'\" te
.\"  Copyright (c) 2006, Sun Microsystems, Inc.  All Rights Reserved
.TH semaphore 9F "7 May 1997" "SunOS 5.11" "Kernel Functions for Drivers"
.SH NAME
semaphore, sema_init, sema_destroy, sema_p, sema_p_sig, sema_v, sema_tryp \- semaphore functions
.SH SYNOPSIS
.LP
.nf
#include <sys/ksynch.h>

\fBvoid\fR \fBsema_init\fR(\fBksema_t *\fR\fIsp\fR, \fBuint_t\fR \fIval\fR, \fBchar *\fR\fIname\fR, \fBksema_type_t\fR \fItype\fR,
      \fBvoid *\fR\fIarg\fR);
.fi

.LP
.nf
\fBvoid\fR \fBsema_destroy\fR(\fBksema_t *\fR\fIsp\fR);
.fi

.LP
.nf
\fBvoid\fR \fBsema_p\fR(\fBksema_t *\fR\fIsp\fR);
.fi

.LP
.nf
\fBvoid\fR \fBsema_v\fR(\fBksema_t *\fR\fIsp\fR);
.fi

.LP
.nf
\fBint\fR \fBsema_p_sig\fR(\fBksema_t *\fR\fIsp\fR);
.fi

.LP
.nf
\fBint\fR \fBsema_tryp\fR(\fBksema_t *\fR\fIsp\fR);
.fi

.SH INTERFACE LEVEL
.sp
.LP
Solaris \fBDDI\fR specific (Solaris \fBDDI\fR).
.SH PARAMETERS
.sp
.ne 2
.mk
.na
\fB\fIsp\fR\fR
.ad
.RS 8n
.rt  
A pointer to a semaphore, type \fBksema_t\fR.
.RE

.sp
.ne 2
.mk
.na
\fB\fIval\fR\fR
.ad
.RS 8n
.rt  
Initial value for semaphore.
.RE

.sp
.ne 2
.mk
.na
\fB\fIname\fR\fR
.ad
.RS 8n
.rt  
Descriptive string. This is obsolete and should be \fINULL\fR. (Non-\fINULL\fR strings are legal, but they are a waste of kernel memory.)
.RE

.sp
.ne 2
.mk
.na
\fB\fItype\fR\fR
.ad
.RS 8n
.rt  
Variant type of the semaphore. Currently, only \fBSEMA_DRIVER\fR is supported.
.RE

.sp
.ne 2
.mk
.na
\fB\fIarg\fR\fR
.ad
.RS 8n
.rt  
Type-specific argument; should be \fINULL\fR.
.RE

.SH DESCRIPTION
.sp
.LP
These functions implement counting semaphores as described by Dijkstra. A semaphore has a value which is atomically decremented by \fBsema_p()\fR and atomically incremented by \fBsema_v()\fR. The value must always be greater than or equal to zero. If \fBsema_p()\fR is called and the value is zero, the calling thread is blocked until another thread performs a \fBsema_v()\fR operation on the semaphore.
.sp
.LP
Semaphores are initialized by calling \fBsema_init()\fR. The argument, \fBval\fR, gives the initial value for the semaphore. The semaphore storage is provided by the caller but more may be dynamically allocated, if necessary, by \fBsema_init()\fR. For this reason, \fBsema_destroy()\fR should be called before deallocating the storage containing the semaphore.
.sp
.LP
The \fBsema_p_sig()\fR function decrements the semaphore, as does \fBsema_p()\fR. However, if the semaphore value is zero, \fBsema_p_sig()\fR will return without decrementing the value if a signal (that is, from \fBkill\fR(2)) is pending for the thread.
.sp
.LP
The \fBsema_tryp()\fR function will decrement the semaphore value only if it is greater than zero, and will not block.
.SH RETURN VALUES
.sp
.ne 2
.mk
.na
\fB\fB0\fR\fR
.ad
.RS 5n
.rt  
\fBsema_tryp()\fR could not decrement the semaphore value because it was zero.
.RE

.sp
.ne 2
.mk
.na
\fB\fB1\fR\fR
.ad
.RS 5n
.rt  
\fBsema_p_sig()\fR was not able to decrement the semaphore value and detected a pending signal.
.RE

.SH CONTEXT
.sp
.LP
These functions can be called from user, interrupt, or kernel context, except for \fBsema_init()\fR and \fBsema_destroy()\fR, 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, \fBsema_v()\fR and \fBsema_p()\fR should not be called from any interrupt context.
.sp
.LP
If \fBsema_p()\fR 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 \fBsema_v()\fR becomes blocked on anything that requires the lower-priority interrupt, the system will hang.
.sp
.LP
For example, the thread that will perform the \fBsema_v()\fR may need to first allocate memory. This memory allocation may require waiting for paging \fBI/O\fR 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.
.SH SEE ALSO
.sp
.LP
\fBkill\fR(2), \fBcondvar\fR(9F), \fBmutex\fR(9F) 
.sp
.LP
\fIWriting Device Drivers for Oracle Solaris 11.2\fR