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From: kushner@ux1.lbl.gov (Gary Kushner)
Newsgroups: comp.os.os2.programmer,connect.audit
Subject: Re: Making a non exclusive semaphore (like on Unix)
Keywords: semaphore, counter, exclusive
Message-ID: <11871@dog.ee.lbl.gov>
Date: 8 Apr 91 02:25:05 GMT
References: <1991Mar12.152033.11880@ibmpcug.co.uk>
Reply-To: kushner@ux1.lbl.gov (Gary Kushner)
Organization: Lawrence Berkeley Laboratory
Lines: 78
X-Local-Date: Sun, 7 Apr 91 19:25:05 PDT

Your code uses polling which should be avoided if at all possible.  Here
is the code I use:
----------
pv.h:
----------

/*------------
  Type for PVsems
  ------------*/

typedef struct _pvsem {
	long count;
	long more;				// sem clear if count > 0
	long mutex;
	} _far PVSEM;


/*-----------
  Protos
  -----------*/
PVSEM _far *PVSemCreate(void);
void _far _fastcall P(PVSEM _far *s);
void _far _fastcall V(PVSEM _far *s);

--------
pv.c
---------

#define INCL_DOSSEMAPHORES

#include <os2.h>
#include <malloc.h>
#include "pv.h"


/**************
*          PVSEMCREATE - allocate sem
**************/
PVSEM _far *PVSemCreate(void)
{
	PVSEM *s;

	if ( (s=calloc(1, sizeof(PVSEM))) != NULL)
		s->count=0;

	return s;
}


/**************
*          P - 
**************/
void _far _fastcall P(PVSEM _far *s)
{
		// any V's pending?  if not then wait...
	DosSemRequest(&s->more, SEM_INDEFINITE_WAIT);
	DosSemRequest(&s->mutex, SEM_INDEFINITE_WAIT);

		// if there are still more V's, then leave more clear
	if (--(s->count) > 0)
		DosSemClear(&s->more);

	DosSemClear(&s->mutex);
}


/**************
*          V - count them up
**************/
void _far _fastcall V(PVSEM _far *s)
{
	DosSemRequest(&s->mutex, SEM_INDEFINITE_WAIT);

	s->count++;
	DosSemClear(&s->more);		// ok guys, come and get it

	DosSemClear(&s->mutex);
}