lcdproc/server/drivers/lcd_sem.c

/** \file server/drivers/lcd_sem.c
 * semaphore code to mediate access to the parallel port.
 */

/*
 * lcd_sem.c  -- semaphore code written for lcdtime and meter to mediate
 * access to the parallel port.
 *
 * Written by Benjamin Tse (blt@mundil.cs.mu.edu.au); August,October 1995
 *
 * Functions in this file:
 *   getkey      returns the key for the semaphore
 *   sem_get     create the semaphore (and initialise) if one doesn't exist
 *                otherwise return its key
 *   sem_wait    wait on the semaphore
 *   sem_signal  signal on the semaphore
 *   sem_remove  remove the semaphore
 *
 * Legal stuff: At no stage was this program written, assembled or compiled on
 * any computer at the University of Melbourne, Australia. This program is
 * Copyright (C) 1995 Benjamin Tse (blt@mundil.cs.mu.oz.au) and covered by
 * GNU's GPL. In particular, this program is free software and comes WITHOUT
 * ANY WARRANTY.
 *
 */

#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>			  /* for semaphore functions */
#include <sys/ipc.h>
#include <sys/sem.h>
#include <errno.h>

#ifdef HAVE_CONFIG_H
# include "config.h"
#endif

#include "lcd_sem.h"

// according to X/OPEN we have to define it ourselves
#ifndef HAVE_UNION_SEMUN
union semun {
	int val;                    /* value for SETVAL */
	struct semid_ds *buf;       /* buffer for IPC_STAT, IPC_SET */
	unsigned short int *array;  /* array for GETALL, SETALL */
	struct seminfo *__buf;      /* buffer for IPC_INFO */
};
#endif

#define SEMAPHORE       "portctrl"
#define SEMKEY          0x706f7274	/* semaphore key */
#define SEMCOUNT        1		  /* number of semaphores to create */
#define WMODE           0660	  /* access permissions */

#define SEM_SIGNAL      0,1,SEM_UNDO
#define SEM_WAIT        0,-1,SEM_UNDO

/* functions local to this file */
static key_t getkey(register char *p);

/* global variables */
static struct sembuf semaphore_wait = { SEM_WAIT };
static struct sembuf semaphore_signal = { SEM_SIGNAL };


/**
 * Return the key for the semaphore.
 */
static key_t
getkey(register char *p)
{
	return((key_t) SEMKEY);
}


/**
 * Get semaphore.
 * If the semaphore does not exist, create it and initialise it to 1,
 * otherwise simply return the semaphore ID.
 * \return     Semaphore ID on success; terminate program with code \c 1 on error.
 */

int
sem_get(void)
{
	int semid;
	union semun semval;

	if ((semid = semget(getkey(SEMAPHORE), SEMCOUNT, IPC_CREAT | IPC_EXCL | WMODE)) < 0) {
		switch (errno) {
		case EEXIST:
			/* semaphore set exists, get id and return it */
			if ((semid = semget(getkey(SEMAPHORE), SEMCOUNT, IPC_EXCL | WMODE)) < 0) {
				perror("semget");
				exit(1);
			}
			return semid;
			break;
		case EACCES:
			/* don't have permissions for semaphore, need to change key */
			perror("semget, can't get permissions for semaphore");
			exit(1);
			break;
		default:
			perror("semget");
			exit(1);
			break;
		}
	} else {
		/* initialise semaphore to 1 */
		semval.val = 1;

		if (semctl(semid, 0, SETVAL, semval) < 0) {
			perror("setval, can't initialise semaphore");
			exit(1);
		}
	}

	return semid;
}


/**
 * Wait on the semaphore.
 * \param sid  Semaphore ID.
 * \return     \c 0 on success; terminate program with code \c 1 on error.
 */
int
sem_wait(int sid)
{
	if (semop(sid, &semaphore_wait, 1) < -1) {
		perror(SEMAPHORE);
		exit(1);
	}

	return 0;
}


/**
 * Signal on the semaphore.
 * \param sid  Semaphore ID.
 * \return     \c 0 on success; terminate program with code \c 1 on error.
 */
int
sem_signal(int sid)
{
	if (semop(sid, &semaphore_signal, 1) < -1) {
		perror(SEMAPHORE);
		exit(1);
	}

	return 0;
}


/**
 * Remove the semaphore
 * \param sid  Semaphore ID.
 * \return     \c 0 on success; terminate program with code \c 1 on error.
 */
int
sem_remove(int sid)
{
#ifdef EIDRM
	int i;
	union semun dummy;

	if ((i = semctl(sid, 0, IPC_RMID, dummy)) < 0) {
		switch (i) {
		case EIDRM:
			/* semaphore removed */
			return 0;
			break;
		default:
			perror("semctl, removing semaphore");
			exit(1);
		}
	}

#endif
	return 0;
}