semaphore man page on SunOS

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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
       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  This  is  obsolete	 and should be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 ALSO
       kill(2), condvar(9F), mutex(9F)

       Writing Device Drivers

SunOS 5.10			  7 May 1997			 semaphore(9F)
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