphore, type ksema_t
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 This is obsolete and should be NULL.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
Variant type of the semaphore. Currently, only SEMA_DRIVER is
Type-specific argument; should be NULL.
These functions implement counting semaphores as described by Dijkstra. A
semaphore has a value which is atomically decremented by sema_p
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
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,
(). For this reason, sema_destroy
() should be called before
deallocating the storage containing the semaphore.
() 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
() function will decrement the semaphore value only if it is
greater than zero, and will not block.
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.
() 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
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.
() could not decrement the semaphore value because it was
() was not able to decrement the semaphore value and
detected a pending signal.
SEE ALSO kill(2)
, mutex(9F) Writing Device Drivers
OmniOS July 30, 2018 OmniOS