mdw@git.distorted.org.uk Git - mLib/blob - lock.c

   1 /* -*-c-*-
   2  *
   3  * $Id: lock.c,v 1.3 1999/06/06 01:23:00 mdw Exp $
   4  *
   5  * Simplified POSIX locking interface
   6  *
   7  * (c) 1997 Straylight/Edgeware
   8  */
   9
  10 /*----- Licensing notice --------------------------------------------------*
  11  *
  12  * This file is part of the mLib utilities library.
  13  *
  14  * mLib is free software; you can redistribute it and/or modify
  15  * it under the terms of the GNU Library General Public License as
  16  * published by the Free Software Foundation; either version 2 of the
  17  * License, or (at your option) any later version.
  18  *
  19  * mLib is distributed in the hope that it will be useful,
  20  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  21  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  22  * GNU Library General Public License for more details.
  23  *
  24  * You should have received a copy of the GNU Library General Public
  25  * License along with mLib; if not, write to the Free
  26  * Software Foundation, Inc., 59 Temple Place - Suite 330, Boston,
  27  * MA 02111-1307, USA.
  28  */
  29
  30 /*----- Revision history --------------------------------------------------*
  31  *
  32  * $Log: lock.c,v $
  33  * Revision 1.3  1999/06/06 01:23:00  mdw
  34  * Fix signal handling.
  35  *
  36  * Revision 1.2  1999/05/26 20:53:40  mdw
  37  * Fixes for stupid bugs.
  38  *
  39  * Revision 1.1  1999/05/15 10:33:53  mdw
  40  * Add simplified locking code.
  41  *
  42  */
  43
  44 /*----- Header files ------------------------------------------------------*/
  45
  46 #include <errno.h>
  47 #include <signal.h>
  48 #include <stdio.h>
  49 #include <stdlib.h>
  50 #include <string.h>
  51 #include <time.h>
  52
  53 #include <sys/types.h>
  54 #include <unistd.h>
  55 #include <fcntl.h>
  56
  57 #include "lock.h"
  58
  59 /*----- Tunable constants -------------------------------------------------*/
  60
  61 #define LOCK_TIMEOUT 10                 /* Maximum time in seconds to wait */
  62
  63 /*----- Main code ---------------------------------------------------------*/
  64
  65 /* --- @lock_alarm@ --- *
  66  *
  67  * Arguments:   @int sig@ = signal number
  68  *
  69  * Returns:     ---
  70  *
  71  * Use:         Makes sure that a @SIGALRM@ signal interrupts system calls.
  72  */
  73
  74 static void lock_alarm(int sig) { ; }
  75
  76 /* --- @lock_file@ --- *
  77  *
  78  * Arguments:   @int fd@ = file descriptor to lock
  79  *              @unsigned how@ = type of lock required
  80  *
  81  * Returns:     0 if OK, -1 if it failed.
  82  *
  83  * Use:         Acquires a lock on the given file.  The value @how@
  84  *              specifies the type of lock to acquire: @LOCK_EXCL@ gets
  85  *              an exclusive (write) lock; @LOCK_NONEXCL@ gets a non-
  86  *              exclusive (read) lock and @LOCK_UNLOCK@ releases any locks.
  87  *              Acquiring a lock gets timed out after a while with an
  88  *              error.
  89  */
  90
  91 int lock_file(int fd, unsigned how)
  92 {
  93   struct flock fk;
  94   struct sigaction sa, oldsa;
  95   int e;
  96
  97   /* --- Fill in the easy bits --- */
  98
  99   fk.l_whence = SEEK_SET;
 100   fk.l_start = 0;
 101   fk.l_len = 0;
 102
 103   /* --- Unlocking is really easy --- */
 104
 105   if (how == LOCK_UNLOCK) {
 106     fk.l_type = F_UNLCK;
 107     return (fcntl(fd, F_SETLK, &fk));
 108   }
 109
 110   /* --- Decide how to do the locking --- */
 111
 112   if (how == LOCK_EXCL)
 113     fk.l_type = F_WRLCK;
 114   else if (how == LOCK_NONEXCL)
 115     fk.l_type = F_RDLCK;
 116   else {
 117     errno = EINVAL;
 118     return (-1);
 119   }
 120
 121   /* --- Set up the alarm --- */
 122
 123   sa.sa_handler = lock_alarm;
 124   sa.sa_flags = 0;
 125   sigemptyset(&sa.sa_mask);
 126   if (sigaction(SIGALRM, &sa, &oldsa))
 127     return (-1);
 128
 129   /* --- Do it --- */
 130
 131   alarm(LOCK_TIMEOUT);
 132   if ((e = fcntl(fd, F_SETLKW, &fk)) != 0) {
 133     if (errno == EINTR)
 134       errno = EAGAIN;
 135   }
 136
 137   /* --- Remove the alarm handling --- */
 138
 139   alarm(0);
 140   sigaction(SIGALRM, &oldsa, 0);
 141   return (e);
 142 }
 143
 144 /*----- That's all, fokls -------------------------------------------------*/