'\" te
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.TH SEMAPHORE 9F "May 7, 1997"
.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
.na
\fB\fIsp\fR\fR
.ad
.RS 8n
A pointer to a semaphore, type \fBksema_t\fR.
.RE

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

.sp
.ne 2
.na
\fB\fIname\fR\fR
.ad
.RS 8n
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 This is obsolete and should be \fINULL\fR.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
.na
\fB\fItype\fR\fR
.ad
.RS 8n
Variant type of the semaphore. Currently, only \fBSEMA_DRIVER\fR is supported.
.RE

.sp
.ne 2
.na
\fB\fIarg\fR\fR
.ad
.RS 8n
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
.na
\fB\fB0\fR\fR
.ad
.RS 5n
\fBsema_tryp()\fR could not decrement the semaphore value because it was zero.
.RE

.sp
.ne 2
.na
\fB\fB1\fR\fR
.ad
.RS 5n
\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\fR