server/drivers/lcd_sem.c

   1 /*
   2  * lcd_sem.c  -- semaphore code written for lcdtime and meter to mediate
   3  * access to the parallel port.
   4  *
   5  * Written by Benjamin Tse (blt@mundil.cs.mu.edu.au); August,October 1995
   6  *
   7  * Functions in this file:
   8  *   getkey      returns the key for the semaphore
   9  *   sem_get     create the semaphore (and initialise) if one doesn't exist
  10  *                otherwise return its key
  11  *   sem_wait    wait on the semaphore
  12  *   sem_signal  signal on the semaphore
  13  *   sem_remove  remove the semaphore
  14  *
  15  * Legal stuff: At no stage was this program written, assembled or compiled on
  16  * any computer at the University of Melbourne, Australia. This program is
  17  * Copyright (C) 1995 Benjamin Tse (blt@mundil.cs.mu.oz.au) and covered by
  18  * GNU's GPL. In particular, this program is free software and comes WITHOUT
  19  * ANY WARRANTY.
  20  *
  21  */
  22
  23 #include <config.h>
  24 #include <stdio.h>
  25 #include <stdlib.h>
  26 #include <sys/types.h>                    /* for semaphore functions */
  27 #include <sys/ipc.h>
  28 #include <sys/sem.h>
  29 #include <errno.h>
  30
  31 #include "lcd_sem.h"
  32
  33 // according to X/OPEN we have to define it ourselves
  34 //#ifdef _SEM_SEMUN_UNDEFINED
  35 #ifndef HAVE_UNION_SEMUN
  36 union semun {
  37         int val;                    /* value for SETVAL */
  38         struct semid_ds *buf;       /* buffer for IPC_STAT, IPC_SET */
  39         unsigned short int *array;  /* array for GETALL, SETALL */
  40         struct seminfo *__buf;      /* buffer for IPC_INFO */
  41 };
  42 #endif
  43
  44 #define SEMAPHORE       "portctrl"
  45 #define SEMKEY          0x706f7274      /* semaphore key */
  46 #define SEMCOUNT        1                 /* number of semaphores to create */
  47 #define WMODE           0660      /* access permissions */
  48
  49 #define SEM_SIGNAL      0,1,SEM_UNDO
  50 #define SEM_WAIT        0,-1,SEM_UNDO
  51
  52 /* functions local to this file */
  53 static key_t getkey (register char *p);
  54
  55 /* global variables */
  56 static struct sembuf semaphore_wait = { SEM_WAIT };
  57 static struct sembuf semaphore_signal = { SEM_SIGNAL };
  58
  59 /*
  60  * getkey  returns the key for the semaphore
  61  */
  62
  63 static key_t
  64 getkey (register char *p)
  65 {
  66         return ((key_t) SEMKEY);
  67 }
  68
  69 /*
  70  * create the semaphore if one doesn't exist and initialise it to 1, else
  71  * return the semaphore set id
  72  */
  73
  74 int
  75 sem_get (void)
  76 {
  77         int semid;
  78         union semun semval;
  79
  80         if ((semid = semget (getkey (SEMAPHORE), SEMCOUNT, IPC_CREAT | IPC_EXCL | WMODE)) < 0) {
  81                 switch (errno) {
  82                 case EEXIST:
  83                         /* semaphore set exists, get id and return it */
  84                         if ((semid = semget (getkey (SEMAPHORE), SEMCOUNT, IPC_EXCL | WMODE)) < 0) {
  85                                 perror ("semget");
  86                                 exit (1);
  87                         }
  88                         return semid;
  89                         break;
  90                 case EACCES:
  91                         /* don't have permissions for semaphore, need to change key */
  92                         perror ("semget, can't get permissions for semaphore");
  93                         exit (1);
  94                         break;
  95                 default:
  96                         perror ("semget");
  97                         exit (1);
  98                         break;
  99                 }
 100         } else {
 101                 /* initialise semaphore to 1 */
 102                 semval.val = 1;
 103
 104                 if (semctl (semid, 0, SETVAL, semval) < 0) {
 105                         perror ("setval, can't initialise semaphore");
 106                         exit (1);
 107                 }
 108         }
 109
 110         return semid;
 111 }
 112
 113 /*
 114  * wait on the semaphore
 115  */
 116
 117 int
 118 sem_wait (int sid)
 119 {
 120         if (semop (sid, &semaphore_wait, 1) < -1) {
 121                 perror (SEMAPHORE);
 122                 exit (1);
 123         }
 124
 125         return 0;
 126 }
 127
 128 /*
 129  * signal on the semaphore
 130  */
 131
 132 int
 133 sem_signal (int sid)
 134 {
 135         if (semop (sid, &semaphore_signal, 1) < -1) {
 136                 perror (SEMAPHORE);
 137                 exit (1);
 138         }
 139
 140         return 0;
 141 }
 142
 143 /*
 144  * remove the semaphore
 145  */
 146
 147 int
 148 sem_remove (int sid)
 149 {
 150 #ifdef EIDRM
 151         int i;
 152         union semun dummy;
 153
 154         if ((i = semctl (sid, 0, IPC_RMID, dummy)) < 0) {
 155                 switch (i) {
 156                 case EIDRM:
 157                         /* semaphore removed */
 158                         return 0;
 159                         break;
 160                 default:
 161                         perror ("semctl, removing semaphore");
 162                         exit (1);
 163                 }
 164         }
 165
 166 #endif
 167         return 0;
 168 }