semaphore man page on SmartOS

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

       #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);

       Solaris DDI specific (Solaris DDI).

	       A pointer to a semaphore, type ksema_t.

	       Initial value for semaphore.

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 This is obsolete and should be NULL.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

	       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() 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.

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

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

       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.

       kill(2), condvar(9F), mutex(9F)

       Writing Device Drivers

				  May 7, 1997			 SEMAPHORE(9F)

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